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Computer pioneers correctly predicted that programmers would want unlimited amounts of fast memory. An economical solution to that desire is a memory hierar- chy, which takes advantage of locality and trade-offs in the cost-performance of memory technologies. The principle of locality, presented in the first chapter says that most programs do not access all code or data uniformly. Locality occurs in time (temporal locality) and in space (spatial locality). This principle plus the guideline that for a given implementation technology and power budget smaller hardware can be made faster led to hierarchies based on memories of different speeds and sizes. Figure 2.1 shows several different multilevel memory hierarchies, including typical sizes and speeds of access. As Flash and next generation memory technol- ogies continue to close the gap with disks in cost per bit such technologies are likely to increasingly replace magnetic disks for secondary storage. As Figure 2.1 shows, these technologies are already used in many personal computers and increasingly in servers, where the advantages in performance, power, and density are significant.Because fast memory is more expensive, a memory hierarchy is organized into several levels-each smaller faster, and more expensive per byte than the next lower level, which is farther from the processor. The goal is to provide a memory system with a cost per byte that is almost as low as the cheapest level of memory and a speed almost as fast as the fastest level. In most cases (but not all), the data contained in a lower level are a superset of the next higher level. This property, called the inclusion property, is always required for the lowest level of the hierar- chy, which consists of main memory in the case of caches and secondary storage (disk or Flash) in the case of virtual memory.The importance of the memory hierarchy has increased with advances in per- formance of processors. Figure 2.2 plots single processor performance projections against the historical performance improvement in time to access main memory. The processor line shows the increase in memory requests per second on average (I .e ., the inverse of the latency between memory references), while the memory line shows the increase in DRAM accesses per second (I .e., the inverse of the DRAM access latency), assuming a single DRAM and a single memory bank. The reality is more complex because the processor request rate is not uniform and the memory system typically has multiple banks of DRAMs and channels. Although the gap in access time increased significantly for many years, the lack of significant perfor- mance improvement in single processors has led to a slowdown in the growth of the gap between processors and DRAM.Because high-end processors have multiple cores the bandwidth requirements are greater than for single cores. Although single-core bandwidth has grown more slowly in recent years, the gap between CPU memory demand and DRAM band- width continues to grow as the numbers of cores grow. A modern high-end desktop processor such as the Intel Core i7 6700 can generate two data memory references per core each clock cycle. With four cores and a 4.2 GHz clock rate, the i7 can generate a peak of 32.8 billion 64-bit data memory references per second, in addi- tion to a peak instruction demand of about 12.8 billion 128-bit instruction references; this is a total peak demand bandwidth of 409.6 GiB/s! This incredible bandwidth is achieved by multiporting and pipelining the caches; by using three levels of caches, with two private levels per core and a shared L3; and by using a separate instruction and data cache at first level. In contrast, the peak band- width for DRAM main memory, using two memory channels, is only 8% of the demand bandwidth (34.1 GiB/s). Upcoming versions are expected to have an L4 DRAM cache using embedded or stacked DRAM (see Sections 2.2 and 2.3). Traditionally designers of memory hierarchies focused on optimizing average memory access time, which is determined by the cache access time, miss rate, and miss penalty. More recently, however, power has become a major consideration. In high-end microprocessors, there may be 60 MiB or more of on-chip cache and a large second- or third-level cache will consume significant power both as leakage when not operating (called static power) and as active power, as when performing a read or write (called dynamic power), as described in Section 2.3. The problem is even more acute in processors in PMDs where the CPU is less aggressive and the power budget may be 20 to 50 times smaller. In such cases, the caches can account for 25% to 50% of the total power consumption. Thus more designs must consider both performance and power trade-offs, and we will examine both in this chapter.
Basics of Memory Hierarchies: A Quick Review The increasing size and thus importance of this gap led to the migration of the basics of memory hierarchy into undergraduate courses in computer architecture, and even to courses in operating systems and compilers. Thus we'll start with a quick review of caches and their operation. The bulk of the chapter, however describes more advanced innovations that attack the processor-memory performance gap.When a word is not found in the cache, the word must be fetched from a lower level in the hierarchy (which may be another cache or the main memory) and placed in the cache before continuing. Multiple words, called a block (or line) are moved for efficiency reasons, and because they are likely to be needed soon due to spatial locality. Each cache block includes a tag to indicate which memory address it corresponds to.A key design decision is where blocks (or lines) can be placed in a cache. The most popular scheme is set associative where a set is a group of blocks in the cache. A block is first mapped onto a set, and then the block can be placed any- where within that set. Finding a block consists of first mapping the block address to the set and then searching the set-usually in parallel-to find the block. The set is chosen by the address of the data:
If there are n blocks in a set, the cache placement is called n-way set associative. The end points of set associativity have their own names. A direct-mapped cache has just one block per set (so a block is always placed in the same location), and a fully associative cache has just one set (so a block can be placed anywhere).Caching data that is only read is easy because the copy in the cache and mem- ory will be identical. Caching writes is more difficult; for example, how can the copy in the cache and memory be kept consistent? There are two main strategies. A write-through cache updates the item in the cache and writes through to update main memory. A write-back cache only updates the copy in the cache. When the block is about to be replaced, it is copied back to memory. Both write strategies can use a write buffer to allow the cache to proceed as soon as the data are placed in the buffer rather than wait for full latency to write the data into memory.One measure of the benefits of different cache organizations is miss rate. Miss rate is simply the fraction cache accesses that result in a miss-that is, the number of accesses that miss divided by the number of accesses.To gain insights into the causes of high miss rates, which can inspire better cache designs, the three Cs model sorts all misses into three simple categories:
- Compulsory-The very first access to a block cannot be in the cache, so the block must be brought into the cache. Compulsory misses are those that occur even if you were to have an infinite-sized cache.
- Capacity-If the cache cannot contain all the blocks needed during execution of a program, capacity misses (in addition to compulsory misses) will occur because of blocks being discarded and later retrieved.
- Conflict-If the block placement strategy is not fully associative, conflict mis- ses (in addition to compulsory and capacity misses) will occur because a block may be discarded and later retrieved if multiple blocks map to its set and accesses to the different blocks are intermingled.
Figure B .8 on page 24 shows the relative frequency of cache misses broken down by the three Cs. As mentioned in Appendix B, the three C's model is conceptual, and although its insights usually hold it is not a definitive model for explaining the cache behavior of individual references.As we will see in Chapters 3 and 5, multithreading and multiple cores add com- plications for caches, both increasing the potential for capacity misses as well as adding a fourth C for coherency misses due to cache flushes to keep multiple caches coherent in a multiprocessor; we will consider these issues in Chapter 5. However, miss rate can be a misleading measure for several reasons. Therefore some designers prefer measuring misses per instruction rather than misses permemory reference (miss rate). These two are related:
(This equation is often expressed in integers rather than fractions as misses per 1000 instructions.)The problem with both measures is that they don't factor in the cost of a miss.A better measure is the average memory access time,Average memory access time ¼ Hit time + Miss rate × Miss penaltywhere hit time is the time to hit in the cache and miss penalty is the time to replace the block from memory (that is, the cost of a miss). Average memory access time is still an indirect measure of performance; although it is a better measure than miss rate, it is not a substitute for execution time. In Chapter 3 we will see that specu- lative processors may execute other instructions during a miss, thereby reducing the effective miss penalty. The use of multithreading (introduced in Chapter 3) also allows a processor to tolerate misses without being forced to idle. As we will exam- ine shortly to take advantage of such latency tolerating techniques, we need caches that can service requests while handling an outstanding miss.If this material is new to you, or if this quick review moves too quickly, see Appendix B. It covers the same introductory material in more depth and includes examples of caches from real computers and quantitative evaluations of their effectiveness.Section B .3 in Appendix B presents six basic cache optimizations, which we quickly review here. The appendix also gives quantitative examples of the benefits of these optimizations. We also comment briefly on the power implications of these trade-offs.
1.Larger block size to reduce miss rate-The simplest way to reduce the miss rate is to take advantage of spatial locality and increase the block size. Larger blocks reduce compulsory misses, but they also increase the miss penalty. Because larger blocks lower the number of tags they can slightly reduce static power. Larger block sizes can also increase capacity or conflict misses, especially in smaller caches. Choosing the right block size is a complex trade-off that depends on the size of cache and the miss penalty.2.Bigger caches to reduce miss rate-The obvious way to reduce capacity misses is to increase cache capacity. Drawbacks include potentially longer hit time of the larger cache memory and higher cost and power. Larger caches increase both static and dynamic power.3.Higher associativity to reduce miss rate-Obviously, increasing associativity reduces conflict misses. Greater associativity can come at the cost of increased hit time. we will see shortly, associativity also increases power consumption.4.Multilevel caches to reduce miss penalty-A difficult decision is whether to make the cache hit time fast, to keep pace with the high clock rate of proces- sors, or to make the cache large to reduce the gap between the processor accesses and main memory accesses. Adding another level of cache between the original cache and memory simplifies the decision. The first-level cache can be small enough to match a fast clock cycle time, yet the second-level (or third-level) cache can be large enough to capture many accesses that would go to main memory. The focus on misses in second-level caches leads to larger blocks, bigger capacity, and higher associativity. Multilevel caches are more power-efficient than a single aggregate cache. If L1 and L2 refer, respectively, to first- and second-level caches, we can redefine the average memory access time:
5.Giving priority to read misses over writes to reduce miss penalty-A write buffer is a good place to implement this optimization. Write buffers create haz- ards because they hold the updated value of a location needed on a read miss- that is a read-after-write hazard through memory. One solution is to check the contents of the write buffer on a read miss. If there are no conflicts, and if the memory system is available sending the read before the writes reduces the miss penalty. Most processors give reads priority over writes. This choice has little effect on power consumption.6.Avoiding address translation during indexing of the cache to reduce hit time- Caches must cope with the translation of a virtual address from the processor to a physical address to access memory. (Virtual memory is covered in Sections 2.4 and B .4.) A common optimization is to use the page offset-the part that is identical in both virtual and physical addresses-to index the cache, as described in Appendix B, page B .38. This virtual index/physical tag method introduces some system complications and/or limitations on the size and struc- ture of the L1 cache, but the advantages of removing the translation lookaside buffer (TLB) access from the critical path outweigh the disadvantages.
Note that each of the preceding six optimizations has a potential disadvantage that can lead to increased, rather than decreased, average memory access time.The rest of this chapter assumes familiarity with the preceding material and the details in Appendix B. In the “Putting It All Together” section we examine the memory hierarchy for a microprocessor designed for a high-end desktop or smaller server, the Intel Core i7 6700, as well as one designed for use in a PMD, the Arm Cortex-53, which is the basis for the processor used in several tablets and smart- phones. Within each of these classes there is a significant diversity in approach because of the intended use of the computer.Although the i7 6700 has more cores and bigger caches than the Intel proces- sors designed for mobile uses, the processors have similar architectures. A proces- sor designed for small servers, such as the i7 6700, or larger servers such as the Intel Xeon processors, typically is running a large number of concurrent processes, often for different users. Thus memory bandwidth becomes more important, and these processors offer larger caches and more aggressive memory systems to boost that bandwidth.In contrast, PMDs not only serve one user but generally also have smaller oper- ating systems usually less multitasking (running of several applications simulta- neously), and simpler applications. PMDs must consider both performance and energy consumption, which determines battery life. Before we dive into more advanced cache organizations and optimizations, one needs to understand the various memory technologies and how they are evolving.
2.2 Memory Technology and Optimizations
... the one single development that put computers on their feet was the invention of a reliable form of memory, namely, the core memory. … Its cost was reasonable, it was reliable and, because it was reliable, it could in due course be made large. (p. 209)Maurice Wilkes.Memoirs of a Computer Pioneer (1985)
This section describes the technologies used in a memory hierarchy, specifically in building caches and main memory. These technologies are SRAM (static random- access memory), DRAM (dynamic random-access memory), and Flash. The last of these is used as an alternative to hard disks, but because its characteristics are based on semiconductor technology it is appropriate to include in this section.Using SRAM addresses the need to minimize access time to caches. When a cache miss occurs, however, we need to move the data from the main memory as quickly as possible which requires a high bandwidth memory. This high memory bandwidth can be achieved by organizing the many DRAM chips that make up the main memory into multiple memory banks and by making the memory bus wider, or by doing both.To allow memory systems to keep up with the bandwidth demands of modern processors memory innovations started happening inside the DRAM chips themselves. This section describes the technology inside the memory chips and those innovative, internal organizations. Before describing the technologies and options, we need to introduce some terminology.With the introduction of burst transfer memories, now widely used in both Flash and DRAM memory latency is quoted using two measures-access time and cycle time. Access time is the time between when a read is requested and when the desired word arrives, and cycle time is the minimum time between unrelated requests to memory.Virtually all computers since 1975 have used DRAMs for main memory and SRAMs for cache with one to three levels integrated onto the processor chip with the CPU. PMDs must balance power and performance, and because they have more modest storage needs, PMDs use Flash rather than disk drives, a decision increasingly being followed by desktop computers as well.
SRAM Technology The first letter of SRAM stands for static. The dynamic nature of the circuits in DRAM requires data to be written back after being read-thus the difference between the access time and the cycle time as well as the need to refresh. SRAMs don't need to refresh so the access time is very close to the cycle time. SRAMs typically use six transistors per bit to prevent the information from being disturbed when read. SRAM needs only minimal power to retain the charge in standby mode. In earlier times, most desktop and server systems used SRAM chips for their primary, secondary or tertiary caches. Today, all three levels of caches are inte- grated onto the processor chip. In high-end server chips, there may be as many as 24 cores and up to 60 MiB of cache; such systems are often configured with 128-256 GiB of DRAM per processor chip. The access times for large, third-level on-chip caches are typically two to eight times that of a second-level cache. Even so, the L3 access time is usually at least five times faster than a DRAM access.On-chip, cache SRAMs are normally organized with a width that matches the block size of the cache with the tags stored in parallel to each block. This allows an entire block to be read out or written into a single cycle. This capability is partic- ularly useful when writing data fetched after a miss into the cache or when writing back a block that must be evicted from the cache. The access time to the cache (ignoring the hit detection and selection in a set associative cache) is proportional to the number of blocks in the cache, whereas the energy consumption depends both on the number of bits in the cache (static power) and on the number of blocks (dynamic power). Set associative caches reduce the initial access time to the mem- ory because the size of the memory is smaller, but increase the time for hit detection and block selection, a topic we will cover in Section 2.3.
DRAM Technology As early DRAMs grew in capacity, the cost of a package with all the necessary address lines was an issue. The solution was to multiplex the address lines, thereby cutting the number of address pins in half. Figure 2.3 shows the basic DRAM orga- nization. One-half of the address is sent first during the row access strobe (RAS). The other half of the address, sent during the column access strobe (CAS), follows it. These names come from the internal chip organization, because the memory is organized as a rectangular matrix addressed by rows and columns.
An additional requirement of DRAM derives from the property signified by its first letter, D, for dynamic. To pack more bits per chip, DRAMs use only a single transistor, which effectively acts as a capacitor, to store a bit. This has two implica- tions: first the sensing wires that detect the charge must be precharged, which sets them “halfway” between a logical 0 and 1, allowing the small charge stored in the cell to cause a 0 or 1 to be detected by the sense amplifiers. On reading, a row is placed into a row buffer where CAS signals can select a portion of the row to read out from the DRAM. Because reading a row destroys the information, it must be written back when the row is no longer needed. This write back happens in overlapped fashion, but in early DRAMs it meant that the cycle time before a new row could be read was larger than the time to read a row and access a portion of that row.In addition, to prevent loss of information as the charge in a cell leaks away (assuming it is not read or written) each bit must be “refreshed” periodically. For- tunately, all the bits in a row can be refreshed simultaneously just by reading that row and writing it back. Therefore every DRAM in the memory system must access every row within a certain time window such as 64 ms. DRAM controllers include hardware to refresh the DRAMs periodically.This requirement means that the memory system is occasionally unavailable because it is sending a signal telling every chip to refresh. The time for a refresh is a row activation and a precharge that also writes the row back (which takes roughly 2/3 of the time to get a datum because no column select is needed), and this is required for each row of the DRAM. Because the memory matrix in a DRAM is conceptually square, the number of steps in a refresh is usually the square root of the DRAM capacity. DRAM designers try to keep time spent refreshing to less than 5% of the total time. So far we have presented main memory as if it operated like a Swiss train, consistently delivering the goods exactly according to schedule. In fact, with SDRAMs, a DRAM controller (usually on the processor chip) tries to optimize accesses by avoiding opening new rows and using block transfer when possible. Refresh adds another unpredictable factor.Amdahl suggested as a rule of thumb that memory capacity should grow linearly with processor speed to keep a balanced system. Thus a 1000 MIPS processor should have 1000 MiB of memory. Processor designers rely on DRAMs to supply that demand. In the past, they expected a fourfold improvement in capacity every three years, or 55% per year. Unfortunately, the performance of DRAMs is growing at a much slower rate. The slower performance improvements arise primarily because of smaller decreases in the row access time, which is determined by issues such as power limitations and the charge capacity (and thus the size) of an individual mem- ory cell. Before we discuss these performance trends in more detail, we need to describe the major changes that occurred in DRAMs starting in the mid-1990s.
Improving Memory Performance Inside a DRAM Chip: SDRAMs Although very early DRAMs included a buffer allowing multiple column accesses to a single row, without requiring a new row access, they used an asynchronous interface, which meant that every column access and transfer involved overhead to synchronize with the controller. In the mid-1990s designers added a clock sig- nal to the DRAM interface so that the repeated transfers would not bear that over- head, thereby creating synchronous DRAM (SDRAM). In addition to reducing overhead, SDRAMs allowed the addition of a burst transfer mode where multiple transfers can occur without specifying a new column address. Typically eight or more 16-bit transfers can occur without sending any new addresses by placing the DRAM in burst mode. The inclusion of such burst mode transfers has meant that there is a significant gap between the bandwidth for a stream of random accesses versus access to a block of data.To overcome the problem of getting more bandwidth from the memory as DRAM density increased, DRAMS were made wider. Initially, they offered a four-bit transfer mode; in 2017, DDR2, DDR3, and DDR DRAMS had up to 4, 8, or 16 bit buses.In the early 2000s, a further innovation was introduced: double data rate (DDR), which allows a DRAM to transfer data both on the rising and the falling edge of the memory clock, thereby doubling the peak data rate.Finally, SDRAMs introduced banks to help with power management, improve access time, and allow interleaved and overlapped accesses to different banks.Access to different banks can be overlapped with each other, and each bank has its own row buffer. Creating multiple banks inside a DRAM effectively adds another segment to the address, which now consists of bank number, row address, and col- umn address. When an address is sent that designates a new bank, that bank must be opened, incurring an additional delay. The management of banks and row buffers is completely handled by modern memory control interfaces, so that when a subsequent access specifies the same row for an open bank, the access can happen quickly, sending only the column address.To initiate a new access, the DRAM controller sends a bank and row number (called Activate in SDRAMs and formerly called RAS-row select). That com- mand opens the row and reads the entire row into a buffer. A column address can then be sent, and the SDRAM can transfer one or more data items, depending on whether it is a single item request or a burst request. Before accessing a new row, the bank must be precharged. If the row is in the same bank, then the pre- charge delay is seen; however, if the row is in another bank, closing the row and precharging can overlap with accessing the new row. In synchronous DRAMs, each of these command cycles requires an integral number of clock cycles.From 1980 to 1995, DRAMs scaled with Moore's Law, doubling capacity every 18 months (or a factor of 4 in 3 years). From the mid-1990s to 2010, capacity increased more slowly with roughly 26 months a doubling. From 2010 to 2016, capacity only doubled! Figure 2.4 shows the capacity and access time for various generations of DDR SDRAMs. From DDR1 to DDR3, access times improved by a factor of about 3, or about 7% per year. DDR4 improves power and bandwidth over DDR3, but has similar access latency.As Figure 2.4 shows, DDR is a sequence of standards. DDR2 lowers power from DDR1 by dropping the voltage from 2.5 to 1.8V and offers higher clock rates: 266, 333, and 400 MHz. DDR3 drops voltage to 1.5V and has a maximum clock speed of 800 MHz. (As we discuss in the next section, GDDR5 is a graphics RAM and is based on DDR3 DRAMs.) DDR4, which shipped in volume in early 2016, but was expected in 2014, drops the voltage to 1-1.2V and has a maximum expected clock rate of 1600 MHz. DDR5 is unlikely to reach production quantities until 2020 or later.
With the introduction of DDR, memory designers increasing focused on band- width, because improvements in access time were difficult. Wider DRAMs, burst transfers, and double data rate all contributed to rapid increases in memory band- width. DRAMs are commonly sold on small boards called dual inline memory modules (DIMMs) that contain 4-16 DRAM chips and that are normally organized to be 8 bytes wide (+ ECC) for desktop and server systems. When DDR SDRAMs are packaged as DIMMs, they are confusingly labeled by the peak DIMM band- width. Therefore the DIMM name PC3200 comes from 200 MHz 2 8 bytes, or 3200 MiB/s; it is populated with DDR SDRAM chips. Sustaining the confusion, the chips themselves are labeled with the number of bits per second rather than their clock rate, so a 200 MHz DDR chip is called a DDR400. Figure 2.5 shows the relationships' I /O clock rate, transfers per second per chip, chip bandwidth, chip name DIMM bandwidth, and DIMM name.
Reducing Power Consumption in SDRAMs Power consumption in dynamic memory chips consists of both dynamic power used in a read or write and static or standby power; both depend on the operating voltage. In the most advanced DDR4 SDRAMs the operating voltage has dropped to 1.2 V, significantly reducing power versus DDR2 and DDR3 SDRAMs. The addition of banks also reduced power because only the row in a single bank is read.
In addition to these changes, all recent SDRAMs support a power-down mode, which is entered by telling the DRAM to ignore the clock. Power-down mode dis- ables the SDRAM, except for internal automatic refresh (without which entering power-down mode for longer than the refresh time will cause the contents of mem- ory to be lost). Figure 2.6 shows the power consumption for three situations in a 2GB DDR3 SDRAM. The exact delay required to return from low power mode depends on the SDRAM, but a typical delay is 200 SDRAM clock cycles.
Graphics Data RAMs GDRAMs or GSDRAMs (Graphics or Graphics Synchronous DRAMs) are a spe- cial class of DRAMs based on SDRAM designs but tailored for handling the higher bandwidth demands of graphics processing units. GDDR5 is based on DDR3 with earlier GDDRs based on DDR2. Because graphics processor units (GPUs; see Chapter 4) require more bandwidth per DRAM chip than CPUs, GDDRs have several important differences:
1.GDDRs have wider interfaces: 32-bits versus 4, 8, or 16 in current designs.2.GDDRs have a higher maximum clock rate on the data pins. To allow a higher transfer rate incurring signaling problems, GDRAMS normally connect directly to the GPU and are attached by soldering them to the board, unlike DRAMs, which are normally arranged in an expandable array DIMMs.
Altogether, these characteristics let GDDRs run at two to five times the bandwidth per DRAM versus DDR3 DRAMs.
Packaging Innovation: Stacked or Embedded DRAMs The newest innovation in 2017 in DRAMs is a packaging innovation, rather than a circuit innovation. It places multiple DRAMs in a stacked or adjacent fashion embedded within the same package as the processor. (Embedded DRAM also is used to refer to designs that place DRAM on the processor chip.) Placing the DRAM and processor in the same package lowers access latency (by shortening the delay between the DRAMs and the processor) and potentially increases band- width by allowing more and faster connections between the processor and DRAM; thus several producers have called it high bandwidth memory (HBM).One version of this technology places the DRAM die directly on the CPU die using solder bump technology to connect them. Assuming adequate heat manage- ment, multiple DRAM dies can be stacked in this fashion. Another approach stacks only DRAMs and abuts them with the CPU in a single package using a substrate (interposer) containing the connections. Figure 2.7 shows these two different inter- connection schemes. Prototypes of HBM that allow stacking of up to eight chips have been demonstrated. With special versions of SDRAMs, such a package could contain 8 GiB of memory and have data transfer rates 1 TB/s. The 2.5D tech- nique is currently available. Because the chips must be specifically manufactured to stack, it is quite likely that most early uses will be in high-end server chipsets.In some applications, it may be possible to internally package enough DRAM to satisfy the needs of the application. For example, a version of an Nvidia GPU used as a node in a special-purpose cluster design is being developed using HBM, and it is likely that HBM will become a successor to GDDR5 for higher-end appli- cations. In some cases, it may be possible to use HBM as main memory, although the cost limitations and heat removal issues currently rule out this technology for some embedded applications. In the next section, we consider the possibility of using HBM as an additional level cache.
Flash Memory Flash memory is a type of EEPROM (electronically erasable programmable read- only memory), which is normally read-only but can be erased. The other key prop- erty of Flash memory is that it holds its contents without any power. We focus on NAND Flash which has higher density than NOR Flash and is more suitable for large-scale nonvolatile memories; the drawback is that access is sequential and writing is slower, as we explain below.Flash is used as the secondary storage in PMDs in the same manner that a disk functions in a laptop or server. In addition because most PMDs have a limited amount of DRAM, Flash may also act as a level of the memory hierarchy, to a much greater extent than it might have to do in a desktop or server with a main memory that might be 10-100 times larger.Flash uses a very different architecture and has different properties than stan- dard DRAM. The most important differences are
1.Reads to Flash are sequential and read an entire page, which can be 512 bytes, 2 KiB, or 4 KiB. Thus NAND Flash has a long delay to access the first byte from a random address (about 25 μS), but can supply the remainder of a page block at about 40 MiB/s. By comparison a DDR4 SDRAM takes about 40 ns to the first byte and can transfer the rest of the row at 4.8 GiB/s. Comparing the time to transfer 2 KiB, NAND Flash takes about 75 μS, while DDR SDRAM takes less than 500 ns, making Flash about 150 times slower. Compared to mag- netic disk, however a 2 KiB read from Flash is 300 to 500 times faster. From these numbers, we can see why Flash is not a candidate to replace DRAM for main memory, but is a candidate to replace magnetic disk.2.Flash memory must be erased (thus the name flash for the “flash” erase process) before it is overwritten and it is erased in blocks rather than individual bytes or words. This requirement means that when data must be written to Flash, an entire block must be assembled, either as new data or by merging the data to be written and the rest of the block's contents. For writing Flash is about 1500 times slower then SDRAM, and about 8-15 times as fast as magnetic disk.3.Flash memory is nonvolatile (I .e., it keeps its contents even when power is not applied) and draws significantly less power when not reading or writing (from less than half in standby mode to zero when completely inactive).4.Flash memory limits the number of times that any given block can be written, typically at least 100,000. By ensuring uniform distribution of written blocks throughout the memory, a system can maximize the lifetime of a Flash memory system. This technique, called write leveling is handled by Flash memory controllers.5.High-density NAND Flash is cheaper than SDRAM but more expensive than disks: roughly $2/GiB for Flash, $20 to $40/GiB for SDRAM, and $0.09/GiB for magnetic disks. In the past five years, Flash has decreased in cost at a rate that is almost twice as fast as that of magnetic disks.
Like DRAM, Flash chips include redundant blocks to allow chips with small numbers of defects to be used; the remapping of blocks is handled in the Flash chip. Flash controllers handle page transfers, provide caching of pages and handle write leveling.The rapid improvements in high-density Flash have been critical to the devel- opment of low-power PMDs and laptops, but they have also significantly changed both desktops, which increasingly use solid state disks, and large servers, which often combine disk and Flash-based storage.
Phase-Change Memory Technology Phase-change memory (PCM) has been an active research area for decades. The technology typically uses a small heating element to change the state of a bulk sub- strate between its crystalline form and an amorphous form which have different resistive properties. Each bit corresponds to a crosspoint in a two-dimensional net- work that overlays the substrate. Reading is done by sensing the resistance between an x and y point (thus the alternative name memristor) and writing is accomplished by applying a current to change the phase of the material. The absence of an active device (such as a transistor) should lead to lower costs and greater density than that of NAND Flash.In 2017 Micron and Intel began delivering Xpoint memory chips that are believed to be based on PCM. The technology is expected to have much better write durability than NAND Flash and, by eliminating the need to erase a page before writing achieve an increase in write performance versus NAND of up to a factor of ten. Read latency is also better than Flash by perhaps a factor of 2-3. Initially, it is expected to be priced slightly higher than Flash, but the advan- tages in write performance and write durability may make it attractive especially for SSDs. Should this technology scale well and be able to achieve additional cost reductions, it may be the solid state technology that will depose magnetic disks, which have reigned as the primary bulk nonvolatile store for more than 50 years.
Enhancing Dependability in Memory Systems Large caches and main memories significantly increase the possibility of errors occurring both during the fabrication process and dynamically during operation. Errors that arise from a change in circuitry and are repeatable are called hard errors or permanent faults. Hard errors can occur during fabrication, as well as from a circuit change during operation (e.g ., failure of a Flash memory cell after many writes). All DRAMs, Flash memory, and most SRAMs are manufactured with spare rows so that a small number of manufacturing defects can be accommodated by programming the replacement of a defective row by a spare row. Dynamic errors, which are changes to a cell's contents not a change in the circuitry, are called soft errors or transient faults.Dynamic errors can be detected by parity bits and detected and fixed by the use of error correcting codes (ECCs). Because instruction caches are read-only, parity suffices. In larger data caches and in main memory ECC is used to allow errors to be both detected and corrected. Parity requires only one bit of overhead to detect a single error in a sequence of bits. Because a multibit error would be undetected with parity, the number of bits protected by a parity bit must be limited. One parity bit per 8 data bits is a typical ratio. ECC can detect two errors and correct a single error with a cost of 8 bits of overhead per 64 data bits.In very large systems, the possibility of multiple errors as well as complete fail- ure of a single memory chip becomes significant. Chipkill was introduced by IBM to solve this problem, and many very large systems, such as IBM and SUN servers and the Google Clusters, use this technology. (Intel calls their version SDDC.) Similar in nature to the RAID approach used for disks, Chipkill distributes the data and ECC information so that the complete failure of a single memory chip can be handled by supporting the reconstruction of the missing data from the remaining memory chips. Using an analysis by IBM and assuming a 10,000 processor server with 4 GiB per processor yields the following rates of unrecoverable errors in three years of operation:
- Parity only: About 90,000, or one unrecoverable (or undetected) failure every 17 minutes.
- ECC only: About 3500, or about one undetected or unrecoverable failure every7.5 hours.
- Chipkill: About one undetected or unrecoverable failure every 2 months.
Another way to look at this is to find the maximum number of servers (each with 4 GiB) that can be protected while achieving the same error rate as demon- strated for Chipkill. For parity even a server with only one processor will have an unrecoverable error rate higher than a 10,000-server Chipkill protected system. For ECC, a 17-server system would have about the same failure rate as a 10,000-server Chipkill system. Therefore Chipkill is a requirement for the 50,000-100,00 servers in warehouse-scale computers (see Section 6.8 of Chapter 6).
2.3 Ten Advanced Optimizations of Cache Performance
The preceding average memory access time formula gives us three metrics for cache optimizations: hit time, miss rate, and miss penalty. Given the recent trends, we add cache bandwidth and power consumption to this list. We can classify the 10 advanced cache optimizations we examine into five categories based on these metrics:
1.Reducing the hit time-Small and simple first-level caches and way-prediction. Both techniques also generally decrease power consumption.2.Increasing cache bandwidth-Pipelined caches, multibanked caches and non- blocking caches. These techniques have varying impacts on power consumption.3.Reducing the miss penalty-Critical word first and merging write buffers. These optimizations have little impact on power.4.Reducing the miss rate-Compiler optimizations. Obviously any improvement at compile time improves power consumption.5.Reducing the miss penalty or miss rate via parallelism-Hardware prefetching and compiler prefetching. These optimizations generally increase power con- sumption, primarily because of prefetched data that are unused.
In general, the hardware complexity increases as we go through these optimi- zations. In addition, several of the optimizations require sophisticated compiler technology and the final one depends on HBM. We will conclude with a summary of the implementation complexity and the performance benefits of the 10 tech- niques presented in Figure 2.18 on page 113. Because some of these are straight- forward, we cover them briefly; others require more description.
First Optimization: Small and Simple First-Level Caches to Reduce Hit Time and Power The pressure of both a fast clock cycle and power limitations encourages limited size for first-level caches. Similarly, use of lower levels of associativity can reduce both hit time and power although such trade-offs are more complex than those involving size.The critical timing path in a cache hit is the three-step process of addressing the tag memory using the index portion of the address, comparing the read tag value to the address and setting the multiplexor to choose the correct data item if the cache is set associative. Direct-mapped caches can overlap the tag check with the transmis- sion of the data, effectively reducing hit time. Furthermore lower levels of associa- tivity will usually reduce power because fewer cache lines must be accessed.Although the total amount of on-chip cache has increased dramatically with new generations of microprocessors, because of the clock rate impact arising from a larger L1 cache the size of the L1 caches has recently increased either slightly or not at all. In many recent processors, designers have opted for more associativity rather than larger caches. An additional consideration in choosing the associativity is the possibility of eliminating address aliases; we discuss this topic shortly.One approach to determining the impact on hit time and power consumption in advance of building a chip is to use CAD tools. CACTI is a program to estimate the access time and energy consumption of alternative cache structures on CMOS microprocessors within 10% of more detailed CAD tools. For a given minimum feature size, CACTI estimates the hit time of caches as a function of cache size, associativity, number of read/write ports, and more complex parameters. Figure 2.8 shows the estimated impact on hit time as cache size and associativity are varied. Depending on cache size, for these parameters, the model suggests that the hit time for direct mapped is slightly faster than two-way set associative and that two-way set associative is 1.2 times as fast four-way and four-way is 1.4 times as fast as eight-way. Of course, these estimates depend on technology as well as the size of the cache, and CACTI must be carefully aligned with the technology; Figure 2.8 shows the relative tradeoffs for one technology.
Energy consumption is also a consideration in choosing both the cache size and associativity, as Figure 2.9 shows. The energy cost of higher associativity ranges from more than a factor of 2 to negligible in caches of 128 or 256 KiB when going from direct mapped to two-way set associative.As energy consumption has become critical designers have focused on ways to reduce the energy needed for cache access. In addition to associativity, the other key factor in determining the energy used in a cache access is the number of blocks in the cache because it determines the number of “rows” that are accessed. A designer could reduce the number of rows by increasing the block size (holding total cache size constant), but this could increase the miss rate, especially in smaller L1 caches.
An alternative is to organize the cache in banks so that an access activates only a portion of the cache, namely the bank where the desired block resides. The primary use of multibanked caches is to increase the bandwidth of the cache an optimization we consider shortly. Multibanking also reduces energy because less of the cache is accessed. The L3 caches in many multicores are logically uni- fied, but physically distributed, and effectively act as a multibanked cache. Based on the address of a request, only one of the physical L3 caches (a bank) is actually accessed. We discuss this organization further in Chapter 5.In recent designs, there are three other factors that have led to the use of higher associativity in first-level caches despite the energy and access time costs. First many processors take at least 2 clock cycles to access the cache and thus the impact of a longer hit time may not be critical. Second, to keep the TLB out of the critical path (a delay that would be larger than that associated with increased associativity) almost all L1 caches should be virtually indexed. This limits the size of the cache to the page size times the associativity because then only the bits within the page are used for the index. There are other solutions to the problem of indexing the cache before address translation is completed, but increasing the associativity which also has other benefits, is the most attractive. Third, with the introduction of multi- threading (see Chapter 3), conflict misses can increase, making higher associativity more attractive.
Second Optimization: Way Prediction to Reduce Hit Time Another approach reduces conflict misses and yet maintains the hit speed of direct- mapped cache. In way prediction, extra bits are kept in the cache to predict the way (or block within the set) of the next cache access. This prediction means the mul- tiplexor is set early to select the desired block and in that clock cycle, only a single tag comparison is performed in parallel with reading the cache data. A miss results in checking the other blocks for matches in the next clock cycle.Added to each block of a cache are block predictor bits. The bits select which of the blocks to try on the next cache access. If the predictor is correct, the cache access latency is the fast hit time. If not, it tries the other block, changes the way predictor, and has a latency of one extra clock cycle. Simulations suggest that set prediction accuracy is in excess of 90% for a two-way set associative cache and 80% for a four-way set associative cache, with better accuracy on I-caches than D-caches. Way prediction yields lower average memory access time for a two- way set associative cache if it is at least 10% faster, which is quite likely. Way prediction was first used in the MIPS R10000 in the mid-1990s. It is popular in processors that use two-way set associativity and was used in several ARM pro- cessors, which have four-way set associative caches. For very fast processors, it may be challenging to implement the one-cycle stall that is critical to keeping the way prediction penalty small.An extended form of way prediction can also be used to reduce power con- sumption by using the way prediction bits to decide which cache block to actually access (the way prediction bits are essentially extra address bits); this approach, which might be called way selection, saves power when the way prediction is cor- rect but adds significant time on a way misprediction, because the access, not just the tag match and selection, must be repeated. Such an optimization is likely to make sense only in low-power processors. Inoue et al. (1999) estimated that using the way selection approach with a four-way set associative cache increases the average access time for the I-cache by 1.04 and for the D-cache by 1.13 on the SPEC95 benchmarks, but it yields an average cache power consumption relative to a normal four-way set associative cache that is 0.28 for the I-cache and 0.35 for the D-cache. One significant drawback for way selection is that it makes it difficult to pipeline the cache access; however, as energy concerns have mounted, schemes that do not require powering up entire cache make increasing sense.
Third Optimization: Pipelined Access and Multibanked Caches to Increase Bandwidth These optimizations increase cache bandwidth either by pipelining the cache access or by widening the cache with multiple banks to allow multiple accesses per clock; these optimizations are the dual to the superpipelined and superscalar approaches to increasing instruction throughput. These optimizations are primarily targeted at L1 where access bandwidth constrains instruction throughput. Multiple banks are also used in L2 and L3 caches, but primarily as a power-management technique.Pipelining L1 allows a higher clock cycle, at the cost of increased latency. For example, the pipeline for the instruction cache access for Intel Pentium processors in the mid-1990s took 1 clock cycle; for the Pentium Pro through Pentium III in the mid-1990s through 2000, it took 2 clock cycles; and for the Pentium 4, which became available in 2000, and the current Intel Core i7 it takes 4 clock cycles. Pipelining the instruction cache effectively increases the number of pipeline stages,leading to a greater penalty on mispredicted branches. Correspondingly, pipelining the data cache leads to more clock cycles between issuing the load and using the data (see Chapter 3). Today, all processors use some pipelining of L1 if only for the simple case of separating the access and hit detection, and many high-speed processors have three or more levels of cache pipelining.
It is easier to pipeline the instruction cache than the data cache because the pro- cessor can rely on high performance branch prediction to limit the latency effects. Many superscalar processors can issue and execute more than one memory refer- ence per clock (allowing a load or store is common and some processors allow multiple loads). To handle multiple data cache accesses per clock, we can divide the cache into independent banks each supporting an independent access. Banks were originally used to improve performance of main memory and are now used inside modern DRAM chips as well as with caches. The Intel Core i7 has four banks in L1 (to support up to 2 memory accesses per clock).Clearly banking works best when the accesses naturally spread themselves across the banks, so the mapping of addresses to banks affects the behavior of the memory system. A simple mapping that works well is to spread the addresses of the block sequentially across the banks, which is called sequential interleaving. For example, if there are four banks bank 0 has all blocks whose address modulo 4 is 0, bank 1 has all blocks whose address modulo 4 is 1, and so on. Figure 2.10 shows this interleaving. Multiple banks also are a way to reduce power consump- tion in both caches and DRAM.Multiple banks are also useful in L2 or L3 caches but for a different reason. With multiple banks in L2, we can handle more than one outstanding L1 miss, if the banks do not conflict. This is a key capability to support nonblocking caches, our next optimization. The L2 in the Intel Core i7 has eight banks while Arm Cortex processors have used L2 caches with 1-4 banks. As mentioned earlier, multibanking can also reduce energy consumption.
Fourth Optimization: Nonblocking Caches to Increase Cache Bandwidth For pipelined computers that allow out-of-order execution (discussed in Chapter 3), the processor need not stall on a data cache miss. For example the processor could continue fetching instructions from the instruction cache while waiting for the data cache to return the missing data. A nonblocking cache or lockup-free cache escalates the potential benefits of such a scheme by allowing the data cache to continue to supply cache hits during a miss. This “hit under miss” optimization reduces effective miss penalty by being helpful during a miss instead of ignoring the requests of the processor. A subtle and complex option is that the cache may further lower the effective miss penalty if it can overlap multiple misses: a “hit under multiple miss” or “miss under miss” optimization. The second option is beneficial only if the memory system can service multiple misses; most high-performance pro- cessors (such as the Intel Core processors) usually support both, whereas many lower-end processors provide only limited nonblocking support in L2.To examine the effectiveness of nonblocking caches in reducing the cache miss penalty, Farkas and Jouppi (1994) did a study assuming 8 KiB caches with a 14-cycle miss penalty (appropriate for the early 1990s). They observed a reduction in the effective miss penalty of 20% for the SPECINT92 benchmarks and 30% for the SPECFP92 benchmarks when allowing one hit under miss.Li et al. (2011) updated this study to use a multilevel cache, more modern assumptions about miss penalties, and the larger and more demanding SPECCPU2006 benchmarks. The study was done assuming a model based on a single core of an Intel i7 (see Section 2.6) running the SPECCPU2006 benchmarks. Figure 2.11 shows the reduction in data cache access latency when allowing 1 2, and 64 hits under a miss; the caption describes further details of the memory system. The larger caches and the addition of an L3 cache since the earlier study have reduced the benefits with the SPECINT2006 benchmarks showing an average reduction in cache latency of about 9% and the SPECFP2006 bench- marks about 12.5%.
The cache access latency (including stalls) for two-way associativity is 0.49/0.52 or 94% of direct-mapped cache. Figure 2.11 caption indicates that a hit under one miss reduces the average data cache access latency floating-point programs to 87.5% of a blocking cache. Therefore, for floating-point programs, the directmapped data cache supporting one hit under one miss gives better performance than a two-way setassociative cache that blocks on a miss.
The real difficulty with performance evaluation of nonblocking caches is that a cache miss does not necessarily stall the processor. In this case it is difficult to judge the impact of any single miss and thus to calculate the average memory access time. The effective miss penalty is not the sum of the misses but the nonoverlapped time that the processor is stalled. The benefit nonblocking caches is complex, as it depends upon the miss penalty when there are multiple misses, the memory reference pattern, and how many instructions the processor can execute with a miss outstanding.In general, out-of-order processors are capable of hiding much of the miss penalty of an L1 data cache miss that hits in the L2 cache but are not capable of hiding a significant fraction of a lower-level cache miss. Deciding how many outstanding misses to support depends on a variety factors:
- The temporal and spatial locality in the miss stream, which determines whether a miss can initiate a new access to a lower-level cache or to memory.
- The bandwidth of the responding memory or cache.
- To allow more outstanding misses at the lowest level of the cache (where the miss time is the longest) requires supporting at least that many misses at a higher level, because the miss must initiate at the highest level cache.
- The latency of the memory system.
The following simplified example illustrates the key idea.
In Li, Chen, Brockman, and Jouppi's study, they found that the reduction in CPI for the integer programs was about 7% for one hit under miss and about 12.7% for 64. For the floating-point programs, the reductions were 12.7% for one hit under miss and 17.8% for 64. These reductions track fairly closely the reductions in the data cache access latency shown in Figure 2.11.
Implementing a Nonblocking Cache Although nonblocking caches have the potential to improve performance, they are nontrivial to implement. Two initial types of challenges arise: arbitrating conten- tion between hits and misses, and tracking outstanding misses so that we know when loads or stores can proceed. Consider the first problem. In a blocking cache misses cause the processor to stall and no further accesses to the cache will occur until the miss is handled. In a nonblocking cache, however, hits can collide with misses returning from the next level of the memory hierarchy. If we allow multiple outstanding misses, which almost all recent processors do it is even possible for misses to collide. These collisions must be resolved, usually by first giving priority to hits over misses, and second by ordering colliding misses (if they can occur). The second problem arises because we need to track multiple outstanding mis- ses. In a blocking cache we always know which miss is returning, because only one can be outstanding. In a nonblocking cache, this is rarely true. At first glance, you might think that misses always return in order, so that a simple queue could be kept to match a returning miss with the longest outstanding request. Consider, however a miss that occurs in L1. It may generate either a hit or miss in L2; if L2 is also nonblocking then the order in which misses are returned to L1 will not necessarily be the same as the order in which they originally occurred. Multi- core and other multiprocessor systems that have nonuniform cache access timesalso introduce this complication.When a miss returns, the processor must know which load or store caused the miss so that instruction can now go forward; and it must know where in the cache the data should be placed (as well as the setting of tags for that block). In recent processors, this information is kept in a set of registers, typically called the Miss Status Handling Registers (MSHRs). If we allow n outstanding misses, there will be n MSHRs, each holding the information about where a miss goes in the cache and the value of any tag bits for that miss, as well as the information indicating which load or store caused the miss (in the next chapter you will see how this is tracked). Thus, when a miss occurs, we allocate an MSHR for handling that miss, enter the appropriate information about the miss, and tag the memory request with the index of the MSHR. The memory system uses that tag when it returns the data allowing the cache system to transfer the data and tag information to the appropri- ate cache block and “notify” the load or store that generated the miss that the data is now available and that it can resume operation. Nonblocking caches clearly require extra logic and thus have some cost in energy. It is difficult, however to assess their energy costs exactly because they may reduce stall time, thereby decreasing execution time and resulting energy consumption.In addition to the preceding issues, multiprocessor memory systems, whetherwithin a single chip or on multiple chips, must also deal with complex implemen- tation issues related to memory coherency and consistency. Also because cache mis- ses are no longer atomic (because the request and response are split and may be interleaved among multiple requests), there are possibilities for deadlock. For the interested reader, Section I .7 in online Appendix I deals with these issues in detail.
Fifth Optimization: Critical Word First and Early Restart to Reduce Miss Penalty This technique is based on the observation that the processor normally needs just one word of the block at a time. This strategy is impatience: don't wait for the full block to be loaded before sending the requested word and restarting the processor. Here are two specific strategies:
- Critical word first-Request the missed word first from memory and send it to the processor as soon as it arrives; let the processor continue execution while filling the rest of the words in the block.
- Early restart-Fetch the words in normal order, but as soon as the requested word of the block arrives, send it to the processor and let the processor continue execution.
Generally, these techniques only benefit designs with large cache blocks because the benefit is low unless blocks are large. Note that caches normally continue to satisfy accesses to other blocks while the rest of the block is being filled.However, given spatial locality there is a good chance that the next reference is to the rest of the block. Just as with nonblocking caches, the miss penalty is not simple to calculate. When there is a second request in critical word first the effec- tive miss penalty is the nonoverlapped time from the reference until the second piece arrives. The benefits of critical word first and early restart depend on the size of the block and the likelihood of another access to the portion of the block that has not yet been fetched. For example, for SPECint2006 running on the i7 6700, which uses early restart and critical word first, there is more than one reference made to a block with an outstanding miss (1.23 references on average with a range from 0.5 to 3.0). We explore the performance of the i7 memory hierarchy in more detail in Section 2.6.
Sixth Optimization: Merging Write Buffer to Reduce Miss Penalty Write-through caches rely on write buffers, as all stores must be sent to the next lower level of the hierarchy. Even write-back caches use a simple buffer when a block is replaced. If the write buffer is empty, the data and the full address are written in the buffer and the write is finished from the processor's perspective; the processor continues working while the write buffer prepares to write the word to memory. If the buffer contains other modified blocks, the addresses can be checked to see if the address of the new data matches the address of a valid write buffer entry. If so the new data are combined with that entry. Write merging is the name of this optimization. The Intel Core i7, among many others, uses write merging.If the buffer is full and there is no address match, the cache (and processor) must wait until the buffer has an empty entry. This optimization uses the memory more efficiently because multiword writes are usually faster than writes performed one word at a time. Skadron and Clark (1997) found that even a merging four-entry write buffer generated stalls that led to a 5%-10% performance loss.
The optimization also reduces stalls because of the write buffer being full. Figure 2.12 shows a write buffer with and without write merging. Assume we had four entries in the write buffer, and each entry could hold four 64-bit words. Without this optimization, four stores to sequential addresses would fill the buffer at one word per entry even though these four words when merged fit exactly within a single entry of the write buffer.Note that input/output device registers are often mapped into the physical address space. These I /O addresses cannot allow write merging because separate I /O registers may not act like array of words in memory. For example, they may require one address and data word per I /O register rather than use multiword writes using a single address. These side effects are typically implemented by marking the pages as requiring nonmerging write through by caches.
Seventh Optimization: Compiler Optimizations to Reduce Miss Rate Thus far, our techniques have required changing the hardware. This next technique reduces miss rates without any hardware changes.This magical reduction comes from optimized software the hardware designer's favorite solution! The increasing performance gap between processors and main memory has inspired compiler writers to scrutinize the memory hierarchy to see if compile time optimizations can improve performance. Once again, research is split between improvements in instruction misses and improvements in data misses. The optimizations presented next are found in many modern compilers.
Loop Interchange Some programs have nested loops that access data in memory in nonsequential order. Simply exchanging the nesting of the loops can make the code access the data in the order in which they are stored. Assuming the arrays do not fit in the cache, this technique reduces misses by improving spatial locality; reordering max- imizes use of data in a cache block before they are discarded. For example, if x is a two-dimensional array of size [5000,100] allocated so that x[ I ,j] and x[ I ,j
- 1] are adjacent (an order called row major because the array is laid out by rows), then the two pieces of the following code show how the accesses can be optimized:
The original code would skip through memory in strides of 100 words, while the revised version accesses all the words in one cache block before going to the next block. This optimization improves cache performance without affecting the num- ber of instructions executed.
Blocking This optimization improves temporal locality to reduce misses. We are again deal- ing with multiple arrays, with some arrays accessed by rows and some by columns. Storing the arrays row by row (row major order) or column by column (column major order) does not solve the problem because both rows and columns are used in every loop iteration. Such orthogonal accesses mean that transformations such as loop interchange still leave plenty of room for improvement.
Instead of operating on entire rows or columns of an array, blocked algorithms operate on submatrices or blocks. The goal is to maximize accesses to the data loaded into the cache before the data are replaced. The following code example, which performs matrix multiplication, helps motivate the optimization:
The two inner loops read all N-by-N elements of z, read the same N elements in a row of y repeatedly, and write one row of N elements of x. Figure 2.13 gives a snapshot of the accesses to the three arrays. A dark shade indicates a recent access, a light shade indicates an older access and white means not yet accessed. The number of capacity misses clearly depends on N and the size of the cache.If it can hold all three N-by-N matrices, then all is well, provided there are no cache conflicts. If the cache can hold one N-by-N matrix and one row of N then at least the ith row of y and the array z may stay in the cache. Less than that and misses may occur for both x and z. In the worst case, there would be 2N3 + N2 memory words accessed for N3 operations.To ensure that the elements being accessed can fit in the cache the original code is changed to compute on a submatrix of size B by B. Two inner loops now compute in steps of size B rather than the full length of x and z. B is called the blocking factor. (Assume x is initialized to zero.)
Figure 2.14 illustrates the accesses to the three arrays using blocking. Looking only at capacity misses, the total number of memory words accessed is 2N3/ B + N2. This total is an improvement by an approximate factor of B. Therefore blocking exploits a combination of spatial and temporal locality because y benefits from spatial locality and z benefits from temporal locality. Although our example uses a square block (BxB), we could also use a rectangular block, which would be nec- essary if the matrix were not square.Although we have aimed at reducing cache misses blocking can also be used to help register allocation. By taking a small blocking size such that the block can be held in registers, we can minimize the number of loads and stores in the program. As we shall see in Section 4.8 of Chapter 4, cache blocking is absolutely nec- essary to get good performance from cache-based processors running applicationsusing matrices as the primary data structure.
Eighth Optimization: Hardware Prefetching of Instructions and Data to Reduce Miss Penalty or Miss Rate Nonblocking caches effectively reduce the miss penalty by overlapping execution with memory access. Another approach is to prefetch items before the processor requests them. Both instructions and data can be prefetched, either directly into the caches or into an external buffer that can be more quickly accessed than main memory.
Instruction prefetch is frequently done in hardware outside of the cache. Typically, the processor fetches two blocks on a miss: the requested block and the next consec- utive block. The requested block is placed in the instruction cache when it returns and the prefetched block is placed in the instruction stream buffer. If the requested block is present in the instruction stream buffer, the original cache request is canceled, the block is read from the stream buffer, and the next prefetch request is issued.A similar approach can be applied to data accesses (Jouppi 1990). Palacharla and Kessler (1994) looked at a set of scientific programs and considered multiple stream buffers that could handle either instructions or data. They found that eight stream buffers could capture 50%-70% of all misses from a processor with two 64 KiB four-way set associative caches one for instructions and the other for data. The Intel Core i7 supports hardware prefetching into both L1 and L2 with the most common case of prefetching being accessing the next line. Some earlier Intel processors used more aggressive hardware prefetching, but that resulted in reduced performance for some applications causing some sophisticated users to turn off thecapability.Figure 2.15 shows the overall performance improvement for a subset of SPEC2000 programs when hardware prefetching is turned on. Note that this figure includes only 2 of 12 integer programs, while it includes the majority of the SPECCPU floating-point programs. We will return to our evaluation of prefetch- ing on the i7 in Section 2.6.
Prefetching relies on utilizing memory bandwidth that otherwise would be unused, but if it interferes with demand misses, it can actually lower performance. Help from compilers can reduce useless prefetching. When prefetching works well, its impact on power is negligible. When prefetched data are not used or useful data are displaced, prefetching will have a very negative impact on power.
Ninth Optimization: Compiler-Controlled Prefetching to Reduce Miss Penalty or Miss Rate An alternative to hardware prefetching is for the compiler to insert prefetch instruc- tions to request data before the processor needs it. There are two flavors of prefetch:
- Register prefetch loads the value into a register.
- Cache prefetch loads data only into the cache and not the register.
Either of these can be faulting or nonfaulting; that is, the address does or does not cause an exception for virtual address faults and protection violations. Using this terminology, a normal load instruction could be considered a “faulting register prefetch instruction." Nonfaulting prefetches simply turn into no-ops if they would normally result in an exception which is what we want.The most effective prefetch is “semantically invisible” to a program: it doesn't change the contents of registers and memory, and it cannot cause virtual memory faults. Most processors today offer nonfaulting cache prefetches. This section assumes nonfaulting cache prefetch also called nonbinding prefetch.Prefetching makes sense only if the processor can proceed while prefetching the data; that is, the caches do not stall but continue to supply instructions and data while waiting for the prefetched data to return. As you would expect, the data cache for such computers is normally nonblocking.Like hardware-controlled prefetching the goal is to overlap execution with the prefetching of data. Loops are the important targets because they lend themselves to prefetch optimizations. If the miss penalty is small, the compiler just unrolls the loop once or twice, and it schedules the prefetches with the execution. If the miss penalty is large, it uses software pipelining (see Appendix H) or unrolls many times to prefetch data for a future iteration.Issuing prefetch instructions incurs an instruction overhead, however, so com- pilers must take care ensure that such overheads do not exceed the benefits. By concentrating on references that are likely to be cache misses, programs can avoid unnecessary prefetches while improving average memory access time significantly.
Although array optimizations are easy to understand, modern programs are more likely to use pointers. Luk and Mowry (1999) have demonstrated that compiler-based prefetching can sometimes be extended to pointers as well. Of 10 programs with recursive data structures prefetching all pointers when a node is visited improved performance by 4%-31% in half of the programs. On the other hand, the remaining programs were still within 2% of their original performance. The issue is both whether prefetches are to data already in the cache and whether they occur early enough for the data to arrive by the time it is needed.Many processors support instructions for cache prefetch, and high-end proces- sors (such as the Intel Core i7) often also do some type of automated prefetch in hardware.
Tenth Optimization: Using HBM to Extend the Memory Hierarchy Because most general-purpose processors in servers will likely want more memory than can be packaged with HBM packaging, it has been proposed that the in- package DRAMs be used to build massive L4 caches, with upcoming technologies ranging from 128 MiB to 1 GiB and more considerably more than current on-chip L3 caches. Using such large DRAM-based caches raises an issue: where do the tags reside? That depends on the number of tags. Suppose we were to use a 64B block size; then a 1 GiB L4 cache requires 96 MiB of tags-far more static memory than exists in the caches on the CPU. Increasing the block size to 4 KiB, yields a dramatically reduced tag store of 256 K entries or less than 1 MiB total storage, which is probably acceptable given L3 caches of 4-16 MiB or more in next-generation, multicore processors. Such large block sizes, however, have two major problems.First, the cache may be used inefficiently when content of many blocks are not needed; this is called the fragmentation problem, and it also occurs in virtual mem- ory systems. Furthermore transferring such large blocks is inefficient if much of the data is unused. Second, because of the large block size, the number of distinct blocks held in the DRAM cache is much lower, which can result in more misses especially for conflict and consistency misses.One partial solution to the first problem is to add sublocking. Subblocking allow parts of the block to be invalid, requiring that they be fetched on a miss. Sub- blocking, however does nothing to address the second problem.The tag storage is the major drawback for using a smaller block size. One pos- sible solution for that difficulty is to store the tags for L4 in the HBM. At first glance this seems unworkable, because it requires two accesses to DRAM for each L4 access: one for the tags and one for the data itself. Because of the long access time for random DRAM accesses, typically 100 or more processor clock cycles, such an approach had been discarded. Loh and Hill (2011) proposed a clever solution to this problem: place the tags and the data in the same row in the HBM SDRAM. Although opening the row (and eventually closing it) takes a large amount of time the CAS latency to access a different part of the row is about one-third the new row access time. Thus we can access the tag portion of the block first, and if it is a hit, then use a column access to choose the correct word. Loh and Hill (L-H) have proposed organizing the L4 HBM cache so that each SDRAM row consists of a set of tags (at the head of the block) and 29 data segments, making a 29-way set associa- tive cache. When L4 is accessed, the appropriate row is opened and the tags are read; a hit requires one more column access to get the matching data.Qureshi and Loh (2012) proposed an improvement called an alloy cache that reduces the hit time. An alloy cache molds the tag and data together and uses a direct mapped cache structure. This allows the L4 access time to be reduced to a single HBM cycle by directly indexing the HBM cache and doing a burst transfer of both the tag and data. Figure 2.16 shows the hit latency for the alloy cache, the L-H scheme, and SRAM based tags. The alloy cache reduces hit time by more than a factor of 2 versus the L-H scheme, in return for increase in the miss rate by a factor of 1.1-1.2. The choice of benchmarks is explained in the caption.Unfortunately, in both schemes, misses require two full DRAM accesses: one to get the initial tag and a follow-on access to the main memory (which is evenslower). If we could speed up the miss detection, we could reduce the miss time. Two different solutions have been proposed to solve this problem: one uses a map that keeps track of the blocks in the cache (not the location of the block, just whether it is present); the other uses a memory access predictor that predicts likely misses using history prediction techniques, similar to those used for global branch prediction (see the next chapter). It appears that a small predictor can predict likely misses with high accuracy, leading to an overall lower miss penalty.
Figure 2.17 shows the speedup obtained on SPECrate for the memory- intensive benchmarks used in Figure 2.16. The alloy cache approach outperforms the LH scheme and even the impractical SRAM tags, because the combination of a fast access time for the miss predictor and good prediction results lead to a shorter time to predict a miss and thus a lower miss penalty. The alloy cache performs close to the Ideal case, an L4 with perfect miss prediction and minimal hit time.
HBM is likely to have widespread use in a variety of different configurations, from containing the entire memory system for some high-performance, special-purpose systems to use as an L4 cache for larger server configurations.
Cache Optimization Summary The techniques to improve hit time, bandwidth, miss penalty, and miss rate gen- erally affect the other components of the average memory access equation as well as the complexity of the memory hierarchy. Figure 2.18 summarizes these tech- niques and estimates the impact on complexity, with + meaning that the technique improves the factor meaning it hurts that factor, and blank meaning it has no impact. Generally, no technique helps more than one category.
2.4 Virtual Memory and Virtual Machines
A virtual machine is taken to be an efficient, isolated duplicate of the real machine. We explain these notions through the idea of a virtual machine monitor (VMM)… a VMM has three essential characteristics. First, the VMM provides an environment for programs which is essentially identical with the original machine; second programs run in this environment show at worst only minor decreases in speed; and last, the VMM is in complete control of system resources.Gerald Popek and Robert Goldberg,“Formal requirements for virtualizable third generation architectures,”Communications of the ACM (July 1974).Section B .4 in Appendix B describes the key concepts in virtual memory. Recall that virtual memory allows the physical memory to be treated as a cache of sec- ondary storage (which may be either disk or solid state). Virtual memory moves pages between the two levels of the memory hierarchy just as caches move blocks between levels. Likewise, TLBs act as caches on the page table eliminating the need to do a memory access every time an address is translated. Virtual memory also provides separation between processes that share one physical memory but have separate virtual address spaces. Readers should ensure that they understand both functions of virtual memory before continuing.In this section, we focus on additional issues in protection and privacy between processes sharing the same processor. Security and privacy are two of the most vexing challenges for information technology in 2017. Electronic burglaries, often involving lists of credit card numbers, are announced regularly, and it's widely believed that many more go unreported. Of course, such problems arise from pro- gramming errors that allow a cyberattack to access data it should be unable to access. Programming errors are a fact of life, and with modern complex software systems, they occur with significant regularity. Therefore both researchers and practitioners are looking for improved ways to make computing systems more secure. Although protecting information is not limited to hardware, in our view real security and privacy will likely involve innovation in computer architecture as well as in systems software.This section starts with a review of the architecture support protecting pro- cesses from each other via virtual memory. It then describes the added protection provided by virtual machines, the architecture requirements of virtual machines, and the performance of a virtual machine. As we will see in Chapter 6, virtual machines are a foundational technology for cloud computing.
Protection via Virtual Memory Page-based virtual memory, including a TLB that caches page table entries, is the primary mechanism that protects processes from each other. Sections B .4 and B .5 in Appendix B review virtual memory, including a detailed description of protec- tion via segmentation and paging in the 80x86. This section acts as a quick review; if it's too quick, please refer to the denoted Appendix B sections.Multiprogramming, where several programs running concurrently share a computer, has led to demands for protection and sharing among programs and to the concept of a process. Metaphorically, a process is a program's breathing air and living space-that is a running program plus any state needed to continue running it. At any instant, it must be possible to switch from one process to another. This exchange is called a process switch or context switch.The operating system and architecture join forces to allow processes to share the hardware yet not interfere with each other. To do this the architecture must limit what a process can access when running a user process yet allow an operating sys- tem process to access more. At a minimum, the architecture must do the following:
1.Provide at least two modes, indicating whether the running process is a user process or an operating system process. This latter process is sometimes called a kernel process or a supervisor process.2.Provide a portion of the processor state that a user process can use but not write. This state includes a user/supervisormode bit, an exceptionenable/disable bit and memory protection information. Users are prevented from writing this state because the operating system cannot control user processes if users can give them- selves supervisor privileges, disable exceptions or change memory protection.3.Provide mechanisms whereby the processor can go from user mode to super- visor mode and vice versa. The first direction is typically accomplished by a system call, implemented as a special instruction that transfers control to a ded- icated location in supervisor code space. The PC is saved from point of the system call, and the processor is placed in supervisor mode. The return to user mode is like a subroutine return that restores the previous user/supervisor mode.4.Provide mechanisms to limit memory accesses to protect the memory state of a process without having to swap the process to disk on a context switch.
Appendix A describes several memory protection schemes, but by far the most popular is adding protection restrictions to each page of virtual memory. Fixed- sized pages, typically 4 KiB, 16 KiB, or larger are mapped from the virtual address space into physical address space via a page table. The protection restrictions are included in each page table entry. The protection restrictions might determine whether a user process can read this page, whether a user process can write to this page and whether code can be executed from this page. In addition, a process can neither read nor write a page if it is not in the page table. Because only the OS can update the page table the paging mechanism provides total access protection.Paged virtual memory means that every memory access logically takes at least twice as long, with one memory access to obtain the physical address and a second access to get the data. This cost would be far too dear. The solution is to rely on the principle of locality; if the accesses have locality, then the address translations for the accesses must also have locality. By keeping these address translations in a spe- cial cache a memory access rarely requires a second access to translate the address. This special address translation cache is referred to as a TLB.A TLB entry is like a cache entry where the tag holds portions of the virtual address and the data portion holds a physical page address, protection field, valid bit and usually a use bit and a dirty bit. The operating system changes these bits by changing the value in the page table and then invalidating the corresponding TLB entry. When the entry is reloaded from the page table, the TLB gets an accurate copy of the bits.Assuming the computer faithfully obeys the restrictions on pages and maps vir- tual addresses to physical addresses, it would seem that we are done. Newspaper headlines suggest otherwise.The reason we're not done is that we depend on the accuracy of the operating system as well as the hardware. Today's operating systems consist of tens of mil- lions of lines of code. Because bugs are measured in number per thousand lines of code, there are thousands of bugs in production operating systems. Flaws in the OS have led vulnerabilities that are routinely exploited.This problem and the possibility that not enforcing protection could be much more costly than in the past have led some to look for a protection model with a much smaller code base than the full OS, such as virtual machines.
Protection via Virtual Machines An idea related to virtual memory that is almost as old are virtual machines (VMs). They were first developed in the late 1960s, and they have remained an important part of mainframe computing over the years. Although largely ignored in the domain of single-user computers in the 1980s and 1990s, they have recently gained popularity because
- the increasing importance of isolation and security in modern systems;
- the failures in security and reliability of standard operating systems;
- the sharing of a single computer among many unrelated users, such as in a data center or cloud; and
- the dramatic increases in the raw speed of processors, which make the overhead of VMs more acceptable.
The broadest definition of VMs includes basically all emulation methods that provide a standard software interface, such as the Java VM. We are interested in VMs that provide a complete system-level environment at the binary instruction set architecture (ISA) level. Most often, the VM supports the same ISA as the under- lying hardware; however it is also possible to support a different ISA, and such approaches are often employed when migrating between ISAs in order to allow software from the departing ISA to be used until it can be ported to the new ISA. Our focus here will be on VMs where the ISA presented by the VM and the underlying hardware match. Such VMs are called (operating) system virtual machines. IBM VM/370, VMware ESX Server, and Xen are examples. They pre- sent the illusion that the users of a VM have an entire computer to themselves, including a copy of the operating system. A single computer runs multiple VMs and can support a number of different operating systems (OSes). On a conventional platform, a single OS “owns” all the hardware resources, but with a VM, multiple OSes all share the hardware resources.The software that supports VMs is called a virtual machine monitor (VMM) orhypervisor; the VMM is the heart of virtual machine technology. The underlying hardware platform is called host, and its resources are shared among the guest VMs. The VMM determines how to map virtual resources to physical resources: A physical resource may be time-shared, partitioned, or even emulated in software. The VMM is much smaller than a traditional OS; the isolation portion of a VMM is perhaps only 10,000 lines of code.In general, the cost of processor virtualization depends on the workload. User- level processor-bound programs, such as SPECCPU2006, have zero virtualization overhead because the OS is rarely invoked, so everything runs at native speeds. Conversely, I /O-intensive workloads generally are also OS-intensive and execute many system calls (which doing I /O requires) and privileged instructions that can result in high virtualization overhead. The overhead is determined by the number of instructions that must be emulated by the VMM and how slowly they are emu- lated. Therefore, when the guest VMs run the same ISA as the host, as we assume here, the goal of the architecture and the VMM is to run almost all instructions directly on the native hardware. On the other hand, if the I /O-intensive workload is also I /O-bound, the cost of processor virtualization can be completely hidden by low processor utilization because it is often waiting for I /O.Although our interest here is in VMs for improving protection, VMs provide two other benefits that are commercially significant:
1.Managing software-VMs provide an abstraction that can run the complete software stack, even including old operating systems such as DOS. A typical deployment might be some VMs running legacy OSes, many running the cur- rent stable OS release and a few testing the next OS release.2.Managing hardware-One reason for multiple servers is to have each applica- tion running with its own compatible version of the operating system on sep- arate computers, as this separation can improve dependability. VMs allow these separate software stacks to run independently yet share hardware thereby consolidating the number of servers. Another example is that most newer VMMs support migration of a running VM to a different computer, either to balance load or to evacuate from failing hardware. The rise of cloud computing has made the ability to swap out an entire VM to another physical processor increasingly useful.
These two reasons are why cloud-based servers, such as Amazon's, rely on virtual machines.
Requirements of a Virtual Machine Monitor What must a VM monitor do? It presents a software interface to guest software, it must isolate the state of guests from each other, and it must protect itself from guest software (including guest OSes). The qualitative requirements are
- Guest software should behave on a VM exactly as if it were running on the native hardware, except for performance-related behavior or limitations of fixed resources shared by multiple VMs.
- Guest software should not be able to directly change allocation of real system resources.
To “virtualize” the processor, the VMM must control just about everything- access to privileged state, address translation, I /O, exceptions and interrupts-even though the guest VM and OS currently running are temporarily using them.For example, in the case of a timer interrupt, the VMM would suspend the cur- rently running guest VM save its state, handle the interrupt, determine which guest VM to run next, and then load its state. Guest VMs that rely on a timer interrupt are provided with a virtual timer and an emulated timer interrupt by the VMM.To be in charge the VMM must be at a higher privilege level than the guest VM, which generally runs in user mode; this also ensures that the execution of any privileged instruction will be handled by the VMM. The basic requirements of sys- tem virtual machines are almost identical to those for the previously mentioned paged virtual memory:
- At least two processor modes, system and user.
- A privileged subset of instructions that is available only in system mode, result- ing in a trap if executed in user mode. All system resources must be controllable only via these instructions.
Instruction Set Architecture Support for Virtual Machines If VMs are planned for during the design of the ISA, it's relatively easy to reduce both the number of instructions that must be executed by a VMM and how long it takes to emulate them. An architecture that allows the VM to execute directly on the hardware earns the title virtualizable and the IBM 370 architecture proudly bears that label.However, because VMs have been considered for desktop and PC-based server applications only fairly recently, most instruction sets were created without virtua- lization in mind. These culprits include 80x86 and most of the original RISC archi- tectures although the latter had fewer issues than the 80x86 architecture. Recent additions to the x 86 architecture have attempted to remedy the earlier shortcom- ings, and RISC V explicitly includes support for virtualization.Because the VMM must ensure that the guest system interacts only with virtual resources a conventional guest OS runs as a user mode program on top of the VMM. Then, if a guest OS attempts to access or modify information related to hardware resources via a privileged instruction-for example reading or writing the page table pointer-it will trap to the VMM. The VMM can then effect the appropriate changes to corresponding real resources.Therefore, if any instruction that tries to read or write such sensitive informa- tion traps when executed in user mode the VMM can intercept it and support a virtual version of the sensitive information as the guest OS expects.In the absence of such support, other measures must be taken. A VMM must take special precautions to locate all problematic instructions and ensure that they behave correctly when executed by a guest OS thereby increasing the complexity of the VMM and reducing the performance of running the VM. Sections 2.5 and2.7 give concrete examples of problematic instructions in the 80x86 architecture. One attractive extension allows the VM and the OS to operate at different privilege levels, each of which is distinct from the user level. By introducing an additional privilege level some OS operations-e.g., those that exceed the permissions granted to a user program but do not require intervention by the VMM (because they cannot affect any other VM)-can execute directly without the overhead of trapping and invoking the VMM. The Xen design, which we examine shortly, makes use of three privilege levels.
Impact of Virtual Machines on Virtual Memory and I /O Another challenge is virtualization of virtual memory, as each guest OS in every VM manages its own set of page tables. To make this work, the VMM separates the notions of real and physical memory (which are often treated synonymously) and makes real memory a separate, intermediate level between virtual memory and physical memory. (Some use the terms virtual memory, physical memory, and machine memory to name the same three levels.) The guest OS maps virtual mem- ory to real memory via its page tables, and the VMM page tables map the guests' real memory to physical memory. The virtual memory architecture is specified either via page tables as in IBM VM/370 and the 80x86, or via the TLB structure, as in many RISC architectures.Rather than pay an extra level of indirection on every memory access the VMM maintains a shadow page table that maps directly from the guest virtual address space to the physical address space of the hardware. By detecting all mod- ifications to the guest's page table, the VMM can ensure that the shadow page table entries being used by the hardware for translations correspond those of the guest OS environment, with the exception of the correct physical pages substituted for the real pages in the guest tables. Therefore the VMM must trap any attempt by the guest OS to change its page table or to access the page table pointer. This is com- monly done by write protecting the guest page tables and trapping any access to the page table pointer by a guest OS. As previously noted, the latter happens naturally if accessing the page table pointer is a privileged operation.The IBM 370 architecture solved the page table problem in the 1970s with an additional level of indirection that is managed by the VMM. The guest OS keeps its page tables as before, so the shadow pages are unnecessary. AMD has implemen- ted a similar scheme for its 80x86.To virtualize the TLB in many RISC computers, the VMM manages the real TLB and has a copy of the contents of TLB of each guest VM. To pull this off, any instructions that access the TLB must trap. TLBs with Process ID tags can sup- port a mix of entries from different VMs and the VMM, thereby avoiding flushing of the TLB on a VM switch. Meanwhile, in the background, VMM supports a mapping between the VMs' virtual Process IDs and the real Process IDs. SectionL.7of online Appendix L describes additional details.The final portion of the architecture to virtualize is I /O. This is by far the most difficult part of system virtualization because of the increasing number of I /O devices attached to the computer and the increasing diversity of I /O device types. Another difficulty is the sharing of a real device among multiple VMs, and yet another comes from supporting the myriad of device drivers that are required, espe- cially if different guest OSes are supported on the same VM system. The VM illu- sion can be maintained by giving each VM generic versions of each type of I /O device driver, and then leaving it to the VMM to handle real I /O.The method for mapping a virtual-to-physical I /O device depends on the type of device. For example, physical disks are normally partitioned by the VMM to create virtual disks for guest VMs, and the VMM maintains the mapping of virtual tracks and sectors to the physical ones. Network interfaces are often shared between VMs in very short time slices, and the job of the VMM is to keep track of messages for the virtual network addresses to ensure that guest VMs receive only messages intended for them.
Extending the Instruction Set for Efficient Virtualization and Better Security In the past 5-10 years, processor designers, including those at AMD and Intel (and to a lesser extent ARM), have introduced instruction set extensions to more effi- ciently support virtualization. Two primary areas of performance improvement have been in handling page tables and TLBs (the cornerstone of virtual memory) and in I /O specifically handling interrupts and DMA. Virtual memory perfor- mance is enhanced by avoiding unnecessary TLB flushes and by using the nested page table mechanism, employed by IBM decades earlier, rather than a complete set of shadow page tables (see Section L.7 in Appendix L). To improve I /O per- formance architectural extensions are added that allow a device to directly use DMA to move data (eliminating a potential copy by the VMM) and allow device interrupts and commands to be handled by the guest OS directly. These extensions show significant performance gains in applications that are intensive either in their memory-management aspects or in the use of I /O.With the broad adoption of public cloud systems for running critical applica- tions, concerns have risen about security of data in such applications. Any mali- cious code that is able to access a higher privilege level than data that must be kept secure compromises the system. For example, if you are running a credit card processing application, you must be absolutely certain that malicious users cannot get access to the credit card numbers, even when they are using the same hardware and intentionally attack the OS or even the VMM. Through the use of virtualiza- tion, we can prevent accesses by an outside user to the data in a different VM, and this provides significant protection compared to a multiprogrammed environment. That might not be enough, however, if the attacker compromises the VMM or can find out information by observations in another VMM. For example, suppose the attacker penetrates the VMM; the attacker can then remap memory so as to access any portion of the data.Alternatively, an attack might rely on a Trojan horse (see Appendix B) intro- duced into the code that can access the credit cards. Because the Trojan horse is running in the same VM as the credit card processing application, the Trojan horse only needs to exploit an OS flaw to gain access to the critical data. Most cyberat- tacks have used some form of Trojan horse, typically exploiting an OS flaw, that either has the effect of returning access to the attacker while leaving the CPU still in privilege mode or allows the attacker to upload and execute code as if it were part of the OS. In either case, the attacker obtains control of the CPU and, using the higher privilege mode, can proceed to access anything the VM. Note that encryption alone does not prevent this attacker. If the data in memory is unen- crypted, which is typical, then the attacker has access to all such data. Furthermore, if the attacker knows where the encryption key is stored, the attacker can freely access the key and then access any encrypted data.More recently, Intel introduced a set of instruction set extensions, called the software guard extensions (SGX), to allow user programs to create enclaves, por- tions of code and data that are always encrypted and decrypted only on use and only with the key provided by the user code. Because the enclave is always encrypted, standard OS operations for virtual memory or I /O can access the enclave (e.g., to move a page) but cannot extract any information. For an enclave to work, all the code and all the data required must be part of the enclave. Although the topic of finer-grained protection has been around for decades it has gotten little traction before because of the high overhead and because other solutions that are more efficient and less intrusive have been acceptable. The rise of cyberattacks and the amount of confidential information online have led to a reexamination of tech- niques for improving such fine-grained security. Like Intel's SGX, IBM and AMD's recent processors support on-the-fly encryption of memory.
An Example VMM: The Xen Virtual Machine Early in the development of VMs, a number of inefficiencies became apparent. For example, a guest OS manages its virtual-to-real page mapping, but this mapping is ignored by the VMM, which performs the actual mapping to physical pages. In other words a significant amount of wasted effort is expended just to keep the guest OS happy. To reduce such inefficiencies, VMM developers decided that it may be worthwhile to allow the guest OS to be aware that it is running on a VM. For example a guest OS could assume a real memory as large as its virtual memory so that no memory management is required by the guest OS .Allowing small modifications to the guest OS to simplify virtualization is referred to as paravirtualization, and the open source Xen VMM is a good exam- ple. The Xen VMM which is used in Amazon's web services data centers, pro- vides a guest OS with a virtual machine abstraction that is similar to the physical hardware, but drops many of the troublesome pieces. For example, to avoid flushing the TLB Xen maps itself into the upper 64 MiB of the address space of each VM. Xen allows the guest OS to allocate pages, checking only to be sure the guest OS does not violate protection restrictions. To protect the guest OS from the user programs in the VM Xen takes advantage of the four protection levels available in the 80x86. The Xen VMM runs at the highest privilege level (0), the guest OS runs at the next level (1), and the applications run at the lowest priv- ilege level (3). Most OSes for the 80x 86 keep everything at privilege levels 0 or 3. For subsetting to work properly, Xen modifies the guest OS to not use prob- lematic portions of the architecture. For example, the port of Linux to Xen changes about 3000 lines, or about 1% of the 80x86-specific code. These changes how-ever, do not affect the application binary interfaces of the guest OS .To simplify the I /O challenge of VMs, Xen assigned privileged virtual machines to each hardware I /O device. These special VMs are called driver domains. (Xen calls VMs “domains.") Driver domains run the physical device drivers although interrupts are still handled by the VMM before being sent to the appropriate driver domain. Regular VMs, called guest domains, run simple vir- tual device drivers that must communicate with the physical device drivers in the driver domains over a channel to access the physical I /O hardware. Data are sent between guest and driver domains by page remapping.
2.5 Cross-Cutting Issues: The Design of Memory Hierarchies
This section describes four topics discussed in other chapters that are fundamental to memory hierarchies.
Protection, Virtualization, and Instruction Set Architecture Protection is a joint effort of architecture and operating systems, but architects had to modify some awkward details of existing instruction set architectures when vir- tual memory became popular. For example, to support virtual memory in the IBM 370 architects had to change the successful IBM 360 instruction set architecture that had been announced just 6 years before. Similar adjustments are being made today to accommodate virtual machines.For example, the 80x86 instruction POPF loads the flag registers from the top of the stack in memory. One of the flags is the Interrupt Enable (IE) flag. Until recent changes to support virtualization, running the POPF instruction in user mode, rather than trapping it, simply changed all the flags except IE. In system mode, it does change the IE flag. Because a guest OS runs in user mode inside a VM, this was a problem as the OS would expect to see a changed IE. Extensions of the 80x86 architecture to support virtualization eliminated this problem.Historically, IBM mainframe hardware and VMM took three steps to improve performance of virtual machines:
1.Reduce the cost of processor virtualization.2.Reduce interrupt overhead cost due to the virtualization.3.Reduce interrupt cost by steering interrupts to the proper VM without invoking VMM.
IBM is still the gold standard of virtual machine technology. For example, an IBM mainframe ran thousands of Linux VMs in 2000, while Xen ran 25 VMs in 2004 (Clark et al ., 2004). Recent versions of Intel and AMD chipsets have added special instructions to support devices in a VM to mask interrupts at lower levels from each VM and to steer interrupts to the appropriate VM.
Autonomous Instruction Fetch Units Many processors with out-of-order execution and even some with simply deep pipelines decouple the instruction fetch (and sometimes initial decode), using a separate instruction fetch unit (see Chapter 3). Typically, the instruction fetch unit accesses the instruction cache to fetch an entire block before decoding it into indi- vidual instructions; such a technique is particularly useful when the instruction length varies. Because the instruction cache is accessed in blocks, it no longer makes sense to compare miss rates to processors that access the instruction cache once per instruction. In addition, the instruction fetch unit may prefetch blocks into the L1 cache; these prefetches may generate additional misses, but may actually reduce the total miss penalty incurred. Many processors also include data prefetch- ing, which may increase the data cache miss rate, even while decreasing the total data cache miss penalty.
Speculation and Memory Access One of the major techniques used in advanced pipelines is speculation, whereby an instruction is tentatively executed before the processor knows whether it is really needed. Such techniques rely on branch prediction, which if incorrect requires that the speculated instructions are flushed from the pipeline. There are two separate issues in a memory system supporting speculation: protection and performance. With speculation, the processor may generate memory references, which will never be used because the instructions were the result of incorrect speculation. Those references, if executed, could generate protection exceptions. Obviously, such faults should occur only if the instruction is actually executed. In the next chapter we will see how such “speculative exceptions” are resolved. Because a speculative processor may generate accesses to both the instruction and data caches, and subsequently not use the results of those accesses, speculation may increase the cache miss rates. As with prefetching, however such speculation may actually lower the total cache miss penalty. The use of speculation, like the use of prefetching, makes it misleading to compare miss rates to those seen in pro- cessors without speculation, even when the ISA and cache structures are otherwise identical.
Special Instruction Caches One of the biggest challenges in superscalar processors is to supply the instruc- tion bandwidth. For designs that translate the instructions into micro-operations, such as most recent Arm and i7 processors, instruction bandwidth demands and branch misprediction penalties can be reduced by keeping a small cache of recently translated instructions. We explore this technique in greater depth in the next chapter.
Coherency of Cached Data Data can be found in memory and in the cache. As long as the processor is the sole component changing or reading the data and the cache stands between the proces- sor and memory, there is little danger in the processor seeing the old or stale copy. As we will see multiple processors and I /O devices raise the opportunity for copies to be inconsistent and to read the wrong copy.The frequency of the cache coherency problem is different for multiprocessors than for I /O. Multiple data copies are a rare event for I /O-one to be avoided when- ever possible-but a program running on multiple processors will want to have copies of same data in several caches. Performance of a multiprocessor pro- gram depends on the performance of the system when sharing data.The I /O cache coherency question is this: where does the I /O occur in the com- puter-between the I /O device and the cache or between the I /O device and main memory? If input puts data into the cache and output reads data from the cache, both I /O and the processor see the same data. The difficulty in this approach is that it interferes with the processor and can cause the processor to stall for I /O. Input may also interfere with the cache by displacing some information with new data that are unlikely to be accessed soon.The goal the I /O system in a computer with a cache is to prevent the stale data problem while interfering as little as possible. Many systems therefore prefer that I /O occur directly to main memory, with main memory acting as an I /O buffer. If a write-through cache were used, then memory would have an up-to-date copy of the information and there would be no stale data issue for output. (This benefit is a reason processors used write through.) However, today write through is usually found only in first-level data caches backed by an L2 cache that uses write back. Input requires some extra work. The software solution is to guarantee that no blocks of the input buffer are in the cache. A page containing the buffer can be marked as noncachable, and the operating system can always input to such a page. Alternatively, the operating system can flush the buffer addresses from the cache before the input occurs. A hardware solution is to check the I /O addresses on input to see if they are in the cache. If there is a match of I /O addresses in the cache, the cache entries are invalidated to avoid stale data. All of these approaches can also beused for output with write-back caches.Processor cache coherency is a critical subject in the age of multicore proces- sors, and we will examine it in detail in Chapter 5.
2.6 Putting It All Together: Memory Hierarchies in the ARM Cortex-A53 and Intel Core i7 6700
This section reveals the ARM Cortex-A53 (hereafter called the A53) and Intel Core i76700 (hereafter called i7) memory hierarchies and shows the performance of their components on a set of single-threaded benchmarks. We examine the Cortex-A53 first because it has a simpler memory system; we go into more detail for the i7 tracing out a memory reference in detail. This section presumes that readers are familiar with the organization of a two-level cache hierarchy using vir- tually indexed caches. The basics of such a memory system are explained in detail in Appendix B, and readers who are uncertain of the organization of such a system are strongly advised to review the Opteron example in Appendix B. Once they understand the organization of the Opteron, the brief explanation of the A53 sys- tem, which is similar, will be easy to follow.
The ARM Cortex-A53 The Cortex-A53 is a configurable core that supports the ARMv8A instruction set architecture, which includes both 32-bit and 64-bit modes. The Cortex-A53 is delivered as an IP (intellectual property) core. IP cores are the dominant form of technology delivery in the embedded, PMD, and related markets; billions of ARM and MIPS processors have been created from these IP cores. Note that IP cores are different from the cores in the Intel i7 or AMD Athlon multicores. An IP core (which may itself be a multicore) is designed to be incorporated with other logic (thus it is the core of a chip) including application-specific processors (such as an encoder or decoder for video), I /O interfaces, and memory interfaces, and then fabricated to yield a processor optimized for a particular application. For example, the Cortex-A53 IP core is used in a variety of tablets and smartphones; it is designed to be highly energy-efficient a key criteria in battery-based PMDs. The A53 core is capable of being configured with multiple cores per chip for use in high-end PMDs; our discussion here focuses on a single core.Generally, IP cores come in two flavors. Hard cores are optimized for a par- ticular semiconductor vendor and are black boxes with external (but still on-chip) interfaces. Hard cores typically allow parametrization only of logic outside the core, such as L2 cache sizes and the IP core cannot be modified. Soft cores are usually delivered in a form that uses a standard library of logic elements. A soft core can be compiled for different semiconductor vendors and can also be modi- fied, although extensive modifications are very difficult because of the complexity of modern-day IP cores. In general hard cores provide higher performance and smaller die area, while soft cores allow retargeting to other vendors and can be more easily modified.The Cortex-A53 can issue two instructions per clock at clock rates up to1.3 GHz. It supports both a two-level TLB and a two-level cache; Figure 2.19 sum- marizes the organization of the memory hierarchy. The critical term is returned first, and the processor can continue while the miss completes; a memory system with up to four banks can be supported. For a D-cache of 32 KiB and a page size of 4 KiB each physical page could map to two different cache addresses; such aliases are avoided by hardware detection on a miss as in Section B .3 of Appendix B. Figure 2.20 shows how the 32-bit virtual address is used to index the TLB and the caches, assuming 32 KiB primary caches and a 1 MiB secondary cache with 16 KiB page size.
Performance of the Cortex-A53 Memory Hierarchy The memory hierarchy of the Cortex-A8 was measured with 32 KiB primary caches and a 1 MiB L2 cache running the SPECInt2006 benchmarks. The instruc- tion cache miss rates for these SPECInt2006 are very small even for just the L1: close to zero for most and under 1% for all of them. This low rate probably results from the computationally intensive nature of the SPECCPU programs and the two- way set associative cache that eliminates most conflict misses.Figure 2.21 shows the data cache results, which have significant L1 and L2 miss rates. The L1 rate varies by a factor of 75, from 0.5% to 37.3% with a median miss rate of 2.4%. The global L2 miss rate varies by a factor of 180, from 0.05% to 9.0% with a median of 0.3%. MCF, which is known as a cache buster, sets the upper bound and significantly affects the mean. Remember that the L2 global miss rate is significantly lower than the L2 local miss rate; for example, the median L2 stand-alone miss rate is 15.1% versus the global miss rate of 0.3%. Using these miss penalties in Figure 2.19, Figure 2.22 shows the average pen- alty per data access. Although the L1 miss rates are about seven times higher than the L2 miss rate, the L2 penalty is 9.5 times as high, leading to L2 misses slightly dominating for the benchmarks that stress the memory system. In the next chapter,we will examine the impact of the cache misses on overall CPI.
The Intel Core i7 6700 The i7 supports the x 86-64 instruction set architecture, a 64-bit extension of the 80x86 architecture. The i7 is an out-of-order execution processor that includes four cores. In this chapter, we focus on the memory system design and performance from the viewpoint of a single core. The system performance of multiprocessor designs, including the i7 multicore is examined in detail in Chapter 5.Each core in an i7 can execute up to four 80x86 instructions per clock cycle, using a multiple issue, dynamically scheduled, 16-stage pipeline, which we describe in detail in Chapter 3. The i7 can also support up to two simultaneous threads per processor using a technique called simultaneous multithreading, described in Chapter 4. In 2017 the fastest i7 had a clock rate of 4.0 GHz (in Turbo Boost mode), which yielded a peak instruction execution rate of 16 billion instruc- tions per second, or 64 billion instructions per second for the four-core design. Of course there is a big gap between peak and sustained performance, as we will see over the next few chapters.The i7 can support up to three memory channels, each consisting of a separate set of DIMMs, and each of which can transfer in parallel. Using DDR3-1066 (DIMM PC8500) the i7 has a peak memory bandwidth of just over 25 GB/s.i7 uses 48-bit virtual addresses and 36-bit physical addresses, yielding a maximum physical memory of 36 GiB. Memory management is handled with a two-level TLB (see Appendix B, Section B .4) summarized in Figure 2.23.Figure 2.24 summarizes the i7's three-level cache hierarchy. The first-level caches are virtually indexed and physically tagged (see Appendix B, Section B .3) while the L2 and L3 caches are physically indexed. Some versions of the i7 6700 will support a fourth-level cache using HBM packaging.Figure 2.25 is labeled with the steps of an access to the memory hierarchy.First, the PC is sent to the instruction cache. The instruction cache index is or 6 bits. The page frame of the instruction's address (36 48 12 bits) is sent to the instruction TLB (step 1). At the same time, the 12-bit page offset from the vir- tual address is sent to the instruction cache (step 2). Notice that for eight-way associative instruction cache, 12 bits are needed for the cache address: 6 bits to index the cache plus 6 bits of block offset for the 64-byte block, so no aliases are possible. The previous versions of the i7 used a four-way set associative I-cache, meaning that a block corresponding to a virtual address could actually be in two different places in the cache, because the corresponding physical address could have either a 0 or 1 in this location. For instructions this did not pose a prob- lem because even if an instruction appeared in the cache in two different locations, the two versions must be the same. If such duplication, or aliasing, of data is allowed, the cache must be checked when the page map is changed, which is an infrequent event. Note that a very simple use of page coloring (see Appendix B, Section B .3) can eliminate the possibility of these aliases. If even-address virtual pages are mapped to even-address physical pages (and the same for odd pages), then these aliases can never occur because the low-order bit in the virtual and phys- ical page number will be identical.
The instruction TLB is accessed to find a match between the address and a valid page table entry (PTE) (steps 3 and 4). In addition to translating the address the TLB checks to see if the PTE demands that this access result in an exception because of an access violation.An instruction TLB miss first goes to the L2 TLB, which contains 1536 PTEs of 4 KiB page sizes and is 12-way set associative. It takes 8 clock cycles to load the L1 TLB from the L2 TLB which leads to the 9-cycle miss penalty including the initial clock cycle to access the L1 TLB. If the L2 TLB misses, a hardware algorithm is used to walk the page table and update the TLB entry. Sections L.5 and L.6 of online Appendix L describe page table walkers and page structure caches. In the worst case the page is not in memory, and the operating system gets the page from secondary storage. Because millions of instructions could execute during a page fault, the operating system will swap in another pro- cess if one is waiting to run. Otherwise, if there is no TLB exception the instruc- tion cache access continues.The index field of the address is sent to all eight banks of the instruction cache (step 5). The instruction cache tag is 36 bits 6 bits (index) 6 bits (block offset) or 24 bits. The four tags and valid bits are compared to the physical page frame from the instruction TLB (step 6). Because the i7 expects 16 bytes each instruction fetch an additional 2 bits are used from the 6-bit block offset to select the appro- priate 16 bytes. Therefore 6+2 or 8 bits are used to send 16 bytes of instructions to the processor. The L1 cache is pipelined, and the latency of a hit is 4 clock cycles (step 7). A miss goes to the second-level cache.As mentioned earlier, the instruction cache is virtually addressed and physi- cally tagged. Because the second-level caches are physically addressed, the phys- ical page address from the TLB is composed with the page offset to make an address to access the L2 cache. The L2 index is
so the 30-bit block address (36-bit physical address 6-bit block offset) is divided into a 20-bit tag and a 10-bit index (step 8). Once again, the index and tag are sent to the four banks of the unified L2 cache (step 9), which are compared in parallel. If one matches and is valid (step 10), it returns the block in sequential order after the initial 12-cycle latency at a rate of 8 bytes per clock cycle.If the L2 cache misses, the L3 cache is accessed. For a four-core i7, which has an 8 MiB L3, the index size is
The 13-bit index (step 11) is sent to all 16 banks of the L3 (step 12). The L3 tag, which is 36 (13 + 6) 17 bits, is compared against the physical address from the TLB (step 13). If a hit occurs the block is returned after an initial latency of 42 clock cycles, at a rate of 16 bytes per clock and placed into both L1 and L3. If L3 misses, a memory access is initiated.If the instruction is not found in the L3 cache the on-chip memory controller must get the block from main memory. The i7 has three 64-bit memory channels that can act as one 192-bit channel, because there is only one memory controller and the same address is sent on both channels (step 14). Wide transfers happen when both channels have identical DIMMs. Each channel supports up to four DDR DIMMs (step 15). When the data return they are placed into L3 and L1 (step16) because L3 is inclusive.The total latency of the instruction miss that is serviced by main memory is approximately 42 processor cycles to determine that an L3 miss has occurred, plus the DRAM latency for the critical instructions. For a single-bank DDR4-2400 SDRAM and 4.0 GHz CPU the DRAM latency is about 40 ns or 160 clock cycles to the first 16 bytes, leading to a total miss penalty of about 200 clock cycles. The memory controller fills the remainder of the 64-byte cache block at a rate of 16 bytes per I /O bus clock cycle which takes another 5 ns or 20 clock cycles.Because the second-level cache is a write-back cache, any miss can lead to an old block being written back to memory. The i7 has a 10-entry merging write buffer that writes back dirty cache lines when the next level in the cache is unused for a read. The write buffer is checked on a miss to see if the cache line exists in the buffer; if so, the miss is filled from the buffer. A similar buffer is used between the L1 and L2 caches. If this initial instruction is a load, the data address is sent to the data cache and data TLBs, acting very much like an instruction cache access. Suppose the instruction is a store instead of a load. When the store issues, it does a data cache lookup just like a load. A miss causes the block to be placed in a write buffer because the L1 cache does not allocate the block on a write miss. On a hit, the store does not update the L1 (or L2) cache until later, after it is known to be nonspeculative. During this time, the store resides in a load-store queue partof the out-of-order control mechanism of the processor.The I7 also supports prefetching for L1 and L2 from the next level in the hierarchy. In most cases, the prefetched line is simply the next block in the cache. By prefetching only for L1 and L2, high-cost unnecessary fetches to memory are avoided.
Performance of the i7 memory system We evaluate the performance of the i7 cache structure using the SPECint2006 benchmarks. The data in this section were collected by Professor Lu Peng and PhD student Qun Liu, both of Louisiana State University. Their analysis is based on earlier work (see Prakash and Peng, 2008).The complexity of the i7 pipeline with its use of an autonomous instruction fetch unit, speculation, and both instruction and data prefetch, makes it hard to compare cache performance against simpler processors. As mentioned on page 110 processors that use prefetch can generate cache accesses independent of the memory accesses performed by the program. A cache access that is generated because of an actual instruction access or data access is sometimes called a demand access to distinguish it from a prefetch access. Demand accesses can come from both speculative instruction fetches and speculative data accesses, some of which are subsequently canceled (see Chapter 3 for a detailed description of speculation and instruction graduation). A speculative processor generates at least as many misses as an in-order nonspeculative processor, and typically more. In addition to demand misses, there are prefetch misses for both instructions and data.The i7's instruction fetch unit attempts to fetch 16 bytes every cycle, which com- plicates comparing instruction cache miss rates because multiple instructions are fetched every cycle (roughly 4.5 on average). In fact, the entire 64-byte cache line is read and subsequent 16-byte fetches do not require additional accesses. Thus misses are tracked only on the basis of 64-byte blocks. The 32 KiB, eight-way set associative instruction cache leads to a very low instruction miss rate for the SPECint2006 programs. If, for simplicity, we measure the miss rate of SPECint2006 as the number of misses for a 64-byte block divided by the number of instructions that complete the miss rates are all under 1% except for one benchmark (XALANCBMK), which has a 2.9% miss rate. Because a 64-byte block typically contains 16-20 instructions, the effective miss rate per instruction is much lower, depending on the degree of spatial locality in the instruction stream.The frequency at which the instruction fetch unit is stalled waiting for the I-cache misses is similarly small (as a percentage of total cycles) increasing to 2% for two benchmarks and 12% for XALANCBMK, which has the highest I-cache miss rate. In the next chapter, we will see how stalls in the IFU contribute to overall reductions in pipeline throughput in the i7.The L1 data cache is more interesting and even trickier to evaluate because in addition to the effects of prefetching and speculation, the L1 data cache is not write-allocated, and writes to cache blocks that are not present are not treated as misses. For this reason, we focus only on memory reads. The performance monitor measurements in the i7 separate out prefetch accesses from demand accesses, but only keep demand accesses for those instructions that graduate. The effect of spec- ulative instructions that do not graduate is not negligible, although pipeline effects probably dominate secondary cache effects caused by speculation; we will return to the issue in the next chapter.
To address these issues, while keeping the amount of data reasonable, Figure 2.26 shows the L1 data cache misses in two ways:
1.The L1 miss rate relative to demand references given by the L1 miss rate includ- ing prefetches and speculative loads/L1 demand read references for those instructions that graduate.2.The demand miss rate given by L1 demand misses/L1 demand read references, both measurements only for instructions that graduate.
On average, the miss rate including prefetches is 2.8 times as high as the demand- only miss rate. Comparing this data to that from the earlier i7 920, which had the same size L1, we see that the miss rate including prefetches is higher on the newer i7, but the number of demand misses which are more likely to cause a stall, are usually fewer.To understand the effectiveness of the aggressive prefetch mechanisms in the i7, let's look at some measurements of prefetching. Figure 2.27 shows both the fraction of L2 requests that are prefetches versus demand requests and the prefetch miss rate. The data are probably astonishing at first glance: there are roughly1.5 times as many prefetches as there are L2 demand requests, which come directly from L1 misses. Furthermore, the prefetch miss rate is amazingly high, with an average miss rate of 58%. Although the prefetch ratio varies considerably, the pre- fetch miss rate is always significant. At first glance you might conclude that the designers made a mistake: they are prefetching too much, and the miss rate is too high. Notice, however, that the benchmarks with the higher prefetch ratios (ASTAR, BZIP2, HMMER, LIBQUANTUM, and OMNETPP) also show the greatest gap between the prefetch miss rate and the demand miss rate, more than a factor of 2 in each case. The aggressive prefetching is trading prefetch misses, which occur earlier, for demand misses, which occur later; and as a result a pipe- line stall is less likely to occur due to the prefetching.Similarly, consider the high prefetch miss rate. Suppose that the majority of the prefetches are actually useful (this is hard to measure because it involves tracking individual cache blocks) then a prefetch miss indicates a likely L2 cache miss in the future. Uncovering and handling the miss earlier via the prefetch is likely to reduce the stall cycles. Performance analysis of speculative superscalars, like the i7, has shown that cache misses tend to be the primary cause of pipeline stalls because it is hard to keep the processor going, especially for longer running L2 and L3 misses. The Intel designers could not easily increase the size of the caches with- out incurring both energy and cycle time impacts; thus the use of aggressive pre- fetching to try to lower effective cache miss penalties is an interesting alternative approach.With the combination of the L1 demand misses and prefetches going to L2, roughly 17% of the loads generate an L2 request. Analyzing L2 performance requires including the effects of writes (because L2 is write-allocated) as well as the prefetch hit rate and the demand hit rate. Figure 2.28 shows the miss rates of the L2 caches for demand and prefetch accesses, both versus the number of L1 references (reads and writes). As with L1, prefetches are a significant contributor generating 75% of the L2 misses. Comparing the L2 demand miss rate with that of earlier i7 implementations (again with the same L2 size) shows that the i7 6700 has a lower L2 demand miss rate by an approximate factor of 2, which may well justify the higher prefetch miss rate.
Because the cost for a miss to memory is over 100 cycles and the average data miss rate in L2 combining both prefetch and demand misses is over 7%, L3 is obvi- ously critical. Without L3 and assuming that about one-third of the instructions are loads or stores L2 cache misses could add over two cycles per instruction to the CPI! Obviously, prefetching past L2 would make no sense without an L3.In comparison the average L3 data miss rate of 0.5% is still significant but less than one-third of the L2 demand miss rate and 10 times less than the L1 demand miss rate. Only in two benchmarks (OMNETPP and MCF) is the L3 miss rate above 0.5%; in those two cases the miss rate of about 2.3% likely dominates all other performance losses. In the next chapter, we will examine the relationship between the i7 CPI and cache misses, as well as other pipeline effects.
2.7 Fallacies and Pitfalls
As the most naturally quantitative of the computer architecture disciplines, mem- ory hierarchy would seem to be less vulnerable to fallacies and pitfalls. Yet we were limited here not by lack of warnings but by lack of space! Fallacy:Predicting cache performance of one program from another.Figure 2.29 shows the instruction miss rates and data miss rates for three programs from the SPEC2000 benchmark suite as cache size varies. Depending on the program, the data misses per thousand instructions for a 4096 KiB cache are 9, 2, or 90 and the instruction misses per thousand instructions for a 4 KiB cache are 55, 19, or 0.0004. Commercial programs such as databases will have significant miss rates even in large second-level caches, which is generally not the case for the SPECCPU programs. Clearly generalizing cache performance from one program to another is unwise. As Figure 2.24 reminds us, there is a great deal of variation, and even predictions about the relative miss rates of integer and floating-point- intensive programs can be wrong, as mcf and sphnix3 remind us!
Pitfall:Simulating enough instructions to get accurate performance measures of the memory hierarchy.There are really three pitfalls here. One is trying to predict performance of a large cache using a small trace. Another is that a program's locality behavior is not con- stant over the run of the entire program. The third is that a program's locality behavior may vary depending on the input.Figure 2.30 shows the cumulative average instruction misses per thousand instructions for five inputs to a single SPEC2000 program. For these inputs, the average memory rate for the first 1.9 billion instructions is very different from the average miss rate for the rest of execution.Pitfall:Not delivering high memory bandwidth in a cache-based system.Caches help with average cache memory latency but may not deliver high memory bandwidth to an application that must go to main memory. The architect must design a high bandwidth memory behind the cache for such applications. We will revisit this pitfall in Chapters 4 and 5.
Pitfall:Implementing a virtual machine monitor on an instruction set architecture that wasn't designed to be virtualizable.Many architects in the 1970s and 1980s weren't careful to make sure that all instructions reading or writing information related to hardware resource informa- tion were privileged. This laissez faire attitude causes problems for VMMs for all of these architectures, including the 80x86 which we use here as an example.Figure 2.31 describes the 18 instructions that cause problems for paravirtuali- zation (Robin and Irvine, 2000). The two broad classes are instructions that
- read control registers in user mode that reveal that the guest operating system is running in a virtual machine (such as POPF mentioned earlier) and
- check protection as required by the segmented architecture but assume that the operating system is running at the highest privilege level.
Virtual memory is also challenging. Because the 80x86 TLBs do not support process ID tags, as do most RISC architectures, it is more expensive for the VMM and guest OSes to share the TLB; each address space change typically requires a TLB flush.
Virtualizing I /O is also a challenge for the 80x86, in part because it supports memory-mapped I /O and has separate I /O instructions, but more importantly because there are a very large number and variety of types of devices and device drivers of PCs for the VMM to handle. Third-party vendors supply their own drivers and they may not properly virtualize. One solution for conventional VM implementations is to load real device drivers directly into the VMM.To simplify implementations of VMMs on the 80x86, both AMD and Intel have proposed extensions to the architecture. Intel's VT-x provides a new execu- tion mode for running VMs, a architected definition of the VM state instructions to swap VMs rapidly, and a large set of parameters to select the circumstances where a VMM must be invoked. Altogether, VT-x adds 11 new instructions for the 80x86. AMD's Secure Virtual Machine (SVM) provides similar functionality.After turning on the mode that enables VT-x support (via the new VMXON instruc- tion) VT-x offers four privilege levels for the guest OS that are lower in priority than the original four (and fix issues like the problem with the POPF instruction mentioned earlier). VT-xcapturesall thestatesofavirtualmachineinthe Virtual Machine Control State (VMCS) and then provides atomic instructions to save and restore a VMCS. In addition to critical state the VMCS includes configuration information to deter- mine when to invoke the VMM and then specifically what caused the VMM to be invoked. To reduce the number of times the VMM must be invoked, this mode adds shadow versions of some sensitive registers and adds masks that check to see whether critical bits of a sensitive register will be changed before trapping. To reduce the cost of virtualizing virtual memory, AMD's SVM adds an additional level of indirection, called nested pagetables, which makes shadow page tables unnecessary (see Section L.7 of Appendix L).
2.8 Concluding Remarks: Looking Ahead
Over the past thirty years there have been several predictions of the eminent [sic] cessation of the rate of improvement in computer performance. Every such pre- diction was wrong. They were wrong because they hinged on unstated assump- tions that were overturned by subsequent events. So, for example the failure to foresee the move from discrete components to integrated circuits led to a predic- tion that the speed of light would limit computer speeds to several orders of mag- nitude slower than they are now. Our prediction of the memory wall is probably wrong too but it suggests that we have to start thinking “out the box.”Wm. A. Wulf and Sally A. McKee,Hitting the Memory Wall: Implications of the Obvious,Department of Computer Science, University of Virginia (December 1994).This paper introduced the term memory wall.The possibility of using a memory hierarchy dates back to the earliest days of general-purpose digital computers in the late 1940s and early 1950s. Virtual mem- ory was introduced in research computers in the early 1960s and into IBM main- frames in the 1970s. Caches appeared around the same time. The basic concepts have been expanded and enhanced over time to help close the access time gap between main memory and processors, but the basic concepts remain.One trend that is causing a significant change in the design of memory hierar- chies is a continued slowdown in both density and access time of DRAMs. In the past 15 years, both these trends have been observed and have been even more obvi- ous over the past 5 years. While some increases in DRAM bandwidth have been achieved, decreases in access time have come much more slowly and almost van- ished between DDR4 and DDR3. The end of Dennard scaling as well as a slow- down in Moore's Law both contributed to this situation. The trenched capacitor design used in DRAMs is also limiting its ability to scale. It may well be the case that packaging technologies such as stacked memory will be the dominant source of improvements in DRAM access bandwidth and latency.Independently of improvements in DRAM, Flash memory has been playing a much larger role. In PMDs, Flash has dominated for 15 years and became the stan- dard for laptops almost 10 years ago. In the past few years, many desktops have shipped Flash as the primary secondary storage. Flash's potential advantage over DRAMs, specifically the absence of a per-bit transistor to control writing, is also its Achilles heel. Flash must use bulk erase-rewrite cycles that are consider- ably slower. As a result, although Flash has become the fastest growing form of secondary storage, SDRAMs still dominate for main memory.Although phase-change materials as a basis for memory have been around for a while, they have never beenserious competitors either formagnetic disksorfor Flash. The recent announcement by Intel and Micron of the cross-point technology may change this. The technology appears tohaveseveral advantagesover Flash, including the elimination of the slow erase-to-write cycle and greater longevity in terms. It could be that this technology will finally be the technology that replaces the electro- mechanical disks that have dominated bulk storage for more than 50 years! For some years, a variety of predictions have been made about the coming memory wall (see previously cited quote and paper), which would lead to serious limits on processor performance. Fortunately, the extension of caches to multiple levels (from 2 to 4), more sophisticated refill and prefetch schemes, greater com- piler and programmer awareness of the importance of locality, and tremendous improvements in DRAM bandwidth (a factor of over 150 times since the mid- 1990s) have helped keep the memory wall at bay. In recent years, the combination of access time constraints on the size of L1 (which is limited by the clock cycle) and energy-related limitations on the size of L2 and L3 have raised new challenges. The evolution of the i7 processor class over 6-7 years illustrates this: the caches are the same size in the i7 6700 as they were in the first generation i7 processors! The more aggressive use of prefetching is an attempt to overcome the inability to increase L2 and L3. Off-chip L4 caches are likely to become more important because they are less energy-constrained than on-chip caches.In addition to schemes relying on multilevel caches, the introduction of out-of- order pipelines with multiple outstanding misses has allowed available instruction- level parallelism to hide the memory latency remaining in a cache-based system. The introduction of multithreading and more thread-level parallelism takes this a step further by providing more parallelism and thus more latency-hiding opportunities. It is likely that the use instruction- and thread-level parallelism will be a more important tool in hiding whatever memory delays are encountered in modern multilevel cache systems.One idea that periodically arises is the use of programmer-controlled scratch- pad or other high-speed visible memories, which we will see are used in GPUs. Such ideas have never made the mainstream in general-purpose processors sev- eral reasons: First, they break the memory model by introducing address spaces with different behavior. Second, unlike compiler-based or programmer-based cache optimizations (such as prefetching), memory transformations with scratch- pads must completely handle the remapping from main memory address space to the scratchpad address space. This makes such transformations more difficult and limited in applicability. In GPUs (see Chapter 4), where local scratchpad memories are heavily used, the burden for managing them currently falls on the programmer. For domain-specific software systems that can use such memories, the perfor- mance gains are very significant. It is likely that HBM technologies will thus be used for caching in large, general-purpose computers and quite possibility as the main working memories in graphics and similar systems. As domain-specific architectures become more important in overcoming the limitations arising from the end of Dennard's Law and the slowdown in Moore's Law (see Chapter 7), scratchpad memories and vector-like register sets are likely to see more use.The implications of the end of Dennard's Law affect both DRAM and proces- sor technology. Thus, rather than a widening gulf between processors and main memory, we are likely to see a slowdown in both technologies, leading to slower overall growth rates in performance. New innovations in computer architecture and in related software that together increase performance and efficiency will be key to continuing the performance improvements seen over the past 50 years.
2.9 Historical Perspectives and References
In Section M.3 (available online) we examine the history of caches, virtual mem- ory, and virtual machines. IBM plays a prominent role in the history of all three. References for further reading are included.
Case Studies and Exercises by Norman P. Jouppi, Rajeev Balasubramonian, Naveen Muralimanohar, and Sheng Li
Case Study 1: Optimizing Cache Performance via Advanced Techniques
Concepts illustrated by this case study
- Nonblocking Caches
- Compiler Optimizations for Caches
- Software and Hardware Prefetching
- Calculating Impact of Cache Performance on More Complex Processors
The transpose of a matrix interchanges its rows and columns; this concept is illustrated here:
Here is a simple C loop to show the transpose:
Assume that both the input and output matrices are stored in the row major order (row major order means that the row index changes fastest). Assume that you are executing a 256 256 double-precision transpose on a processor with a 16KB fully associative (don't worry about cache conflicts) least recently used (LRU) replace- ment L1 data cache with 64-byte blocks. Assume that the L1 cache misses or pre- fetches require 16 cycles and always hit in the L2 cache, and that the L2 cache can process a request every 2 processor cycles. Assume that each iteration of the pre- ceding inner loop requires 4 cycles if the data are present in the L1 cache. Assume that the cache has a write-allocate fetch-on-write policy for write misses. Unreal- istically, assume that writing back dirty cache blocks requires 0 cycles.
2.1[10/15/15/12/20] <2.3> For the preceding simple implementation, this execution order would be nonideal for the input matrix; however, applying a loop interchangeoptimization would create a nonideal order for the output matrix. Because loop interchange is not sufficient to improve its performance, it must be blocked instead. a. <2.3> What should be the minimum size of the cache to take advantage of blocked execution? B . <2.3> How do the relative number of misses in the blocked and unblocked versions compare in the preceding minimum-sized cache? c. <2.3> Write code to perform a transpose with a block size parameter Bthat uses B· B blocks. d. <2.3> What is the minimum associativity required of the L1 cache for consistent performance independent of both arrays' position in memory? e. <2.3> Try out blocked and nonblocked 256 256 matrix transpositions on a computer. How closely do the results match your expectations based on whatyou know about the computer's memory system? Explain any discrepancies if possible.2.2 <2.3> Assume you are designing a hardware prefetcher for the preceding unblocked matrix transposition code. The simplest type of hardware prefetcher only prefetches sequential cache blocks after a miss. More complicated “nonunit stride”hardware prefetchers can analyze a miss reference stream and detect and prefetch nonunit strides. In contrast, software prefetching can determine nonunit strides as eas- ily as it can determine unit strides. Assume prefetches write directly into the cache and that there is no “pollution” (overwriting data that must be used before the data that are prefetched). For best performance given a nonunit stride prefetcher, in the steady state the inner loop, how many prefetches must be outstanding at a given time? 2.3[15/20] <2.3> With software prefetching, it is important to be careful to have the prefetches occur in time for use but also to minimize the number of outstandingprefetches to live within the capabilities of the microarchitecture and minimize cache pollution. This is complicated by the fact that different processors have dif- ferent capabilities and limitations. a. <2.3> Create a blocked version of the matrix transpose with software prefetching. B . <2.3> Estimate and compare the performance of the blocked and unblocked transpose codes both with and without software prefetching.
Case Study 2: Putting It All Together: Highly Parallel Memory Systems
Concept illustrated by this case study
- Cross-Cutting Issues: The Design of Memory Hierarchies
The program in Figure 2.32 can be used to evaluate the behavior of a memory sys- tem. The key is having accurate timing and then having the program stride through memory to invoke different levels of the hierarchy. Figure 2.32 shows the code inC. The first part is a procedure that uses a standard utility to get an accurate measure of the user CPU time; this procedure may have to be changed to work on some systems. The second part is a nested loop to read and write memory at different strides and cache sizes. To get accurate cache timing, this code is repeated many times. The third part times the nested loop overhead only so that it can be subtracted from overall measured times to see how long the accesses were. The results are output in .csv file format to facilitate importing into spreadsheets. You may need to change CACHE_MAX depending on the question you are answer- ing and the size of memory on the system you are measuring. Running the program in single-user mode or at least without other active applications will give more con- sistent results. The code in Figure 2.32 was derived from a program written by Andrea Dusseau at the University of California-Berkeley and was based on a detailed description found in Saavedra-Barrera (1992). It has been modified to fix a number of issues with more modern machines and to run under Microsoft Visual C++++. It can be downloaded from http://www.hpl.hp.com/research/cacti/ aca_ch2_cs2.c.
The preceding program assumes that program addresses track physical addresses, which is true on the few machines that use virtually addressed caches, such as the Alpha 21264. In general, virtual addresses tend to follow physical addresses shortly after rebooting so you may need to reboot the machine in order to get smooth lines in your results. To answer the following questions, assume that the sizes of all components of the memory hierarchy are powers of 2. Assume that the size of the page is much larger than the size of a block in a second-level cache (if there is one) and that the size of a second-level cache block is greater than or equal to the size of a block in a first-level cache. An example of the output of the program is plotted in Figure 2.33; the key lists the size of the array that is exercised.2.4[12/12/12/10/12] <2.6> Using the sample program results in Figure 2.33: a. <2.6> What are the overall size and block size of the second-level cache? B . <2.6> What is the miss penalty of the second-level cache? c. <2.6> What is the associativity of the second-level cache? d. <2.6> What is the size of the main memory? e. <2.6> What is the paging time if the page size is 4KB?
2.5[12/15/15/20] <2.6> If necessary, modify the code in Figure 2.32 to measure the following system characteristics. Plot the experimental results with elapsed time on the y-axis and the memory stride on the x-axis. Use logarithmic scales for bothaxes, and draw a line for each cache size. a. <2.6> What is the system page size? B . <2.6> How many entries are there in the TLB? c. <2.6> What is the miss penalty for the TLB? d. <2.6> What is the associativity of the TLB? 2.6[20/20] <2.6> In multiprocessor memory systems lower levels of the memory hierarchy may not be able to be saturated by a single processor but should be ableto be saturated by multiple processors working together. Modify the code in Figure 2.32, and run multiple copies at the same time. Can you determine: a. <2.6> How many actual processors are in your computer system and how many system processors are just additional multithreaded contexts? B . <2.6> How many memory controllers does your system have? 2.7 <2.6> Can you think of a way to test some of the characteristics of an instruc- tion cache using a program? Hint: The compiler may generate a large number of nonobvious instructions from a piece of code. Try to use simple arithmetic instruc-tions of known length in your instruction set architecture (ISA).
Case Study 3: Studying the Impact of Various Memory System Organizations
Concepts illustrated by this case study
- DDR3 memory systems
- Impact of ranks, banks, row buffers on performance and power
- DRAM timing parameters
A processor chip typically supports a few DDR3 or DDR4 memory channels. We will focus on a single memory channel in this case study and explore how its per- formance and power are impacted by varying several parameters. Recall that the channel is populated with one or more DIMMs. Each DIMM supports one or more ranks-a rank is a collection of DRAM chips that work in unison to service a single command issued by the memory controller. For example, a rank may be composed of 16 DRAM chips, where each chip deals with a 4-bit input or output on every channel clock edge. Each such chip is referred to as a 4 (by four) chip. In other examples, a rank may be composed of 8 8 chips or 4 16 chips-note that in each case, a rank can handle data that are being placed on a 64-bit memory channel. A rank is itself partitioned into 8 (DDR3) or 16 (DDR4) banks. Each bank has a row buffer that essentially remembers the last row read out of a bank. Here's an example of a typical sequence of memory commands when performing a read from a bank:( I )The memory controller issues a Precharge command to get the bank ready to access a new row. The precharge is completed after time tRP.(ii)The memory controller then issues an Activate command to read the appro- priate row out of the bank. The activation is completed after time tRCD and the row is deemed to be part of the row buffer.(iii)The memory controller can then issue a column-read or CAS command that places a specific subset of the row buffer on memory channel. After time CL, the first 64 bits of the data burst are placed on the memory channel. A burst typically includes eight 64-bit transfers on the memory channel, per- formed on the rising and falling edges of 4 memory clock cycles (referred to as transfer time).(iv)If the memory controller wants to then access data in a different row of the bank, referred to as a row buffer miss, it repeats steps (I )-(iii). For now, we will assume that after CL has elapsed, the Precharge in step (I) can be issued; in some cases, an additional delay must be added but we will ignore that delay here. If the memory controller wants to access another block of data in the same row, referred to as a row buffer hit, it simply issues another CAS command. Two back-to-back CAS commands have to be separated by at least 4 cycles so that the first data transfer is complete before the second data transfer can begin.Note that a memory controller can issue commands to different banks in successive cycles so that it can perform many memory reads/writes in parallel and it is not sitting idle waiting for tRP, tRCD, and CL to elapse in a single bank. For the sub- sequent questions assume that tRP tRCD CL 13 ns, and that the memory channel frequency is 1 GHz, that is, a transfer time of 4 ns.2.8 <2.2> What is the read latency experienced by a memory controller on a row buffer miss? 2.9 <2.2> What is the latency experienced by a memory controller on a row buffer hit? 2.10 <2.2> If the memory channel supports only one bank and the memory access pattern is dominated by row buffer misses, what is the utilization of the memory channel? 2.11 <2.2> Assuming a 100% row buffer miss rate, what is the minimum number of banks that the memory channel should support in order to achieve a 100% mem- ory channel utilization? 2.12 <2.2> Assuming a 50% row buffer miss rate, what is the minimum number of banks that the memory channel should support in order to achieve a 100% memory channel utilization? 2.13 <2.2> Assume that we are executing an application with four threads and the threads exhibit zero spatial locality, that is, a 100% row buffer miss rate. Every 200 ns, each of the four threads simultaneously inserts a read operation into the memory controller queue. What is the average memory latency experienced if the memory channel supports only one bank? What if the memory channel supported four banks? 2.14 <2.2> From these questions, what have you learned about the benefits and downsides of growing the number of banks? 2.15 <2.2> Now let's turn our attention to memory power. Download a copy of the Micron power calculator from this link: https://www.micron.com/ /media/ documents/products/power-calculator/ddr3_power_calc.xlsm. This spreadsheetis preconfigured to estimate the power dissipation in a single 2 Gb 8 DDR3 SDRAM memory chip manufactured by Micron. Click on the “Summary” tab to see the power breakdown in a single DRAM chip under default usage conditions (reads occupy the channel for 45% of all cycles, writes occupy the channel for 25% of all cycles, and the row buffer hit rate is 50%). This chip consumes 535 mW, and the breakdown shows that about half of that power is expended in Activate oper- ations, about 38% in CAS operations, and 12% in background power. Next, click on the “System Config” tab. Modify the read/write traffic and the row buffer hit rate and observe how that changes the power profile. For example, what is the decrease in power when channel utilization is 35% (25% reads and 10% writes), or when row buffer hit rate is increased to 80%? 2.16 <2.2> In the default configuration, a rank consists of eight 8 2 Gb DRAM chips. A rank can also comprise16 4 chips or 4 16 chips. You can also vary the capacity of each DRAM chip-1 Gb, 2 Gb, and 4 Gb. These selections can bemade in the “DDR3 Config” tab of the Micron power calculator. Tabulate the total power consumed for each rank organization. What is the most power-efficient approach to constructing a rank a given capacity?
2.17[12/12/15] <2.3> The following questions investigate the impact of small and simple caches using CACTI and assume a 65 nm (0.065 m) technology. (CACTI is available in an online form at http://quid.hpl.hp.com:9081/cacti/ .) a. <2.3> Compare the access times of 64KB caches with 64-byte blocks and a single bank. What are the relative access times of two-way and four-way set associative caches compared to a direct mapped organization? B . <2.3> Compare the access times of four-way set associative caches with 64-byte blocks and a single bank. What are the relative access times of 32 and 64KB caches compared to a 16KB cache? c. <2.3> For a 64KB cache find the cache associativity between 1 and8 with the lowest average memory access time given that misses per instructionfor a certain workload suite is 0.00664 for direct-mapped, 0.00366 for two-way set associative, 0.000987 for four-way set associative, and 0.000266 for eight- way set associative cache. Overall there are 0.3 data references per instruction. Assume cache misses take 10 ns in all models. To calculate the hit time in cycles, assume the cycle time output using CACTI, which corresponds to the maximum frequency a cache can operate without any bubbles in the pipeline.2.18[12/15/15/10] <2.3> You are investigating the possible benefits of a way- predicting L1 cache. Assume that a 64KB four-way set associative single-banked L1 data cache is the cycle time limiter in a system. For an alternative cache orga-nization, you are considering a way-predicted cache modeled as a 64KB direct- mapped cache with 80% prediction accuracy. Unless stated otherwise, assume that a mispredicted way access that hits in the cache takes one more cycle. Assume the miss rates and the miss penalties in question 2.8 part (c). a. <2.3> What is the average memory access time of the current cache (in cycles) versus the way-predicted cache? B . <2.3> If all other components could operate with the faster way-predicted cache cycle time (including the main memory), what would be the impact on performance from using the way-predicted cache? c. <2.3> Way-predicted caches have usually been used only for instruction caches that feed an instruction queue or buffer. Imagine that you want to try out way prediction on a data cache. Assume that you have 80% prediction accuracyand that subsequent operations (e.g., data cache access of other instructions, dependent operations) are issued assuming a correct way prediction. Thus a way misprediction necessitates a pipe flush and replay trap, which requires 15 cycles. Is the change in average memory access time per load instruction with data cache way prediction positive or negative, and how much is it? d. <2.3> As an alternative to way prediction, many large associative L2 caches serialize tag and data access so that only the required dataset array needs to be activated. This saves power but increases the access time. UseCACTI's detailed web interface for a 0.065 m process 1MB four-way set associative cache with 64-byte blocks, 144 bits read out, 1 bank, only 1 read/write port, 30 bit tags, and ITRS-HP technology with global wires. What is the ratio of the access times for serializing tag and data access compared to parallel access? 2.19[10/12] <2.3> You have been asked to investigate the relative performance of a banked versus pipelined L1 data cache for a new microprocessor. Assume a 64KB two-way set associative cache with 64-byte blocks. The pipelined cache wouldconsist of three pipe stages, similar in capacity to the Alpha 21264 data cache. A banked implementation would consist of two 32KB two-way set associative banks. Use CACTI and assume a 65 nm (0.065 m) technology to answer the fol- lowing questions. The cycle time output in the web version shows at what frequency a cache can operate without any bubbles in the pipeline. a. <2.3> What is the cycle time of the cache in comparison to its access time, and how many pipe stages will the cache take up (to two decimal places)? B . <2.3> Compare the area and total dynamic read energy per access of the pipelined design versus the banked design. State which takes up less area and which requires more power, and explain why that might be.2.20[12/15] <2.3> Consider the usage of critical word first and early restart on L2 cache misses. Assume a 1MB L2 cache with 64-byte blocks and a refill path that is 16 bytes wide. Assume that the L2 can be written with 16 bytes every 4processor cycles, the time to receive the first 16 byte block from the memory con- troller is 120 cycles, each additional 16 byte block from main memory requires 16 cycles, and data can be bypassed directly into the read port of the L2 cache. Ignore any cycles to transfer the miss request to the L2 cache and the requested data to the L1 cache. a. <2.3> How many cycles would it take to service an L2 cache miss with and without critical word first and early restart? B . <2.3> Do you think critical word first and early restart would be more important for L1 caches or L2 caches, and what factors would contribute to their relative importance? 2.21[12/12] <2.3> You are designing a write buffer a write-through L1 cache and a write-back L2 cache. The L2 cache write data bus is 16 B wide and can per- form a write to an independent cache address every four processor cycles. a. <2.3> How many bytes wide should each write buffer entry be? B . <2.3> What speedup could be expected in the steady state by using a merging write buffer instead of a nonmerging buffer when zeroing memory by the execution of 64-bit stores if all other instructions could be issued inparallel with the stores and the blocks are present in the L2 cache? c. <2.3> What would the effect of possible L1 misses be on the number of required write buffer entries for systems with blocking and nonblocking caches? 2.22 <2.1, 2.2, 2.3> A cache acts as a filter. For example, for every 1000 instruc- tions of a program, an average of 20 memory accesses may exhibit low enough locality that they cannot be serviced by a 2MB cache. The 2MB cache is saidto have an MPKI (misses per thousand instructions) of 20, and this will be largely true regardless of the smaller caches that precede the 2MB cache. Assume the fol- lowing cache/latency/MPKI values: 32 KB/1/100, 128 KB/2/80, 512 KB/4/50, 2 MB/8/40, 8 MB/16/10. Assume that accessing the off-chip memory system requires 200 cycles on average. For the following cache configurations, calculate the average time spent accessing the cache hierarchy. What do you observe about the downsides of a cache hierarchy that is too shallow or too deep? a.32 KB L1; 8MB L2; off-chip memory B .32KB L1; 512KB L2; 8MB L3; off-chip memory c.32 KB L1; 128KB L2; 2MB L3; 8MB L4; off-chip memory2.23 <2.1, 2.2, 2.3> Consider a 16MB 16-way L3 cache that is shared by two programs A and B. There is a mechanism in the cache that monitors cache miss rates for each program and allocates 1-15 ways to each program such that the over-all number of cache misses is reduced. Assume that program A has an MPKI of 100 when it is assigned 1MB of the cache. Each additional 1MB assigned to program A reduces the MPKI by 1. Program B has an MPKI of 50 when it is assigned 1MB of cache; each additional 1MB assigned to program B reduces its MPKI by 2. What is the best allocation of ways to programs A and B? 2.24 <2.1, 2.6> You are designing a PMD and optimizing it for low energy. The core, including an 8KB L1 data cache, consumes 1W whenever it is not in hiber- nation. If the core has a perfect L1 cache hit rate, it achieves an average CPI of 1 for a given task, that is, 1000 cycles to execute 1000 instructions. Each additional cycle accessing the L2 and beyond adds a stall cycle for the core. Based on the following specifications what is the size of L2 cache that achieves the lowest energy for the PMD (core, L1, L2, memory) for that given task? a.The core frequency is 1 GHz, and the L1 has an MPKI of 100. B .A 256KB L2 has a latency of 10 cycles, an MPKI of 20 a background power of0.2 W, and each L2 access consumes 0.5 nJ. c.A 1MB L2 has a latency of 20 cycles, an MPKI of 10, a background power of0.8 W, and each L2 access consumes 0.7 nJ. d.The memory system has an average latency of 100 cycles, a background power of 0.5W and each memory access consumes 35 nJ.2.25 <2.1, 2.6> You are designing a PMD that is optimized for low power. Qual- itatively explain the impact on cache hierarchy (L2 and memory) power and overall application energy if you design an L2 cache with: a.Small block size B .Small cache size c.High associativity2.30[10/10] <2.1 2.2, 2.3> The ways of a set can be viewed as a priority list, ordered from high priority to low priority. Every time the set is touched, the list can be reorganized to change block priorities. With this view, cache management policies can be decomposed into three sub-policies: Insertion, Promotion, and Victim Selection. Insertion defines where newly fetched blocks are placed in the priority list. Promotion defines how a block's position in the list is changed every time it is touched (a cache hit). Victim Selection defines which entry of the list is evicted to make room for a new block when there is a cache miss. a.Can you frame the LRU cache policy in terms of the Insertion, Promotion, and Victim Selection sub-policies? B .Can you define other Insertion and Promotion policies that may be competitive and worth exploring further? 2.31 <2.1, 2.3> In a processor that is running multiple programs, the last-level cache is typically shared by all the programs. This leads interference, where one program's behavior and cache footprint can impact the cache available to other
- First, this is a problem from a quality-of-service (QoS) perspective, where the interference leads to a program receiving fewer resources and lower performance than promised, say by the operator of a cloud service. Second, this is a problem in terms of privacy. Based on the interference it sees a program can infer the memory access patterns of other programs. This is referred to as a timing chan- nel, a form of information leakage from one program to others that can be exploited to compromise data privacy or to reverse-engineer a competitor's algorithm. What policies can you add to your last-level cache so that the behavior one program is immune to the behavior of other programs sharing the cache? 2.32 <2.3> A large multimegabyte L3 cache can take tens of cycles to access because of the long wires that have to be traversed. For example, it may take 20 cycles to access a 16MB L3 cache. Instead of organizing the 16MB cache Such
that every access takes 20 cycles, we can organize the cache so that it is an array of smaller cache banks. Some of these banks may be closer to the processor core, while others may be further. This leads to nonuniform cache access (NUCA) where 2MB of the cache may be accessible in 8 cycles, the next 2MB in 10 cycles, and so on until the last 2MB is accessed in 22 cycles. What new policies can you introduce to maximize performance in a NUCA cache? 2.33[10/10/10] <2.2> Consider a desktop system with a processor connected to a 2GB DRAM with error-correcting code (ECC). Assume that there is only one memory channel of width 72 bits (64 bits for data and 8 bits for ECC). a. <2.2> How many DRAM chips are on the DIMM if 1 Gb DRAM chips are used and how many data I/OS must each DRAM have if only one DRAM connects to each DIMM data pin? B . <2.2> What burst length is required to support 32 B L2 cache blocks? c. <2.2> Calculate the peak bandwidth for DDR2-667 and DDR2-533 DIMMs for reads from an active page excluding the ECC overhead.2.34[10/10] <2.2> A sample DDR2 SDRAM timing diagram is shown in Figure 2.34. tRCD is the time required to activate a row in a bank, and column address strobe (CAS) latency (CL) is the number of cycles required to read out a columnin a row. Assume that the RAM is on a standard DDR2 DIMM with ECC, having 72 data lines. Also assume burst lengths of 8 that read out 8 bits, or a total of 64 B from the DIMM. Assume tRCD = CAS (or CL) clock_frequency, and clock_frequency = transfers_per_second/2. The on-chip latency on a cache miss through levels 1 and 2 and back, not including the DRAM access, is 20 ns.a. <2.2> How much time is required from presentation of the activate command until the last requested bit of data from the DRAM transitions from valid to invalid for the DDR2-667 1 Gb CL 5 DIMM? Assume that for every request, we automatically prefetch another adjacent cache line in the same page. B . <2.2> What is the relative latency when using the DDR2-667 DIMM of a read requiring a bank activate versus one to an already open page, including the time required to process the miss inside the processor? 2.35 <2.2> Assume that a DDR2-667 2GB DIMM with CL 5 is available for 130 and a DDR2-533 2GB DIMM with CL 4 is available for 100. Assume that two DIMMs are used in a system, and the rest of the system costs 800. Consider the performance of the system using the DDR2-667 and DDR2-533 DIMMs on a workload with 3.33 L2 misses per 1K instructions, and assume that 80% of all DRAM reads require an activate. What is the cost-performance of the entire system when using the different DIMMs, assuming only one L2 miss is outstanding at a time and an in-order core with a CPI of 1.5 not including L2 cache miss memory access time? 2.36 <2.2> You are provisioning a server with eight-core 3 GHz CMP that can execute a workload with an overall CPI of 2.0 (assuming that L2 cache miss refills are not delayed). The L2 cache line size is 32 bytes. Assuming the system uses DDR2-667 DIMMs how many independent memory channels should be provided so the system is not limited by memory bandwidth if the bandwidth required is sometimes twice the average? The workloads incur, on average, 6.67 L2 misses per 1 K instructions. 2.37 <2.2> Consider a processor that has four memory channels. Should consec- utive memory blocks be placed in the same bank, or should they be placed in dif- ferent banks on different channels? 2.38[12/12] <2.2> A large amount (more than a third) of DRAM power can be due to page activation (see http://download.micron.com/pdf/technotes/ddr2/TN4704.pdf and http://www.micron.com/systemcalc). Assume you are building a system with 2GB of memory using either 8-bank 2 Gb 8 DDR2 DRAMs or 8-bank 1 Gb 8 DRAMs, both with the same speed grade. Both use a page size of 1 KB, and the last- level cache line size is 64 bytes. Assume that DRAMs that are not active are in precharged standby and dissipate negligible power. Assume that time to transition from standby to active is not significant. a. <2.2> Which type of DRAM would be expected to provide the higher system performance? Explain why. B . <2.2> How does a 2GB DIMM made of 1 Gb 8 DDR2 DRAMs com- pare with a DIMM with similar capacity made of 1 Gb 4 DDR2 DRAMs in terms of power? 2.39[20/15/12] <2.2> To access data from a typical DRAM, we first have to activate the appropriate row. Assume that this brings an entire page of size 8KB to the row buffer. Then we select a particular column from the row buffer. If subsequent accesses to DRAM are to the same page, then we can skip the activation step; oth- erwise, we have to close the current page and precharge the bitlines for the next activation. Another popular DRAM policy is to proactively close a page and precharge bitlines as soon as an access is over. Assume that every read or write to DRAM is of size 64 bytes and DDR bus latency (data from Figure 2.33) for sending 512 bits is Tddr. a. <2.2> Assuming DDR2-667, if it takes five cycles to precharge, five cycles to activate, and four cycles to read a column, for what value of the row buffer hit rate (r) will you choose one policy over another to get the best access time? Assume that every access to DRAM is separated by enough time to finish a random new access. B . <2.2> If 10% of the total accesses to DRAM happen back to back or contiguously without any time gap, how will your decision change? c. <2.2> Calculate the difference in average DRAM energy per access between the two policies using the previously calculated row buffer hit rate. Assume that precharging requires 2 nJ and activation requires 4 nJ and that 100 pJ/bit are required to read or write from the row buffer. 2.40 <2.2> Whenever a computer is idle, we can either put it in standby (where DRAM is still active) or we can let it hibernate. Assume that, to hibernate, we have to copy just the contents of DRAM to a nonvolatile medium such as Flash. If read- ing or writing a cache line of size 64 bytes to Flash requires 2.56 J and DRAM requires 0.5 nJ, and if idle power consumption for DRAM is 1.6W (for 8 GB), how long should a system be idle to benefit from hibernating? Assume a main memory of size 8 GB. 2.41[10/10/10/10/10] <2.4> Virtual machines (VMs) have the potential for adding many beneficial capabilities to computer systems, such as improved total cost of ownership (TCO) or availability. Could VMs be used to provide the following capabilities? If so, how could they facilitate this? a. <2.4> Test applications in production environments using development machines? B . <2.4> Quick redeployment of applications in case of disaster or failure? c. <2.4> Higher performance in I /O-intensive applications? d. <2.4> Fault isolation between different applications, resulting in higher availability for services? e. <2.4> Performing software maintenance on systems while applications are running without significant interruption? 2.42[10/10/12/12]<2.4> Virtualmachinescanloseperformancefromanumberofevents such as the execution of privileged instructions, TLB misses, traps, and I /O.These events are usually handled in system code. Thus one way of estimating the slowdown when running under a VM is the percentage of application execution time in system versus user mode. For example, an application spending 10% of its execution in system mode might slow down by 60% when running on a VM. Figure 2.35 lists the early performance of various system calls under native execu- tion, pure virtualization, and paravirtualization for LMbench using Xen on an Itanium system with times measured in microseconds (courtesy of Matthew Chapman of the University of New South Wales).
a. <2.4> What types of programs would be expected to have smaller slowdowns when running under VMs? B . <2.4> If slowdowns were linear as a function of system time, given the preceding slowdown, how much slower would a program spending 20% of its execution in system time be expected to run? c. <2.4> What is the median slowdown of the system calls in the table above under pure virtualization and paravirtualization? d. <2.4> Which functions in the table above have the largest slowdowns? What do you think the cause of this could be? 2.43 <2.4> Popek and Goldberg's definition of a virtual machine said that it would be indistinguishable from a real machine except for its performance. In this question, we will use that definition to find out if we have access to native execution on a processor or are running on a virtual machine. The Intel VT-x technology effec- tively provides a second set of privilege levels for the use of the virtual machine. What would a virtual machine running on top of another virtual machine have to do, assuming VT-x technology? 2.44[20/25] <2.4> With the adoption of virtualization support on the x86 architecture, virtual machines are actively evolving and becoming mainstream. Compare and contrast the Intel VT-x and AMD's AMD-V virtualization technologies.(Information on AMD-V can be found at http://sites.amd.com/us/business/itsolutions/virtualization/Pages/resources.aspx .) a. <2.4> Which one could provide higher performance for memory- intensive applications with large memory footprints? B . <2.4> Information on AMD's IOMMU support for virtualized I /O can be found at http://developer.amd.com/documentation/articles/pages/892006101. aspx. What do Virtualization Technology and an input/output memory manage-ment unit (IOMMU) do to improve virtualized I /O performance? 2.45 <2.2, 2.3> Since instruction-level parallelism can also be effectively exploited on in-order superscalar processors and very long instruction word (VLIW) processors with speculation, one important reason for building an out-of-order (OOO) superscalar processor is the ability to tolerate unpredictable mem- ory latency caused by cache misses. Thus you can think about hardware supporting OOO issue as being part of the memory system. Look at the floorplan of the Alpha 21264 in Figure 2.36 to find the relative area of the integer and floating-point issue queues and mappers versus the caches. The queues schedule instructions for issue,and the mappers rename register specifiers. Therefore these are necessary additions to support OOO issue. The 21264 only has L1 data and instruction caches on chip, and they are both 64KB two-way set associative. Use an OOO superscalar sim- ulator such as SimpleScalar (http://www.cs.wisc.edu/ mscalar/simplescalar. html) on memory-intensive benchmarks to find out how much performance is lost if the area of the issue queues and mappers is used for additional L1 data cache area in an in-order superscalar processor, instead of OOO issue in a model of the 21264. Make sure the other aspects of the machine are similar as possible to make the comparison fair. Ignore any increase in access or cycle time from larger caches and effects of the larger data cache on the floorplan of the chip. (Note that this com- parison will not be totally fair, as the code will not have been scheduled for the in-order processor by the compiler.)
2.46 <2.2, 2.7> As discussed in Section 2.7, the Intel i7 processor has an aggres- sive prefetcher. What are potential disadvantages in designing a prefetcher that is extremely aggressive? 2.47[20/20/20] <2.6> The Intel performance analyzer VTune can be used to make many measurements of cache behavior. A free evaluation version of VTune on both Windows and Linux can be downloaded from http://software.intel.com/en-us/articles/intel-vtune-amplifier-xe/. The program (aca_ch2_cs2.c) used in Case Study 2 has been modified so that it can work with VTune out of the box on Microsoft Visual C++. The program can be downloaded from http://www.hpl.hp.com/research/cacti/aca_ch2_cs2_vtune.c. Special VTune functions have been inserted to exclude initialization and loop overhead during the performance analysis process. Detailed VTune setup directions are given in the README sec- tion in the program. The program keeps looping for 20 seconds for every config- uration. In the following experiment, you can find the effects of data size on cache and overall processor performance. Run the program in VTune on an Intel proces- sor with the input dataset sizes of 8 KB, 128 KB, 4 MB, and 32 MB, and keep a stride of 64 bytes (stride one cache line on Intel i7 processors). Collect statistics on overall performance and L1 data cache, L2, and L3 cache performance. a. <2.6> List the number of misses per 1K instruction of L1 data cache, L2, and L3 for each dataset size and your processor model and speed. Based on the results, what can you say about the L1 data cache, L2, and L3 cache sizes onyour processor? Explain your observations. B . <2.6> List the instructions per clock (IPC) for each dataset size and your processor model and speed. Based on the results, what can you say about the L1, L2, and L3 miss penalties on your processor? Explain your observations. c. <2.6> Run the program in VTune with input dataset size of 8KB and 128KB on an Intel OOO processor. List the number of L1 data cache and L2 cache misses per 1K instructions and the CPI for both configurations. Whatcan you say about the effectiveness of memory latency hiding techniques in high-performance OOO processors? Hint: You need to find the L1 data cache miss latency for your processor. For recent Intel i7 processors, it is approxi- mately 11 cycles.
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