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Understanding Data Caching

Caching is an efficient way to capture interactions between your application and the data storage location.

Understanding_Data_Caching

Introduction

Caching is an efficient and easy way to capture interactions between your application and the data storage location. To accomplish this effectively, you need to understand the various implementations of cache and their effects on your application. In this article, we will be focusing on the various usage scenarios of data caching without discussing the details of the technical implementation.

Data Caching Features and Application Scenarios

All businesses rely on data, but the relevance of different types of data varies by industry. This blog emphasizes the importance of increasing post-classification data in proportion with business needs.

When data size reaches a certain degree of magnitude, developers must consider how to quickly retrieve the user-required data while minimizing the time taken to produce an output.

Data access optimization observes a law similar to the funnel law, which entails:

  • Reducing data access
  • Returning less data
  • Reducing interactions with the under layer
  • Reducing Central Processing Unit (CPU) overhead and using more machine resources

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Data caching is introduced when all existing optimization tools reach a bottleneck and are still unsuccessful in meeting the current business needs. A point worth mentioning here is that the use of caching is closer to the "reduced data access" layer.

Advantages and Disadvantages of Caching

Two types of cache exist in Java development: local cache and cluster cache.

Local cache

Advantages:

  • Data is directly taken from Java Virtual Machine (JVM)
  • Access speed is very fast
  • Uses very low threshold
  • Well-developed plug-ins like ehcache, oscache, and abd guava cache are easily available in the market. (The ehcache and oscache already provide the cluster cache.)

Disadvantages:

  • Inconsistency and disparity problems may occur between different machines in the server cluster
  • When the local cache is too large, it is easy to trigger Garbage Collection (GC) of servers

Cluster cache

Advantages:

  • The data consistency is guaranteed, and the capacity expansion is convenient, transparent, and unperceivable to users.
  • It engages more mature middleware tools like Tair, Redis, and Memcached.

Disadvantages:

  • You need to select mature and business-complying cluster caching middleware and cluster framework to establish and maintain a set of cache cluster servers
  • The threshold for use is high, and cache hotspot issues may occur, like the Tair hotspot issues.

Cache Usage Methods

Cache usage varies for different data sizes and scenarios. Three usage methods exist, which are as follows:

  • Passive cache processing: In this approach, results return immediately if the query hits a record. If the query does not hit a record, it queries the underlying cached result sets and returns the result to the user
  • Full data cache warming or key data cache warming: In this method, results are already available, and the query is run against the data source. The results are retrieved from the cache with no need for recalculation
  • Implementation of Sentinel Caching: Sentinel caching uses multiple integrated layers to retrieve data
    Passive cache processing is the simplest to implement and suitable for scenarios with a small data size. Full data cache warming requires a sound design and is suitable for hotspot access scenarios with a medium data size. Sentinel caching integrates various layers, but the all-round application involves complex technologies.

Local Cache Bottleneck

The decision to use a local cache or cluster cache does not restrict the cache usage methods. While extensions are available, this article does not focus on the cluster cache implementation but on the bottleneck that local cache usage causes. In most cases, a local cache of JVM refers to heap memory.

The typical configuration of Alibaba Cloud's standard application servers is comprised of stand-alone virtual 4-core CPUs with 8GB memory. This configuration allocates the 8GB between two parts: JVM and daily overhead of servers. The JVM memory consists of the following:

  • 2GB for the young generation, 2GB for the old generation, maximum 1GB for off-heap memory
  • Several hundred megabytes for persistent generation PermSize

The memory for the young generation is subdivided, based on a 10:1:1 ratio, between Eden, so1, and so2, with about 170MB dedicated to each.

You might wonder, what is the significance of such a division? What happens if we allot 3GB to the young generation? What happens if we adjust the allocated size for the young generation, or change the ratio of the subdivision?

One of the biggest differences between Java development and C development is that Java developers avoid addressing memory application and release, which are managed dynamically by JVM's garbage collection mechanism. When the JVM heap memory usage reaches a certain percentage, it will trigger the Stop-The-World (STW) GC. JVM's GC is not perceivable to the application. When a user requests data and the query hits a record in the cache, but GC activates in the JVM, the entire application will freeze, and the application holds the user's request until the GC completes. The GC may not exceed the maximum timeout value of the request.

This is the first influence of JVM heap on local cache usage. Some have suggested that we can extend the GC cycle by increasing the JVM heap size. However, we must also consider the impact of the ratio of memory size allocated to the JVM young generation in local memory.

Currently, the JVM of Taobao servers adopts the Concurrent Mark-Sweep (CMS) mechanism for GC.

The corresponding collection mechanism of the young generation is tag-copy.

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The corresponding collection mechanism of the old generation is tag-clear (or tag-trimming).

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The benefit of using the "tag-copy" algorithm on the young generation is that the majority of the young generation's data survival cycles are not lengthy. Similar to the temporary variables in the method body, such data is discarded immediately after use. Few young-generation data objects survive after the YGC (GC events in young area), and the cost for transferring these surviving objects to another is minimal. Therefore, the "tag-copy" algorithm is used to the maximum effect, and the impact to the user is minimal.

Compared with YGC, developers pay more attention to FGC (number of full GC events occurred) because FGC's suspension duration is longer than YGC's, as shown in the figure below.

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This is a comparison between FGC & YGC time consumption.

In general, YGC takes about 10ms to 200ms, while FGC may take several seconds, under normal circumstances. When the memory size allocated for the young generation is in accordance with the ratio of 10:1:1, the maximum size of an So is 170MB. After the collection, the size of the remaining objects copied from so1 to so2 is smaller than 170MB or a few megabytes. What will happen if we adjust the ratio of Eden and So, downsizing Eden and enlarging So, to the ratio 1:1:1?

In that case, the maximum size of So will be 682MB. Thus, more objects may survive after the object collection, and the time consumption for copying objects from so1 to so2 will also increase. In severe cases, the time may exceed several seconds, or even exceed that by FGC.

If you use a USB drive to copy large files, you must have noticed that the copying speed from memory is much higher than that from a hard disk. Copying files from memory only takes several minutes, but copying files from a hard disk takes a longer time. The time elapsed for copying several gigabytes of files from the USB drive to the hard disk is similar.

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This is the YGC impact caused by a similar problem and the time consumption level reached by FGC.

If we minimize the memory distribution ratio of the young generation, the space in so2 may not be enough after copying objects from so1. Also, objects may enter the old generation prematurely. This will trigger FGC too early and large objects will lead to more memory fragments for the old generation. These fragments cannot be effectively used, resulting in early FGC. Therefore, the ratio setting is an issue of data balance. Through numerous experiments and verifications, engineers at Alibaba indicate that the 10:1:1 ratio is the optimal setting.

The ratio settings of the young generation will influence the local cache access. Also, improper use of the local cache will impact the JVM. This is known as the large object. For example, declaring a List attribute in a class to store several million bytes of data will trigger vicious GC if the List occupies 1GB to 2GB of the overall heap memory.

Generally, the object may occupy tens to hundreds of megabytes. Since the List serves as the local cache, the survival cycle of the young generation is longer. If the size of the List does not exceed the size of So, JVM will not throw the object to the old generation in advance. As a result, the So copying process repeats multiple times during the collection of the young generation (15 times by default). Returning to the USB drive example, we can see that the repeated copying operations produce bad results, even if you downsize or enlarge the ratio of Eden and So.

To summarize:

  • Improper young generation ratio settings prevent the YGC from functioning "short, smooth and fast," and the "tag-copy" algorithm becomes a hindrance.
  • The "tag-copy" algorithm also becomes cumbersome when local cache usage is too high and too much heap memory is unavailable. Also, when the local cache is used improperly leading to a large object resulting in a compromise of the YGC.
  • Developers often encounter the JVM GC bottleneck at the end of code writing. This causes confrontation and compromise with JVM garbage collection. Workarounds include writing temporary variables of a List and inserting objects into the List. Splitting large objects means investing more designing effort and writing a lot of business logic-independent code.

Problems with Using Multiple Threads

Why does the improper use of multiple threads lead to GC problems?

Java can use Xmx and Xms to set heap memory size. In a broad sense, the off-heap memory refers to the VM memory that remains after the Java heap and permanent generation memories are removed, including:

  • Direct memory: Set through -XX: MaxDirectMemorySize
  • Thread stack: Set through -Xss
  • Socket buffer cache
  • Java Native Interface (JNI) code
  • Virtual machine and GC: The execution of virtual machine and GC code also consumes part of the memory

Allocation of threads in the off-heap memory occurs by default. Xss is also known as ThreadStackSize (thread stack space). The default size of Xss of JDK1.4 is 256KB, and the default size of Xss of JDK1.5+ is 1 MB.

When virtual machine memory is limited, larger heap memory leaves less space for off-heap memory. Off-heap memory size also limits the number of threads. When the number of threads exceeds the off-heap memory size, System.gc () warns JVM to perform GC. If the -XX: DisableExplicitGC parameter appears in JVM, the role of this parameter is equivalent to invalidating "System.gc."

In this case, the off-heap memory can only watch itself blow up and then throw StackOverFlowError or OutOfMemoryError: unable to create new native thread. In general, Taobao servers will use -XX: ExplicitGCInvokesConcurrent in place of DisableExplicitGC to turn the original FGC into the concurrent GC of the CMS.

The improper use of threads leads to frequent GC of applications. Theoretically, you can set a smaller Xss or reduce the heap memory size to increase the number of threads, depending on the circumstances.

You can specify the -XX: + UseTLAB parameter to set allocating threads directly in the heap memory. The Thread Local Allocation Buffer (TLAB) is used to allocate the buffer for local threads. This parameter will directly assign a space in the heap memory.

Preliminary Study on Off-Heap Memory

Remember the following key points when using local cache:

  • Avoid the generation of large objects and split large objects as much as possible
  • Avoid excessive local cache
  • Adjust JVM configuration as appropriate to meet the current project characteristics

While important, these points are not always easy to follow. Splitting large objects is not a simple matter in many business processes because it involves dynamic resizing for the splitting. Additionally, you will need to invest more on design for the same type of data, which increases structural complexity.

There is no getting around JVM for development. Some developers pin their eyes on the non-heap memory of JVM. Non-heap memory does not have the garbage collection mechanism and the data is usually collected at a GC together. Thus, storing the data in non-heap memory seems to be a good way to bypass the JVM-GC without increasing design complexity.

While studying the mature usage practices of non-heap memory in the market, many mature products can be found. However, none of them have seamless code access, long-term experiments, or even multi-party cooperation. These are generally required to achieve a satisfactory result. Some mature middleware products of Java non-heap memory are listed below.

  • MapDB: It provides mature APIs, and its data operations reach the atomic level. It also enables quasi-SQL committing and data rollback. However, MapDB has its own set of disadvantages. Data cannot be resident in the memory in the form of ordinary maps. You need to connect to the non-heap memory every time you want to get the data from the non-heap memory and must commit it to make the data valid. In addition, its reading efficiency has no advantage over ordinary concurrentHashMap (unless the heap GC has seriously slowed down the HashMap reading).
  • Imcache: It supports n-level caching hierarchy where it supports various caching methods like heap, offheap, and more. Imcache also supports the well-known caching solution - Redis.
  • SharedHashMap: SharedHashMap is targeted at high-performance off-heap memory storage of entries for low latency applications, to avoid GC overheads on that data
  • EHcache: Only the enterprise edition supports non-heap memory
  • BigMemory: The free edition has a limited service life. The enterprise edition has mature applications, but it is not free.

All of the above non-heap memories have the following limitations:

  • The required size of non-heap memory is large, usually above 4GB.
  • When data storage is in the non-heap memory, although avoidance of GC's impact is possible, developers need to focus on non-heap memory data overflow problems.
  • When data storage is in the non-heap memory, and there is an access request from the client, the data needs to be retrieved from the non-heap memory to the heap memory, and then returned to the client. The process of retrieving the data still needs to take up a portion of the heap memory.
  • A majority of non-heap memories require the application to have the read and write permissions to a server file directory since the file houses the non-heap memory.

While the above limitations stand true, non-heap memory is the future of the local cache system.

Summary

This blog focuses on the concept of caching and its applications. Two types of cache – local and cluster – were discussed along with their advantages and disadvantages. This article also sheds light on GC and large local caching, as well as the potential of non-heap memory.

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