Memcg global minimum watermark rating - Images| Alibaba Cloud Documentation Center

Background information

Global memory reclaim has a great impact on system performance in Linux kernels. When latency-sensitive tasks and resource-consuming tasks are deployed together, resource-consuming tasks request a large amount of memory instantly, which causes the free memory of the system to reach the global minimum watermark (global wmark_min). As a result, direct memory reclaim is enabled for all system tasks, which causes performance jitters for latency-sensitive tasks. Both global kswapd backend reclaim and memcg backend reclaim cannot resolve this problem.

In this case, Alibaba Cloud Linux 2 provides the memcg global minimum watermark rating feature. The global minimum watermark of resource-consuming tasks is increased to trigger direct memory reclaim. The global minimum watermark of latency-sensitive tasks is decreased to avoid direct memory reclaim. When a resource-consuming task requests a large amount of memory, an increase in the global minimum watermark throttles the resources used by the task for a short period to avoid direct memory reclaim for latency-sensitive tasks. After a certain amount of memory is reclaimed through global kswapd backend reclaim, the resource-consuming task is not throttled.

Interface description

The interface that implements the memcg global minimum watermark rating feature is memory.wmark_min_adj. The value of this interface indicates the percent of adjustment over the global minimum watermark. Valid values: -25 to 50.

This interface inherits a value of 0 from its parent group when the interface is created. Therefore, the default value is 0.
A negative value is a percent of adjustment over the [0, WMARK_MIN] range, where WMARK_MIN is the value of global wmark_min. Example:
```
memory.wmark_min_adj=-25, memcg WMARK_MIN is "WMARK_MIN + (WMARK_MIN - 0) * (-25%)"
```
Note A negative value also indicates a decrease of the global minimum watermark. This can increase the memcg quality of service (QoS) of latency-sensitive tasks.
A positive value is a percent of adjustment over the [WMARK_MIN, WMARK_LOW] range, where WMARK_MIN and WMARK_LOW are values of global wmark_min and global wmark_low. Example:
```
memory.wmark_min_adj=50, memcg WMARK_MIN is "WMARK_MIN + (WMARK_LOW - WMARK_MIN) * 50%"
```
Note A positive value also indicates an increase of the global minimum watermark. This can decrease the memcg QoS of resource-consuming tasks.
When the offset global minimum watermark is triggered, throttling is performed, and the throttling time is linearly proportional to the excessive memory usage. Valid values of the throttling time: 1 to 1000. Unit: ms.

Note This interface file is not stored in the memcg root directory.

Precautions

A multi-level memcg contains effective memory.wmark_min_adj, which is the final effective value of memory.wmark_min_adj. The values of memory.wmark_min_adj at all levels are traversed to obtain the maximum value. Intermediate nodes that have 0 as the default value are excluded. The following hierarchy provides an example:

The following table describes the mapping between the values of memory.wmark_min_adj at each level and the final effective value.


Level	Value at each level	Final effective value
A	-10	-10
B	-25	-10
C	0	0
D	50	50
E	-25	-10
F	50	50

Note

The value displayed in the output of the cat /sys/fs/cgroup/memory/<Memcg path>/memory.wmark_min_adj command is the final effective value. In the command, <Memcg path> indicates the root path of the memcg.
We recommend that you use the global minimum watermark rating feature together with global wmark_min. For example, you can set global wmark_min to 2 GB or more in /proc/sys/vm/min_free_kbytes.

Configuration examples

Example 1: Configure global minimum watermark rating for the memcg of latency-sensitive tasks.

Run the mkdir /sys/fs/cgroup/memory/test-lc command to create a test file.
Run the echo -25 > /sys/fs/cgroup/memory/test-lc/memory.wmark_min_adj command to write the value of -25 to the interface to increase the memcg QoS of latency-sensitive tasks.

Example 2: Configure global minimum watermark rating for the memcg of resource-consuming tasks.

Run the mkdir /sys/fs/cgroup/memory/test-be command to create a test file.
Run the echo 25 > /sys/fs/cgroup/memory/test-be/memory.wmark_min_adj command to write the value of 25 to the interface to decrease the memcg QoS of resource-consuming tasks.