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Updated on 2024-07-02 GMT+08:00
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What Is OOM? Why Does OOM Occur?

OOM Concepts

Out of Memory (OOM) occurs when all available memory is exhausted and the system is unable to allocate memory for processes, which will trigger a kernel panic or OOM killer.

On Linux, OOM killer is a process that prevents other processes from collectively exhausting the host's memory. When the system is critically low on memory, the processes that use more memory than available will be killed to ensure the overall availability of the system.

OOM Parameters

Table 1 OOM parameters

Parameter

Description

Value

How to Change

panic_on_oom

Controls how the system reacts when OOM occurs. When OOM occurs, the system has two options:

  • Triggers a kernel panic, during which the system may break down frequently.
  • Selects one or more processes and triggers OOM killer to end the selected processes and to release the memory so that the system can be used normally.

You can run either of the following commands to view the parameter value:

cat /proc/sys/vm/panic_on_oom

sysctl -a | grep panic_on_oom

  • If the value is set to 0, OOM killer is triggered when the memory is insufficient.
  • If the value is set to 1, either OOM killer or kernel panic will be triggered.
  • If the value is set to 2, a kernel panic will be forcibly triggered. As a result, the system restarts.
    NOTE:

    The default value of this parameter in Huawei Cloud EulerOS is 1.

For example, to set the value to 0, use either of the following methods:

  • Temporary configuration: The configuration takes effect immediately. However, after the system is restarted, the value changes to the default one.

    sysctl -w vm.panic_on_oom=0

  • Persistence configuration: The configuration still takes effect after the system is restarted.

    Run vim /etc/sysctl.conf, add vm.panic_on_oom =0 to the configuration file, and then run sysctl –p or restart the system for the configuration to take effect.

oom_kill_allocating_task

Determines which processes are selected when the system triggers OOM killer and attempts to end some processes. The options are as follows:

  • Process that triggers OOM
  • Process with the highest oom_score value

You can run either of the following commands to view the parameter value:

cat /proc/sys/vm/oom_kill_allocating_task

sysctl -a | grep oom_kill_allocating_task

  • If the value is set to 0, the process with the highest oom_score value is selected.
  • If the value is a non-zero value, the process that triggers OOM is selected.
NOTE:

The default value of this parameter in Huawei Cloud EulerOS is 0.

For example, to set the value to 1, use either of the following methods:

  • Temporary configuration: The configuration takes effect immediately. However, after the system is restarted, the value changes to the default one.

    sysctl -w vm.oom_kill_allocating_task=1

  • Persistence configuration: The configuration still takes effect after the system is restarted.

    Run vim /etc/sysctl.conf, add vm.oom_kill_allocating_task=1 to the configuration file, and then run sysctl –p or restart the system for the configuration to take effect.

oom_score

Indicates the score of a process, which consists of two parts:

  • System score: The system automatically calculates the score based on the memory usage of the process.
  • User score: This is the oom_score_adj score, which can be customized.

You can adjust the value of oom_score_adj to adjust the final score of a process. You can run the following command to view the parameter value:

cat /proc/Process ID/oom_score_adj

  • If the value is set to 0, the oom_score value is not adjusted.
  • If the value is negative, the probability of the process getting picked and terminated by OOM killer is reduced.
  • If the value is positive, the probability of the process getting picked and terminated by OOM killer is increased.
NOTE:

The value of oom_score_adj ranges from -1000 to 1000.

If this parameter is set to OOM_SCORE_ADJ_MIN or -1000, OOM killer is not allowed to end the process.

For example, to set oom_score_adj to 1000 for the process whose ID is 2939, run the following command:

echo 1000 > /proc/2939/oom_score_adj

oom_dump_tasks

Specifies whether to record the system process information and OOM killer information when OOM occurs, for example, dump information about all processes in the system like the process ID, memory used by the process, and page table information of the process. Such information helps you understand the cause of OOM.

You can run either of the following commands to view the parameter value:

cat /proc/sys/vm/oom_dump_tasks

sysctl -a | grep oom_dump_tasks

  • If the value is set to 0, related information is not printed when OOM occurs.
  • If the value is not 0, a system-wide task dump is produced to print the memory usage of all tasks in the system in the following scenarios:
    • A kernel panic is triggered due to OOM.
    • The process to be terminated is not found.
    • The process is found and terminated.
NOTE:

The default value of this parameter in Huawei Cloud EulerOS is 1.

For example, to set the value to 0, use either of the following methods:

  • Temporary configuration: The configuration takes effect immediately. However, after the system is restarted, the value changes to the default one.

    sysctl –w vm.oom_dump_tasks=0

  • Persistence configuration: The configuration still takes effect after the system is restarted.

    Run vim /etc/sysctl.conf, add vm.oom_dump_tasks=0 to the configuration file, and then run sysctl –p or restart the system for the configuration to take effect.

Example of OOM Killer

  1. Set Huawei Cloud EulerOS system parameters by referring to Table 1. The following is an example configuration: 
    [root@localhost ~]# cat /proc/sys/vm/panic_on_oom
0
[root@localhost ~]# cat /proc/sys/vm/oom_kill_allocating_task
0
[root@localhost ~]# cat /proc/sys/vm/oom_dump_tasks
1
    • panic_on_oom=0 indicates that OOM killer is triggered when OOM occurs.
    • oom_kill_allocating_task=0 indicates that the process with the highest oom_score value is preferentially terminated when OOM killer is triggered.
    • oom_dump_tasks=1 indicates that the process and OOM killer information is recorded when OOM occurs.
  2. Start the process.

    Start three same test processes (test, test1, and test2) in the system at the same time, continuously request new memory for the three processes, and set oom_score_adj of process test1 to 1000 (indicating that OOM killer will terminate this process first), until the memory is used up and OOM occurs.

    [root@localhost ~]# ps -ef | grep test
root        2938    2783  0 19:08 pts/2    00:00:00 ./test
root        2939    2822  0 19:08 pts/3    00:00:00 ./test1
root        2940    2918  0 19:08 pts/5    00:00:00 ./test2
[root@localhost ~]# echo 1000 > /proc/2939/oom_score_adj
[root@localhost ~]# cat /proc/2939/oom_score_adj
1000
  3. View the OOM information.

    After a period of time, OOM occurs in the system, and OOM killer is triggered. At the same time, the memory information of all processes in the system is printed in /var/log/messages, and process test1 is terminated.

Possible Causes

  • The cgroup memory is insufficient.

    The memory exceeds the value of memory.limit_in_bytes in cgroup. Suppose memory.limit_in_bytes is set to 80 MB and 100 MB of memory is allocated to memhog. As shown in the logs (stored in the /var/log/messages directory), the memhog process (PID: 2021820) uses 81,920 KB of memory, which exceeds the memory specified by memory.limit_in_bytes and triggers OOM.

    warning|kernel[-]|[2919920.414131] memhog invoked oom-killer: gfp_mask=0xcc0(GFP_KERNEL), order=0, oom_score_adj=0
info|kernel[-]|[2919920.414220] memory: usage 81920kB, limit 81920kB, failcnt 30
err|kernel[-]|[2919920.414272] Memory cgroup out of memory: Killed process 2021820 (memhog) total-vm:105048kB, anon-rss:81884kB, file-rss:1544kB, shmem-rss:0kB, UID:0 pgtables:208kB oom_score_adj:0
  • The parent cgroup memory is insufficient.

    The memory of child cgroups is sufficient, but the memory of the parent cgroup is insufficient and exceeds the memory limit. In the following example, memory.limit_in_bytes is set to 80 MB for the parent cgroup and to 50 MB for the two child cgroups, respectively. A program is used to cyclically allocate memory in the two child cgroups to trigger OOM. Some logs in /var/log/messages are as follows:

    warning|kernel[-]|[2925796.529231] main invoked oom-killer: gfp_mask=0xcc0(GFP_KERNEL), order=0, oom_score_adj=0
info|kernel[-]|[2925796.529315] memory: usage 81920kB, limit 81920kB, failcnt 199
err|kernel[-]|[2925796.529366] Memory cgroup out of memory: Killed process 3238866 (main) total-vm:46792kB, anon-rss:44148kB, file-rss:1264kB, shmem-rss:0kB, UID:0 pgtables:124kB oom_score_adj:0
  • The system memory is insufficient.

    The free memory of the OS is insufficient, and some programs keep requesting memory. Even some memory can be reclaimed, the memory is still insufficient, and OOM is triggered. In the following example, a program is used to cyclically allocate memory in the OS to trigger OOM. The logs in /var/log/messages show that the free memory of Node 0 is lower than the minimum memory (the value of low), triggering OOM.

    kernel: [ 1475.869152] main invoked oom: gfp_mask=0x100dca(GFP_HIGHUSER_MOVABLE|__GFP_ZERO), order=0
kernel: [ 1477.959960] Node 0 DMA32 free:22324kB min:44676kB low:55844kB high:67012kB reserved_highatomic:0KB active_anon:174212kB inactive_anon:1539340kB active_file:0kB inactive_file:64kB unevictable:0kB writepending:0kB present:2080636kB managed:1840628kB mlocked:0kB pagetables:7536kB bounce:0kB free_pcp:0kB local_pcp:0kB free_cma:0kB
kernel: [ 1477.960064] oom-kill:constraint=CONSTRAINT_NONE,nodemask=(null),cpuset=/,mems_allowed=0,global_oom,task_memcg=/system.slice/sshd.service,task=main,pid=1822,uid=0
kernel: [ 1477.960084] Out of memory: Killed process 1822 (main) total-vm:742748kB, anon-rss:397884kB, file-rss:4kB, shmem-rss:0kB, UID:0 pgtables:1492kB oom_score_adj:1000
  • The memory of the memory nodes is insufficient.

    In a NUMA system, an OS has multiple memory nodes. If a program uses up the memory of a specific memory node, OOM may be triggered even when the OS memory is sufficient. In the following example, there are two memory nodes, and a program is used to cyclically allocate memory on Node 1. As a result, the memory of Node 1 is insufficient, but the OS memory is sufficient. Some logs in /var/log/messages are as follows:

    kernel: [  465.863160] main invoked oom: gfp_mask=0x100dca(GFP_HIGHUSER_MOVABLE|__GFP_ZERO), order=0
kernel: [  465.878286] active_anon:218 inactive_anon:202527 isolated_anon:0#012 active_file:5979 inactive_file:5231 isolated_file:0#012 unevictable:0 dirty:0 writeback:0#012 slab_reclaimable:6164 slab_unreclaimable:9671#012 mapped:4663 shmem:2556 pagetables:846 bounce:0#012 free:226231 free_pcp:36 free_cma:0
kernel: [  465.878292] Node 1 DMA32 free:34068kB min:32016kB low:40020kB high:48024kB reserved_highatomic:0KB active_anon:188kB inactive_anon:778076kB active_file:20kB inactive_file:40kB unevictable:0kB writepending:0kB present:1048444kB managed:866920kB mlocked:0kB pagetables:2752kB bounce:0kB free_pcp:144kB local_pcp:0kB free_cma:0kB
kernel: [  933.264779] oom-kill:constraint=CONSTRAINT_MEMORY_POLICY,nodemask=1,cpuset=/,mems_allowed=0-1,global_oom,task_memcg=/system.slice/sshd.service,task=main,pid=1733,uid=0
kernel: [  465.878438] Out of memory: Killed process 1734 (main) total-vm:239028kB, anon-rss:236300kB, file-rss:200kB, shmem-rss:0kB, UID:0 pgtables:504kB oom_score_adj:1000
  • Other possible cause

    During memory allocation, if the memory of the buddy system is insufficient, OOM killer is triggered to release the memory to the buddy system.

Solutions

  • Check if there is memory leak, which causes OOM.
  • Check whether the cgroup limit_in_bytes configuration matches the memory plan. If any modification is required, run the following command: 
    echo <value> > /sys/fs/cgroup/memory/<cgroup_name>/memory.limit_in_bytes
  • If more memory is required, upgrade the ECS flavors.