2.8. Swapping and Out of Memory Tips

Memory Swapping

Swapping pages out to disk can introduce latency in any environment. To ensure low latency, the best strategy is to have enough memory in your systems so that swapping is not necessary. Always size the physical RAM as appropriate for your application and system. Use vmstat to monitor memory usage and watch the si (swap in) and so (swap out) fields. It is optimal that they remain zero as much as possible.

Procedure 2.8. Out of Memory (OOM)

Out of Memory (OOM) refers to a computing state where all available memory, including swap space, has been allocated. Normally this will cause the system to panic and stop functioning as expected. There is a switch that controls OOM behavior in /proc/sys/vm/panic_on_oom. When set to 1 the kernel will panic on OOM. The default setting is 0 which instructs the kernel to call a function named oom_killer on an OOM. Usually, oom_killer can kill rogue processes and the system will survive.
  1. The easiest way to change this is to echo the new value to /proc/sys/vm/panic_on_oom. 
    ~]# cat /proc/sys/vm/panic_on_oom
0

~]# echo 1 > /proc/sys/vm/panic_on_oom

~]# cat /proc/sys/vm/panic_on_oom
1

    Note

    It is recommended that you make the $RT; kernel panic on OOM. When the system has encountered an OOM state, it is no longer deterministic.
  2. It is also possible to prioritize which processes get killed by adjusting the oom_killer score. In /proc/PID/ there are two files named oom_adj and oom_score. Valid scores for oom_adj are in the range -16 to +15. This value is used to calculate the 'badness' of the process using an algorithm that also takes into account how long the process has been running, among other factors. To see the current oom_killer score, view the oom_score for the process. oom_killer will kill processes with the highest scores first.
    This example adjusts the oom_score of a process with a PID of 12465 to make it less likely that oom_killer will kill it. 
    ~]# cat /proc/12465/oom_score
79872

~]# echo -5 > /proc/12465/oom_adj

~]# cat /proc/12465/oom_score
78
  3. There is also a special value of -17, which disables oom_killer for that process. In the example below, oom_score returns a value of O, indicating that this process would not be killed. 
    ~]# cat /proc/12465/oom_score
78

~]# echo -17 > /proc/12465/oom_adj

~]# cat /proc/12465/oom_score
0
Related Manual Pages

For more information, or for further reading, the following man pages are related to the information given in this section.

  • swapon(2)
  • swapon(8)
  • vmstat(8)