How does the OOM killer decide which process to kill first?

This answer explains the actions taken by the kernel when an OOM situation is encountered based on the value of sysctl vm.overcommit_memory.

When overcommit_memory is set to 0 or 1, overcommit is enabled, and programs are allowed to allocate more memory than is really available.

Now what happens when we run out of memory in this situation? How does the OOM killer decide which process to kill first?

Asked By: Ramesh


If memory is exhaustively used up by processes, to the extent which can possibly threaten the stability of the system, then the OOM killer comes into the picture.

NOTE: It is the task of the OOM Killer to continue killing processes until enough memory is freed for the smooth functioning of the rest of the process that the Kernel is attempting to run.

The OOM Killer has to select the best process(es) to kill. Best here refers to that process which will free up the maximum memory upon killing and is also the least important to the system.

The primary goal is to kill the least number of processes that minimizes the damage done and at the same time maximizing the amount of memory freed.

To facilitate this, the kernel maintains an oom_score for each of the processes. You can see the oom_score of each of the processes in the /proc filesystem under the pid directory.

$ cat /proc/10292/oom_score

The higher the value of oom_score of any process, the higher is its likelihood of getting killed by the OOM Killer in an out-of-memory situation.

How is the OOM_Score calculated?

In David’s patch set, the old badness() heuristics are almost entirely
gone. Instead, the calculation turns into a simple question of what
percentage of the available memory is being used by the process. If
the system as a whole is short of memory, then "available memory" is
the sum of all RAM and swap space available to the system.

If instead, the OOM situation is caused by exhausting the memory allowed
to a given cpuset/control group, then "available memory" is the total
amount allocated to that control group. A similar calculation is made
if limits imposed by a memory policy have been exceeded. In each case,
the memory use of the process is deemed to be the sum of its resident
set (the number of RAM pages it is using) and its swap usage.

This calculation produces a percent-times-ten number as a result; a
process which is using every byte of the memory available to it will
have a score of 1000, while a process using no memory at all will get
a score of zero. There are very few heuristic tweaks to this score,
but the code does still subtract a small amount (30) from the score of
root-owned processes on the notion that they are slightly more
valuable than user-owned processes.

One other tweak which is applied is to add the value stored in each
process’s oom_score_adj variable, which can be adjusted via /proc.
This knob allows the adjustment of each process’s attractiveness to
the OOM killer in user space; setting it to -1000 will disable OOM
kills entirely, while setting to +1000 is the equivalent of painting a
large target on the associated process.


Answered By: Ramesh
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