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SubjectRe: [take3] OOM documentation update [was: Linux killed Kenny, bastard!]
On Thu, 15 Jan 2009, Evgeniy Polyakov wrote:

> diff --git a/Documentation/filesystems/proc.txt b/Documentation/filesystems/proc.txt
> index d105eb4..eed2fbb 100644
> --- a/Documentation/filesystems/proc.txt
> +++ b/Documentation/filesystems/proc.txt
> @@ -2311,6 +2311,32 @@ increase the likelihood of this process being killed by the oom-killer. Valid
> values are in the range -16 to +15, plus the special value -17, which disables
> oom-killing altogether for this process.
> +The process to be killed in an out-of-memory situation is selected among all others
> +based on its badness score. This value equals the original memory size of the process
> +and is then updated according to its CPU time (utime + stime) and the
> +run time (uptime - start time). The longer it runs the smaller is the score.
> +Badness score is divided by the square root of the CPU time and then by
> +the double square root of the run time.
> +
> +Swapped out tasks are killed first. Half of each child's memory size is added to
> +the parent's score if they do not share the same memory. Thus forking servers
> +are the prime candidates to be killed. Having only one 'hungry' child will make
> +parent less preferable than the child.
> +
> +/proc/<pid>/oom_score shows process' current badness score.
> +
> +The following heuristics are then applied:
> + * if the task was reniced, its score doubles
> + * superuser or direct hardware access tasks (CAP_SYS_ADMIN, CAP_SYS_RESOURCE
> + or CAP_SYS_RAWIO) have their score divided by 4
> + * if oom condition happened in one cpuset and checked task does not belong
> + to it, its score is divided by 8
> + * the resulting score is multiplied by two to the power of oom_adj, i.e.
> + points <<= oom_adj when it is positive and
> + points >>= -(oom_adj) otherwise
> +
> +The task with the highest badness score is then killed.
> +

Not quite, even after a task is selected for oom kill, the oom killer
still prefers to kill one of its children first if any have a different
mm. See oom_kill_process().

You also don't mention the exception of OOM_DISABLE (oom_adj score of -17)
in your formula for how oom_adj impacts the points value. Although its
already explained earlier, it should be mentioned here since a oom_adj is
an int and a right shift of 17 does not guarantee `points' will be 0.

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