linux-kernel - Re: [take3] OOM documentation update [was: Linux killed Kenny, bastard!]

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Date:	Wed, 14 Jan 2009 16:58:41 -0800 (PST)
From:	David Rientjes <rientjes@...gle.com>
To:	Evgeniy Polyakov <zbr@...emap.net>
cc:	linux-kernel@...r.kernel.org, Bryan Donlan <bdonlan@...il.com>,
	Balbir Singh <balbir@...ux.vnet.ibm.com>,
	Alan Cox <alan@...rguk.ukuu.org.uk>,
	Dave Jones <davej@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Theodore Tso <tytso@....edu>,
	Matthias Andree <matthias.andree@....de>,
	Randy Dunlap <randy.dunlap@...cle.com>
Subject: Re: [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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