linux-kernel - Re: Memory overcommit

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Date:	Mon, 02 Nov 2009 20:58:50 +0100
From:	Vedran Furač <vedran.furac@...il.com>
To:	David Rientjes <rientjes@...gle.com>
CC:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	Hugh Dickins <hugh.dickins@...cali.co.uk>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	minchan.kim@...il.com, Andrew Morton <akpm@...ux-foundation.org>,
	Andrea Arcangeli <aarcange@...hat.com>
Subject: Re: Memory overcommit

David Rientjes wrote:

> On Fri, 30 Oct 2009, Vedran Furac wrote:
> 
>>> The problem you identified in http://pastebin.com/f3f9674a0, however, is a 
>>> forkbomb issue where the badness score should never have been so high for 
>>> kdeinit4 compared to "test".  That's directly proportional to adding the 
>>> scores of all disjoint child total_vm values into the badness score for 
>>> the parent and then killing the children instead.
>> Could you explain me why ntpd invoked oom killer? Its parent is init. Or
>> syslog-ng?
>>
> 
> Because it attempted an order-0 GFP_USER allocation and direct reclaim 
> could not free any pages.
> 
> The task that invoked the oom killer is simply the unlucky task that tried 
> an allocation that couldn't be satisified through direct reclaim.  It's 
> usually unrelated to the task chosen for kill unless 
> /proc/sys/vm/oom_kill_allocating_task is enabled (which SGI requested to 
> avoid excessively long tasklist scans).

Oh, well, I didn't know that. Maybe rephrasing of that part of the
output would help eliminating future misinterpretation.

>> OK then, if you have a solution, I would be glad to test your patch. I
>> won't care much if you don't change total_vm as a baseline. Just make
>> random killing history.
> 
> The only randomness is in selecting a task that has a different mm from 
> the parent in the order of its child list.  Yes, that can be addressed by 
> doing a smarter iteration through the children before killing one of them.
> 
> Keep in mind that a heuristic as simple as this:
> 
>  - kill the task that was started most recently by the same uid, or
> 
>  - kill the task that was started most recently on the system if a root
>    task calls the oom killer,
> 
> would have yielded perfect results for your testcase but isn't necessarily 
> something that we'd ever want to see.

Of course, I want algorithm that works well in all possible situations.

Regards,

Vedran

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