linux-kernel - Re: [PATCH] oom_kill: use rss value instead of vm size for badness

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Date:	Mon, 30 Nov 2009 15:09:44 -0800 (PST)
From:	David Rientjes <rientjes@...gle.com>
To:	Andrea Arcangeli <aarcange@...hat.com>
cc:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	linux-mm@...ck.org, linux-kernel@...r.kernel.org,
	Andrew Morton <akpm@...ux-foundation.org>,
	Hugh Dickins <hugh.dickins@...cali.co.uk>,
	vedran.furac@...il.com,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>
Subject: Re: [PATCH] oom_kill: use rss value instead of vm size for badness

On Fri, 27 Nov 2009, Andrea Arcangeli wrote:

> Ok I can see the fact by being dynamic and less predictable worries
> you. The "second to last" tasks especially are going to be less
> predictable, but the memory hog would normally end up accounting for
> most of the memory and this should increase the badness delta between
> the offending tasks (or tasks) and the innocent stuff, so making it
> more reliable. The innocent stuff should be more and more paged out
> from ram. So I tend to think it'll be much less likely to kill an
> innocent task this way (as demonstrated in practice by your
> measurement too), but it's true there's no guarantee it'll always do
> the right thing, because it's a heuristic anyway, but even total_vm
> doesn't provide guarantee unless your workload is stationary and your
> badness scores are fixed and no virtual memory is ever allocated by
> any task in the system and no new task are spawned.
> 

The purpose of /proc/pid/oom_adj is not always to polarize the heuristic 
for the task it represents, it allows userspace to define when a task is 
rogue.  Working with total_vm as a baseline, it is simple to use the 
interface to tune the heuristic to prefer a certain task over another when 
its memory consumption goes beyond what is expected.  With this interface, 
I can easily define when an application should be oom killed because it is 
using far more memory than expected.  I can also disable oom killing 
completely for it, if necessary.  Unless you have a consistent baseline 
for all tasks, the adjustment wouldn't contextually make any sense.  Using 
rss does not allow users to statically define when a task is rogue and is 
dependent on the current state of memory at the time of oom.

I would support removing most of the other heuristics other than the 
baseline and the nodes intersection with mems_allowed to prefer tasks in 
the same cpuset, though, to make it easier to understand and tune.
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