linux-kernel - Re: [patch 4/7 -mm] oom: badness heuristic rewrite

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Message-Id: <20100211134343.4886499c.akpm@linux-foundation.org>
Date:	Thu, 11 Feb 2010 13:43:43 -0800
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	David Rientjes <rientjes@...gle.com>
Cc:	Rik van Riel <riel@...hat.com>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	Nick Piggin <npiggin@...e.de>,
	Andrea Arcangeli <aarcange@...hat.com>,
	Balbir Singh <balbir@...ux.vnet.ibm.com>,
	Lubos Lunak <l.lunak@...e.cz>, linux-kernel@...r.kernel.org,
	linux-mm@...ck.org
Subject: Re: [patch 4/7 -mm] oom: badness heuristic rewrite

On Thu, 11 Feb 2010 01:14:43 -0800 (PST)
David Rientjes <rientjes@...gle.com> wrote:

> On Wed, 10 Feb 2010, Rik van Riel wrote:
> 
> > > OOM_ADJUST_MIN and OOM_ADJUST_MAX have been exported to userspace since
> > > 2006 via include/linux/oom.h.  This alters their values from -16 to -1000
> > > and from +15 to +1000, respectively.
> > 
> > That seems like a bad idea.  Google may have the luxury of
> > being able to recompile all its in-house applications, but
> > this will not be true for many other users of /proc/<pid>/oom_adj
> > 
> 
> Changing any value that may have a tendency to be hardcoded elsewhere is 
> always controversial, but I think the nature of /proc/pid/oom_adj allows 
> us to do so for two specific reasons:
> 
>  - hardcoded values tend not the fall within a range, they tend to either
>    always prefer a certain task for oom kill first or disable oom killing
>    entirely.  The current implementation uses this as a bitshift on a
>    seemingly unpredictable and unscientific heuristic that is very 
>    difficult to predict at runtime.  This means that fewer and fewer
>    applications would hardcode a value of '8', for example, because its 
>    semantics depends entirely on RAM capacity of the system to begin with
>    since badness() scores are only useful when used in comparison with
>    other tasks.

You'd be amazed what dumb things applications do.  Get thee to
http://google.com/codesearch?hl=en&lr=&q=[^a-z]oom_adj[^a-z]&sbtn=Search
and start reading.  All 641 matches ;)

Here's one which which writes -16:
http://google.com/codesearch/p?hl=en#eN5TNOm7KtI/trunk/wlan/vendor/asus/eeepc/init.rc&q=[^a-z]oom_adj[^a-z]&sa=N&cd=70&ct=rc

Let's not change the ABI please.

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