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

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Date:	Thu, 11 Feb 2010 14:31:05 -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 13:51:36 -0800 (PST)
David Rientjes <rientjes@...gle.com> wrote:

> On Thu, 11 Feb 2010, Andrew Morton wrote:
> 
> > > 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.
> > 
> 
> Sigh, this is going to require the amount of system memory to be 
> partitioned into OOM_ADJUST_MAX, 15, chunks and that's going to be the 
> granularity at which we'll be able to either bias or discount memory usage 
> of individual tasks by: instead of being able to do this with 0.1% 
> granularity we'll now be limited to 100 / 15, or ~7%.  That's ~9GB on my 
> 128GB system just because this was originally a bitshift.  The upside is 
> that it's now linear and not exponential.

Can you add newly-named knobs (rather than modifying the existing
ones), deprecate the old ones and then massage writes to the old ones
so that they talk into the new framework?

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