linux-kernel - Re: [patch] mm: oom_kill: revert 3% system memory bonus for privileged tasks

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Message-ID: <20140116070709.GM6963@cmpxchg.org>
Date:	Thu, 16 Jan 2014 02:07:09 -0500
From:	Johannes Weiner <hannes@...xchg.org>
To:	David Rientjes <rientjes@...gle.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Michal Hocko <mhocko@...e.cz>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org
Subject: Re: [patch] mm: oom_kill: revert 3% system memory bonus for
 privileged tasks

On Wed, Jan 15, 2014 at 04:18:47PM -0800, David Rientjes wrote:
> On Wed, 15 Jan 2014, Johannes Weiner wrote:
> 
> > With a63d83f427fb ("oom: badness heuristic rewrite"), the OOM killer
> > tries to avoid killing privileged tasks by subtracting 3% of overall
> > memory (system or cgroup) from their per-task consumption.  But as a
> > result, all root tasks that consume less than 3% of overall memory are
> > considered equal, and so it only takes 33+ privileged tasks pushing
> > the system out of memory for the OOM killer to do something stupid and
> > kill sshd or dhclient.  For example, on a 32G machine it can't tell
> > the difference between the 1M agetty and the 10G fork bomb member.
> > 
> > The changelog describes this 3% boost as the equivalent to the global
> > overcommit limit being 3% higher for privileged tasks, but this is not
> > the same as discounting 3% of overall memory from _every privileged
> > task individually_ during OOM selection.
> > 
> > Revert back to the old priority boost of pretending root tasks are
> > only a quarter of their actual size.
> > 
> 
> Unfortunately, I think this could potentially be too much of a bonus.  On 
> your same 32GB machine, if a root process is using 18GB and a user process 
> is using 14GB, the user process ends up getting selected while the current 
> discount of 3% still selects the root process.
> 
> I do like the idea of scaling this bonus depending on points, however.  I 
> think it would be better if we could scale the discount but also limit it 
> to some sane value.

I just reverted to the /= 4 because we had that for a long time and it
seemed to work.  I don't really mind either way as long as we get rid
of that -3%.  Do you have a suggestion?
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