linux-kernel - Re: [PATCH] mm: memcg: fix over reclaiming mem cgroup

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Message-ID: <20120124082352.GA26289@tiehlicka.suse.cz>
Date:	Tue, 24 Jan 2012 09:23:53 +0100
From:	Michal Hocko <mhocko@...e.cz>
To:	Hillf Danton <dhillf@...il.com>
Cc:	Ying Han <yinghan@...gle.com>, linux-mm@...ck.org,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	Hugh Dickins <hughd@...gle.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Johannes Weiner <hannes@...xchg.org>
Subject: Re: [PATCH] mm: memcg: fix over reclaiming mem cgroup

On Tue 24-01-12 11:26:05, Hillf Danton wrote:
> Hi all
> 
> On Tue, Jan 24, 2012 at 3:14 AM, Ying Han <yinghan@...gle.com> wrote:
> > On Mon, Jan 23, 2012 at 5:02 AM, Michal Hocko <mhocko@...e.cz> wrote:
> >> On Sat 21-01-12 22:49:23, Hillf Danton wrote:
> >>> In soft limit reclaim, overreclaim occurs when pages are reclaimed from mem
> >>> group that is under its soft limit, or when more pages are reclaimd than the
> >>> exceeding amount, then performance of reclaimee goes down accordingly.
> >>
> >> First of all soft reclaim is more a help for the global memory pressure
> >> balancing rather than any guarantee about how much we reclaim for the
> >> group.
> >> We need to do more changes in order to make it a guarantee.
> >> For example you implementation will cause severe problems when all
> >> cgroups are soft unlimited (default conf.) or when nobody is above the
> >> limit but the total consumption triggers the global reclaim. Therefore
> >> nobody is in excess and you would skip all groups and only bang on the
> >> root memcg.
> 
> If soft limits are set to be limited and there are no excessors, who
> are consuming physical pages? The consumers maybe those with soft
> unlimited.

You might have many small groups which are all under their soft limit
but their total usage triggers global reclaim for example.

> If so, they should be punished first, based on the assumption that the
> unlimited is treated with no guarantee.
> Then soft limit guarantee could be assured without changes in the
> current default setting of soft limit, no?

How would you prioritize between over soft limit groups (which should
give at least some protection from over reclaim AFAIU from your
suggestion) from those that are unlimited?

Also, when we start talking about guarantees, why would somebody who
is soft unlimited be punished at all? Soft unlimited basically means
soft_limit >= hard_limit which says please do not reclaim from me unless
really really really necessary.

> With soft limit available, victims are only selected from excessors, I think.
> 
> >>
> >> Ying Han has a patch which basically skips all cgroups which are under
> >> its limit until we reach a certain reclaim priority but even for this we
> >> need some additional changes - e.g. reverse the current default setting
> >> of the soft limit.
> >>
> >> Anyway, I like the nr_to_reclaim reduction idea because we have to do
> >> this in some way because the global reclaim starts with ULONG
> >> nr_to_scan.
> >
> > Agree with Michal where there are quite a lot changes we need to get
> > in for soft limit before any further optimization.
> >
> > Hillf, please refer to the patch from Johannes
> > https://lkml.org/lkml/2012/1/13/99 which got quite a lot recent
> > discussions. I am expecting to get that in before further soft limit
> > changes.
> >
> 
> Johannes did great cleanup, why barriered?

Barriered?

-- 
Michal Hocko
SUSE Labs
SUSE LINUX s.r.o.
Lihovarska 1060/12
190 00 Praha 9    
Czech Republic
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