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

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Date:	Tue, 24 Jan 2012 09:18:21 +0530
From:	Balbir Singh <bsingharora@...il.com>
To:	Ying Han <yinghan@...gle.com>
Cc:	Michal Hocko <mhocko@...e.cz>, Hillf Danton <dhillf@...il.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, Jan 24, 2012 at 12:44 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.
>>

True, ideally soft reclaim should not turn on and allow global reclaim
to occur in the scenario mentioned.

>> 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.
>>

I'd be wary of that approach, because it might be harder to explain
the working of soft limits,I'll look at the discussion thread
mentioned earlier for the benefits of that approach.

>> 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.

Balbir
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