linux-kernel - Re: [rfc patch 4/6] memcg: reclaim statistics

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Date:	Tue, 17 May 2011 09:55:46 -0400
From:	Rik van Riel <riel@...hat.com>
To:	Johannes Weiner <hannes@...xchg.org>
CC:	Ying Han <yinghan@...gle.com>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	Daisuke Nishimura <nishimura@....nes.nec.co.jp>,
	Balbir Singh <balbir@...ux.vnet.ibm.com>,
	Michal Hocko <mhocko@...e.cz>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Minchan Kim <minchan.kim@...il.com>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	Mel Gorman <mgorman@...e.de>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org
Subject: Re: [rfc patch 4/6] memcg: reclaim statistics

On 05/17/2011 03:42 AM, Johannes Weiner wrote:

> It does hierarchical soft limit reclaim once triggered, but I meant
> that soft limits themselves have no hierarchical meaning.  Say you
> have the following hierarchy:
>
>                  root_mem_cgroup
>
>               aaa               bbb
>
>             a1  a2             b1  b2
>
>          a1-1
>
> Consider aaa and a1 had a soft limit.  If global memory arose, aaa and
> all its children would be pushed back with the current scheme, the one
> you are proposing, and the one I am proposing.
>
> But now consider aaa hitting its hard limit.  Regular target reclaim
> will be triggered, and a1, a2, and a1-1 will be scanned equally from
> hierarchical reclaim.  That a1 is in excess of its soft limit is not
> considered at all.
>
> With what I am proposing, a1 and a1-1 would be pushed back more
> aggressively than a2, because a1 is in excess of its soft limit and
> a1-1 is contributing to that.

Ying, I think Johannes has a good point.  I do not see
a way to enforce the limits properly with the scheme we
came up with at LSF, in the hierarchical scenario above.

There may be a way, but until we think of it, I suspect
it will be better to go with Johannes's scheme for now.

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