Message-ID: <4DD27E62.50806@redhat.com>
Date: Tue, 17 May 2011 09:55:46 -0400
From: Rik van Riel <riel@redhat.com>
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To: Johannes Weiner <hannes@cmpxchg.org>
CC: Ying Han <yinghan@google.com>,
        KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>,
        Daisuke Nishimura <nishimura@mxp.nes.nec.co.jp>,
        Balbir Singh <balbir@linux.vnet.ibm.com>,
        Michal Hocko <mhocko@suse.cz>,
        Andrew Morton <akpm@linux-foundation.org>,
        Minchan Kim <minchan.kim@gmail.com>,
        KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>,
        Mel Gorman <mgorman@suse.de>, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org
Subject: Re: [rfc patch 4/6] memcg: reclaim statistics
References: <1305212038-15445-1-git-send-email-hannes@cmpxchg.org> <1305212038-15445-5-git-send-email-hannes@cmpxchg.org> <BANLkTi=yCyAsOc_uTQLp1kWp5w0i9gomxg@mail.gmail.com> <20110516231028.GV16531@cmpxchg.org> <BANLkTimLNZfc-jcA3yBG5D3k2u=0_JnrhQ@mail.gmail.com> <20110517074230.GY16531@cmpxchg.org>
In-Reply-To: <20110517074230.GY16531@cmpxchg.org>
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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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