linux-kernel - Re: [PATCH] memcg: fix mapcount check in move charge code for anonymous page

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Date:	Tue, 6 Mar 2012 17:29:46 -0800 (PST)
From:	Hugh Dickins <hughd@...gle.com>
To:	Naoya Horiguchi <n-horiguchi@...jp.nec.com>
cc:	Daisuke Nishimura <nishimura@....nes.nec.co.jp>,
	linux-mm@...ck.org, Andrew Morton <akpm@...ux-foundation.org>,
	Andrea Arcangeli <aarcange@...hat.com>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	Hillf Danton <dhillf@...il.com>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] memcg: fix mapcount check in move charge code for
 anonymous page

On Tue, 6 Mar 2012, Naoya Horiguchi wrote:
> 
> IMO, ideally the charge of shared (both file and anon) pages should
> be accounted for all cgroups to which the processes mapping the pages
> belong to, where each charge is weighted by inverse number of mapcount.
> I think accounting total number of mapcount with another counter does
> not work, because the weight of charge depends on each page and the
> total count of mapcount doesn't describe the proportion among cgroups.
> But anyway, it adds more complexity and needs much work, so is not
> a short term fix.

That "ideal" complexity was considered before the current memcg approach
went in.  We elected to go with the less satisfying, but much simpler,
single-owner approach, and it does seem to have paid off.  I believe
that even those who had successfully developed a more complex approach
have since abandoned it for performance scalability reasons.

Hugh
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