linux-kernel - Re: [patch 7/8] vmscan: memcg-aware unevictable page rescue scanner

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Date:	Fri, 3 Jun 2011 00:19:06 +0200
From:	Johannes Weiner <hannes@...xchg.org>
To:	Hiroyuki Kamezawa <kamezawa.hiroyuki@...il.com>
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>,
	Rik van Riel <riel@...hat.com>,
	Minchan Kim <minchan.kim@...il.com>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	Mel Gorman <mgorman@...e.de>, Greg Thelen <gthelen@...gle.com>,
	Michel Lespinasse <walken@...gle.com>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [patch 7/8] vmscan: memcg-aware unevictable page rescue scanner

On Fri, Jun 03, 2011 at 07:01:34AM +0900, Hiroyuki Kamezawa wrote:
> 2011/6/3 Ying Han <yinghan@...gle.com>:
> > On Thu, Jun 2, 2011 at 6:27 AM, Hiroyuki Kamezawa
> > <kamezawa.hiroyuki@...il.com> wrote:
> >> 2011/6/1 Johannes Weiner <hannes@...xchg.org>:
> >>> Once the per-memcg lru lists are exclusive, the unevictable page
> >>> rescue scanner can no longer work on the global zone lru lists.
> >>>
> >>> This converts it to go through all memcgs and scan their respective
> >>> unevictable lists instead.
> >>>
> >>> Signed-off-by: Johannes Weiner <hannes@...xchg.org>
> >>
> >> Hm, isn't it better to have only one GLOBAL LRU for unevictable pages ?
> >> memcg only needs counter for unevictable pages and LRU is not necessary
> >> to be per memcg because we don't reclaim it...
> >
> > Hmm. Are we suggesting to keep one un-evictable LRU list for all
> > memcgs? So we will have
> > exclusive lru only for file and anon. If so, we are not done to make
> > all the lru list being exclusive
> > which is critical later to improve the zone->lru_lock contention
> > across the memcgs
> >
> considering lrulock, yes, maybe you're right.

That's one of the complications.

> > Sorry If i misinterpret the suggestion here
> >
> 
> My concern is I don't know for what purpose this function is used ..

I am not sure how it's supposed to be used, either.  But it's
documented to be a 'really big hammer' and it's kicked off from
userspace.  So I suppose having the thing go through all memcgs bears
a low risk of being a problem.  My suggestion is we go that way until
someone complains.
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