linux-kernel - Re: [patch] memcg: skip scanning active lists based on individual size

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Date:	Thu, 1 Sep 2011 08:15:40 +0200
From:	Johannes Weiner <jweiner@...hat.com>
To:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
Cc:	Minchan Kim <minchan.kim@...il.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Rik van Riel <riel@...hat.com>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	Daisuke Nishimura <nishimura@....nes.nec.co.jp>,
	Balbir Singh <bsingharora@...il.com>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org
Subject: Re: [patch] memcg: skip scanning active lists based on individual
 size

On Thu, Sep 01, 2011 at 09:09:31AM +0900, KAMEZAWA Hiroyuki wrote:
> On Wed, 31 Aug 2011 19:13:34 +0900
> Minchan Kim <minchan.kim@...il.com> wrote:
> 
> > On Wed, Aug 31, 2011 at 6:08 PM, Johannes Weiner <jweiner@...hat.com> wrote:
> > > Reclaim decides to skip scanning an active list when the corresponding
> > > inactive list is above a certain size in comparison to leave the
> > > assumed working set alone while there are still enough reclaim
> > > candidates around.
> > >
> > > The memcg implementation of comparing those lists instead reports
> > > whether the whole memcg is low on the requested type of inactive
> > > pages, considering all nodes and zones.
> > >
> > > This can lead to an oversized active list not being scanned because of
> > > the state of the other lists in the memcg, as well as an active list
> > > being scanned while its corresponding inactive list has enough pages.
> > >
> > > Not only is this wrong, it's also a scalability hazard, because the
> > > global memory state over all nodes and zones has to be gathered for
> > > each memcg and zone scanned.
> > >
> > > Make these calculations purely based on the size of the two LRU lists
> > > that are actually affected by the outcome of the decision.
> > >
> > > Signed-off-by: Johannes Weiner <jweiner@...hat.com>
> > > Cc: Rik van Riel <riel@...hat.com>
> > > Cc: KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>
> > > Cc: KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
> > > Cc: Daisuke Nishimura <nishimura@....nes.nec.co.jp>
> > > Cc: Balbir Singh <bsingharora@...il.com>
> > 
> > Reviewed-by: Minchan Kim <minchan.kim@...il.com>
> > 
> > I can't understand why memcg is designed for considering all nodes and zones.
> > Is it a mistake or on purpose?
> 
> It's purpose. memcg just takes care of the amount of pages.

This mechanism isn't about memcg at all, it's an aging decision at a
much lower level.  Can you tell me how the old implementation is
supposed to work?

> But, hmm, this change may be good for softlimit and your work.

Yes, I noticed those paths showing up in a profile with my patches.
Lots of memcgs on a multi-node machine will trigger it too.  But it's
secondary, my primary reasoning was: this does not make sense at all.

> I'll ack when you add performance numbers in changelog.

It's not exactly a performance optimization but I'll happily run some
workloads.  Do you have suggestions what to test for?  I.e. where
would you expect regressions?
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