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

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Message-Id: <20110901153148.70452287.kamezawa.hiroyu@jp.fujitsu.com>
Date:	Thu, 1 Sep 2011 15:31:48 +0900
From:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
To:	Johannes Weiner <jweiner@...hat.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, 1 Sep 2011 08:15:40 +0200
Johannes Weiner <jweiner@...hat.com> wrote:

> 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?
> 
Old implemenation was supporsed to make vmscan to see only memcg and
ignore zones. memcg doesn't take care of any zones. Then, it uses
global numbers rather than zones.

Assume a system with 2 nodes and the whole memcg's inactive/active ratio
is unbalaned. 

   Node      0     1
   Active   800M   30M
   Inactive 100M   200M

If we judge 'unbalance' based on zones, Node1's Active will not rotate
even if it's not accessed for a while.
If we judge unbalance based on total stat, Both of Node0 and Node 1
will be rotated.

Hmm, old one doesn't work as I expexted ?

But okay, as time goes, I think Node1's inactive will decreased
and then, rotate will happen even with zone based ones.



> > 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.
> 

your word sounds always too strong to me ;) please be soft.

> > 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?
> 
Some comparison about amount of swap-out before/after change will be good.

Hm. If I do...
  - set up x86-64 NUMA box. (fake numa is ok.)
  - create memcg with 500M limit.
  - running kernel make with make -j 6(or more)

see time of make and amount of swap-out.


Thanks,
-Kame

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