Date:	Tue, 6 Sep 2011 19:52:11 +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 Tue, 6 Sep 2011 12:43:47 +0200
Johannes Weiner <jweiner@...hat.com> wrote:

> On Tue, Sep 06, 2011 at 06:33:58PM +0900, KAMEZAWA Hiroyuki wrote:
> > On Mon, 5 Sep 2011 20:25:14 +0200
> > Johannes Weiner <jweiner@...hat.com> wrote:
> > 
> > > On Thu, Sep 01, 2011 at 03:31:48PM +0900, KAMEZAWA Hiroyuki wrote:
> > > > On Thu, 1 Sep 2011 08:15:40 +0200
> > > > Johannes Weiner <jweiner@...hat.com> wrote:
> > > > 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.
> > > 
> > > But why should we deactivate on Node 1?  We have good reasons not to
> > > on the global level, why should memcgs silently behave differently?
> > 
> > One reason was I thought that memcg should behave as to have one LRU list,
> > which is not devided by zones and wanted to ignore zones as much
> > as possible. Second reason was that I don't want to increase swap-out
> > caused by memcg limit.
> 
> You can think of it like this: if every active list is only balanced
> when its inactive counterpart is too small, then the memcg-wide
> proportion of inactive vs. active pages is as desired, too.  So even
> after my change, the 'one big LRU' has the right inactive/active ratio.
> 
> On the other hand, the old way could allow for the memcg-level to have
> the right proportion and still thrash one workload that is bound to
> another node even though its inactive > active, but is overshadowed by
> inactive < active workloads on other nodes.
> 
ok.

> > > I mostly don't understand it on a semantic level.  vmscan needs to
> > > know whether a certain inactive LRU list has enough reclaim candidates
> > > to skip scanning its corresponding active list.  The global state is
> > > not useful to find out if a single inactive list has enough pages.
> > 
> > Ok, I agree to this. I should add other logic to do what I want.
> > In my series,
> >   - passing nodemask
> >   - avoid overscan
> >   - calculating node weight
> > These will allow me to see what I want.
> 
> What /do/ you want? :) I just don't see your concern.  I mean, yes, no
> increased swapout, but in what concrete scenario could you suspect
> swapping to increase because of this change?
> 

Ah, sorry. Maybe I was merged other concerns and this talk. This change
itseld doesn't related to my scenario.

Please forget. I'm a bit tired in these days...



> > > > > > 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.
> > > 
> > > 4G ram, 500M swap on SSD, numa=fake=16, 10 runs of make -j11 in 500M
> > > memcg, standard deviation in parens:
> > > 
> > > 		seconds		pswpin			pswpout
> > > vanilla:	175.359(0.106)	6906.900(1779.135)	8913.200(1917.369)
> > > patched:	176.144(0.243)	8581.500(1833.432)	10872.400(2124.104)
> > 
> > Hmm. swapin/out seems increased. But hmm...stddev is large.
> > Is this expected ? reason ?
> 
> It's kind of expected because there is only a small number of parallel
> jobs that have bursty memory usage, so the slightest timing variations
> can make the difference between an episode of heavy thrashing and the
> tasks having their bursts at different times and getting along fine.
> 
> So we are basically looking at test results that are clustered around
> not one, but several different mean values.  The arithmetic mean is
> not really meaningful for these samples.
> 
ok.

> > Anyway, I don't want to disturb you more. Thanks.
> 
> I am happy to test if my changes introduce regressions, I don't want
> that, obviously.  But do you have a theory behind your concern that
> swapping could increase?  Just saying, this test request seemed a bit
> random because I don't see where my change would affect this
> particular workload.
> 
> 

the patch is on mm queue and this may be too late, but..

Acked-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>


Thanks,
-Kame

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