linux-kernel - Re: Found the commit that causes the OOMs

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Message-ID: <20090705105158.GA1804@localhost>
Date:	Sun, 5 Jul 2009 18:51:58 +0800
From:	Wu Fengguang <fengguang.wu@...il.com>
To:	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>
Cc:	Rik van Riel <riel@...hat.com>,
	David Woodhouse <dwmw2@...radead.org>,
	David Howells <dhowells@...hat.com>,
	Minchan Kim <minchan.kim@...il.com>,
	Mel Gorman <mel@....ul.ie>,
	Johannes Weiner <hannes@...xchg.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Christoph Lameter <cl@...ux-foundation.org>,
	"peterz@...radead.org" <peterz@...radead.org>,
	"tytso@....edu" <tytso@....edu>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	"elladan@...imo.com" <elladan@...imo.com>,
	"npiggin@...e.de" <npiggin@...e.de>,
	"Barnes, Jesse" <jesse.barnes@...el.com>
Subject: Re: Found the commit that causes the OOMs

On Sun, Jul 05, 2009 at 07:38:54PM +0900, KOSAKI Motohiro wrote:
> > >> OK. thanks.
> > >> I plan to submit this patch after small more tests. it is useful for OOM analysis.
> > >
> > > It is also useful for throttling page reclaim.
> > >
> > > If more than half of the inactive pages in a zone are
> > > isolated, we are probably beyond the point where adding
> > > additional reclaim processes will do more harm than good.
> > 
> > Maybe we can try limiting the isolation phase of direct reclaims to
> > one per CPU?
> > 
> >         mutex_lock(per_cpu_lock);
> >         isolate_pages();
> >         shrink_page_list();
> >         put_back_pages();
> >         mutex_unlock(per_cpu_lock);
> > 
> > This way the isolated pages as well as major parts of direct reclaims
> > will be bounded by CPU numbers. The added overheads should be trivial
> > comparing to the reclaim costs.
> 
> hm, this idea makes performance degression on few CPU machine, I think.

Yes, this is also my big worry. But one possible workaround is to
allow N direct reclaims per CPU.

> e.g.
> if system have only one cpu and sysmtem makes lumpy reclaim, lumpy reclaim
> makes synchronous pageout and it makes very long waiting time.

We can temporarily drop the lock during the writeback waiting.
0-order reclaims shall not be blocked by ongoing high order reclaims.

> I suspect per-cpu decision is not useful in this area.

Maybe. I'm just proposing one more possible way to choose from :)

Thanks,
Fengguang

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