linux-kernel - Re: [PATCH 55 of 66] select CONFIG_COMPACTION if TRANSPARENT

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Message-ID: <20101214160637.GB5638@random.random>
Date:	Tue, 14 Dec 2010 17:06:37 +0100
From:	Andrea Arcangeli <aarcange@...hat.com>
To:	Mel Gorman <mel@....ul.ie>
Cc:	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	linux-mm@...ck.org, Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org,
	Marcelo Tosatti <mtosatti@...hat.com>,
	Adam Litke <agl@...ibm.com>, Avi Kivity <avi@...hat.com>,
	Hugh Dickins <hugh.dickins@...cali.co.uk>,
	Rik van Riel <riel@...hat.com>,
	Dave Hansen <dave@...ux.vnet.ibm.com>,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	Ingo Molnar <mingo@...e.hu>, Mike Travis <travis@....com>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	Christoph Lameter <cl@...ux-foundation.org>,
	Chris Wright <chrisw@...s-sol.org>, bpicco@...hat.com,
	Balbir Singh <balbir@...ux.vnet.ibm.com>,
	"Michael S. Tsirkin" <mst@...hat.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Johannes Weiner <hannes@...xchg.org>,
	Daisuke Nishimura <nishimura@....nes.nec.co.jp>,
	Chris Mason <chris.mason@...cle.com>,
	Borislav Petkov <bp@...en8.de>
Subject: Re: [PATCH 55 of 66] select CONFIG_COMPACTION if
 TRANSPARENT_HUGEPAGE enabled

Hi Mel,

On Tue, Dec 14, 2010 at 09:45:56AM +0000, Mel Gorman wrote:
> On Thu, Dec 09, 2010 at 08:04:07PM +0100, Andrea Arcangeli wrote:
> > On Thu, Nov 18, 2010 at 04:22:45PM +0000, Mel Gorman wrote:
> > > Just to confirm - by hang, you mean grinds to a slow pace as opposed to
> > > coming to a complete stop and having to restart?
> > 
> > Hmm it's like if you're gigabytes in swap and apps hangs for a while
> > and system is not really usable and it swaps for most new memory
> > allocations despite there's plenty of memory free, but it's not a
> > deadlock of course.
> > 
> 
> Ok, but it's likely to be kswapd being very aggressive because it's
> woken up frequently and tries to balance all zones. Once it's not
> deadlocking entirely, there isn't a more fundamental bug hiding in there
> somewhere.

kswapd isn't activated by transhuge allocations because there's
khugepaged for that and it's throttle to try a 2m alloc only once per
minute if there's fragmentation.

So the reason of the trashing is the direct lumpy.
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