linux-kernel - Re: [PATCH 0/3] Reduce watermark-related problems with the per-cpu allocator V2

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Date:	Mon, 23 Aug 2010 14:39:48 +0100
From:	Mel Gorman <mel@....ul.ie>
To:	Christoph Lameter <cl@...ux.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Linux Kernel List <linux-kernel@...r.kernel.org>,
	linux-mm@...ck.org, Rik van Riel <riel@...hat.com>,
	Johannes Weiner <hannes@...xchg.org>,
	Minchan Kim <minchan.kim@...il.com>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>
Subject: Re: [PATCH 0/3] Reduce watermark-related problems with the per-cpu
	allocator V2

On Mon, Aug 23, 2010 at 08:38:25AM -0500, Christoph Lameter wrote:
> On Mon, 23 Aug 2010, Mel Gorman wrote:
> 
> > > The maximum time for which the livelock can exists is the vm stat
> > > interval. By default the counters are brought up to date at least once per
> > > second or if a certain delta was violated. Drifts are controlled by the
> > > delta configuration.
> > >
> >
> > While there is a maximum time (2 seconds I think) the drift can exist
> > in, a machine under enough pressure can make a mess of the watermarks
> > during that time. If it wasn't the case, these livelocks with 0 pages
> > free wouldn't be happening.
> 
> So because we go way beyond the watermarks we reach a state in which a
> livelock exists that does not go away when the counters are finally
> updated?
> 

That appears to be the case. The system has already gotten into a state
where there are 0 pages free. Just because the NR_FREE_PAGES counter
gets updated to reflect the accurate count of 0 does not mean the system
can recover from it.

-- 
Mel Gorman
Part-time Phd Student                          Linux Technology Center
University of Limerick                         IBM Dublin Software Lab
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