linux-kernel - Re: [PATCH 3/4] Count the number of times zone

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Message-ID: <4A2EB04F.2090906@redhat.com>
Date:	Tue, 09 Jun 2009 14:56:15 -0400
From:	Rik van Riel <riel@...hat.com>
To:	Mel Gorman <mel@....ul.ie>
CC:	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	Christoph Lameter <cl@...ux-foundation.org>,
	yanmin.zhang@...el.com, Wu Fengguang <fengguang.wu@...el.com>,
	linuxram@...ibm.com, linux-mm <linux-mm@...ck.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 3/4] Count the number of times zone_reclaim() scans and
 fails

Mel Gorman wrote:
> On NUMA machines, the administrator can configure zone_reclaim_mode that
> is a more targetted form of direct reclaim. On machines with large NUMA
> distances for example, a zone_reclaim_mode defaults to 1 meaning that clean
> unmapped pages will be reclaimed if the zone watermarks are not being met.
> 
> There is a heuristic that determines if the scan is worthwhile but it is
> possible that the heuristic will fail and the CPU gets tied up scanning
> uselessly. Detecting the situation requires some guesswork and experimentation
> so this patch adds a counter "zreclaim_failed" to /proc/vmstat. If during
> high CPU utilisation this counter is increasing rapidly, then the resolution
> to the problem may be to set /proc/sys/vm/zone_reclaim_mode to 0.
> 
> Signed-off-by: Mel Gorman <mel@....ul.ie>

Reviewed-by: Rik van Riel <riel@...hat.com>

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