linux-kernel - Re: [PATCH 2/3] mm: page allocator: Calculate a better estimate of NR_FREE_PAGES when memory is low and kswapd is awake

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Date:	Mon, 23 Aug 2010 14:55:59 +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 2/3] mm: page allocator: Calculate a better estimate of
	NR_FREE_PAGES when memory is low and kswapd is awake

On Mon, Aug 23, 2010 at 08:41:56AM -0500, Christoph Lameter wrote:
> On Mon, 23 Aug 2010, Mel Gorman wrote:
> 
> > > The delta of the counters could also be reduced to increase accuracy.
> > > See refresh_zone_stat_thresholds().
> > True, but I thought that would introduce a constant performance penalty
> > for a corner case which I didn't like.
> 
> Sure, an increased frequency of updates would increase the chance of
> bouncing cachelines. But the bouncing cacheline scenario for the vm
> counters was tuned for applications that continually allocate pages in
> parallel.
> 
> When the vm gets into a state where continual reclaim is necessary then
> the counters are not that frequently updated. If the machine is already
> slowing down due to reclaim then the vm can likely affort more frequent
> counter updates.
> 

Ok, but is that better than this patch? Decreasing the size of the window by
reducing the threshold still leaves a window. There is still a small amount
of drift by summing up all the deltas but you get a much more accurate count
at the point of time it was important to know.

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