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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Message-ID: <20100902085118.GA7670@csn.ul.ie>
Date:	Thu, 2 Sep 2010 09:51:18 +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 Wed, Sep 01, 2010 at 07:43:41PM -0500, Christoph Lameter wrote:
> On Tue, 31 Aug 2010, Mel Gorman wrote:
> 
> > +#ifdef CONFIG_SMP
> > +/* Called when a more accurate view of NR_FREE_PAGES is needed */
> > +unsigned long zone_nr_free_pages(struct zone *zone)
> > +{
> > +	unsigned long nr_free_pages = zone_page_state(zone, NR_FREE_PAGES);
> 
> You cannot call zone_page_state here because zone_page_state clips the
> counter at zero. The nr_free_pages needs to reflect the unclipped state
> and then the deltas need to be added. Then the clipping at zero can be
> done.
> 

Good point. This justifies the use of a generic helper that is co-located
with vmstat.h. I've taken your zone_page_state_snapshot() patch, am using
the helper to take a more accurate reading of NR_FREE_PAGES and preparing
for a test.  Thanks Christoph.

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