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Date: Fri, 19 Feb 2010 23:09:32 +0530
From: Balbir Singh <balbir@linux.vnet.ibm.com>
To: Christoph Lameter <cl@linux-foundation.org>
Subject: Re: [PATCH] powerpc: Set a smaller value for RECLAIM_DISTANCE to
	enable zone reclaim
Message-ID: <20100219173932.GX5612@balbir.in.ibm.com>
References: <20100218222923.GC31681@kryten> <20100219000730.GD31681@kryten>
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Cc: Mel Gorman <mel@csn.ul.ie>, linuxppc-dev@lists.ozlabs.org,
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* Christoph Lameter <cl@linux-foundation.org> [2010-02-19 09:51:12]:

> On Fri, 19 Feb 2010, Balbir Singh wrote:
> 
> > >> zone_reclaim. The others back off and try the next zone in the zonelist
> > >> instead. I'm not sure what the original intention was but most likely it
> > >> was to prevent too many parallel reclaimers in the same zone potentially
> > >> dumping out way more data than necessary.
> > >
> > > Yes it was to prevent concurrency slowing down reclaim. At that time the
> > > number of processors per NUMA node was 2 or so. The number of pages that
> > > are reclaimed is limited to avoid tossing too many page cache pages.
> > >
> >
> > That is interesting, I always thought it was to try and free page
> > cache first. For example with zone->min_unmapped_pages, if
> > zone_pagecache_reclaimable is greater than unmapped pages, we start
> > reclaim the cached pages first. The min_unmapped_pages almost sounds
> > like the higher level watermark - or am I misreading the code.
> 
> Indeed the purpose is to free *old* page cache pages.
> 
> The min_unmapped_pages is to protect a mininum of the page cache pages /
> fs metadata from zone reclaim so that ongoing file I/O is not impacted.

Thanks for the explanation!

-- 
	Three Cheers,
	Balbir