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Date: Thu, 11 Aug 2011 14:18:54 -0400
From: Rik van Riel <riel@redhat.com>
MIME-Version: 1.0
Subject: Re: [PATCH 5/7] mm: vmscan: Do not writeback filesystem pages in
	kswapd except in high priority
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To: Mel Gorman <mgorman@suse.de>
Cc: Jan Kara <jack@suse.cz>, LKML <linux-kernel@vger.kernel.org>, XFS <xfs@oss.sgi.com>, Christoph Hellwig <hch@infradead.org>, Linux-MM <linux-mm@kvack.org>, Minchan Kim <minchan.kim@gmail.com>, Wu Fengguang <fengguang.wu@intel.com>, Johannes Weiner <jweiner@redhat.com>

On 08/10/2011 06:47 AM, Mel Gorman wrote:
> It is preferable that no dirty pages are dispatched for cleaning from
> the page reclaim path. At normal priorities, this patch prevents kswapd
> writing pages.
>
> However, page reclaim does have a requirement that pages be freed
> in a particular zone. If it is failing to make sufficient progress
> (reclaiming<  SWAP_CLUSTER_MAX at any priority priority), the priority
> is raised to scan more pages. A priority of DEF_PRIORITY - 3 is
> considered to be the point where kswapd is getting into trouble
> reclaiming pages. If this priority is reached, kswapd will dispatch
> pages for writing.
>
> Signed-off-by: Mel Gorman<mgorman@suse.de>
> Reviewed-by: Minchan Kim<minchan.kim@gmail.com>

My only worry with this patch is that maybe we'll burn too
much CPU time freeing pages from a zone.  However, chances
are we'll have freed pages from other zones when scanning
one zone multiple times (the page cache dirty limit is global,
the clean pages have to be _somewhere_).

Since the bulk of the allocators are not too picky about
which zone they get their pages from, I suspect this patch
will be an overall improvement pretty much all the time.

Acked-by: Rik van Riel <riel@redhat.com>

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