linux-kernel - Re: [PATCH 11/12] vmscan: Write out dirty pages in batch

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Message-ID: <20100615101857.GB26788@csn.ul.ie>
Date:	Tue, 15 Jun 2010 11:18:58 +0100
From:	Mel Gorman <mel@....ul.ie>
To:	Rik van Riel <riel@...hat.com>
Cc:	linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
	linux-mm@...ck.org, Dave Chinner <david@...morbit.com>,
	Chris Mason <chris.mason@...cle.com>,
	Nick Piggin <npiggin@...e.de>,
	Johannes Weiner <hannes@...xchg.org>,
	Christoph Hellwig <hch@...radead.org>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH 11/12] vmscan: Write out dirty pages in batch

On Mon, Jun 14, 2010 at 05:13:37PM -0400, Rik van Riel wrote:
> On 06/14/2010 07:17 AM, Mel Gorman wrote:
>> Page reclaim cleans individual pages using a_ops->writepage() because from
>> the VM perspective, it is known that pages in a particular zone must be freed
>> soon, it considers the target page to be the oldest and it does not want
>> to wait while background flushers cleans other pages. From a filesystem
>> perspective this is extremely inefficient as it generates a very seeky
>> IO pattern leading to the perverse situation where it can take longer to
>> clean all dirty pages than it would have otherwise.
>
> Reclaiming clean pages should be fast enough that this should
> make little, if any, difference.
>

Indeed, this was a bit weak. The original point of the patch was to write
contiguous pages belonging to the same inode when they were encountered in
that batch which made a bit more sense but didn't work out at first
pass.

>> This patch queues all dirty pages at once to maximise the chances that
>> the write requests get merged efficiently. It also makes the next patch
>> that avoids writeout from direct reclaim more straight-forward.
>
> However, this is a convincing argument :)
>

Thanks.

>> Signed-off-by: Mel Gorman<mel@....ul.ie>
>
> Reviewed-by: Rik van Riel <riel@...hat.com>
>

Thanks again :)

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