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

From: Mel Gorman
Date: Tue Jun 15 2010 - 06:19:25 EST

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

>> 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 :)


>> Signed-off-by: Mel Gorman<mel@xxxxxxxxx>
> Reviewed-by: Rik van Riel <riel@xxxxxxxxxx>

Thanks again :)

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