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

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Date:	Tue, 15 Jun 2010 06:57:40 -0400
From:	Christoph Hellwig <hch@...radead.org>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	Dave Chinner <david@...morbit.com>, Mel Gorman <mel@....ul.ie>,
	linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
	linux-mm@...ck.org, Chris Mason <chris.mason@...cle.com>,
	Nick Piggin <npiggin@...e.de>, Rik van Riel <riel@...hat.com>,
	Johannes Weiner <hannes@...xchg.org>,
	Christoph Hellwig <hch@...radead.org>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
Subject: Re: [PATCH 11/12] vmscan: Write out dirty pages in batch

On Mon, Jun 14, 2010 at 04:21:43PM -0700, Andrew Morton wrote:
> Yup.
> 
> But then, this all really should be done at the block layer so other
> io-submitting-paths can benefit from it.
> 
> IOW, maybe "the sort queue is the submission queue" wasn't a good idea.

Even if has not effect on the actual I/O patters it has a massive
benefit for the filesystem.  When probing delalloc/unwritten space at
least XFS does try to convert a larger extent forward from the index,
but doesn't bother to go backwards.  By providing the trivial sort here
we make life a lot easier for the filesystem.

In addition to that we do get better I/O patters especially with short
queues as smart writepage implementatons will also submit the next few
pages, which is essentially free given how the storage works.
This means we already have a page cleaned before we might even submit it
without sorting.

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