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

[Christoph Hellwig]

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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