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    SubjectRe: [PATCH 1/4] vmscan: delegate pageout io to flusher thread if current is kswapd
    On Thu, Apr 15, 2010 at 05:26:27PM +0900, KOSAKI Motohiro wrote:
    > Cc to Johannes
    >
    > > >
    > > > On Apr 14, 2010, at 9:11 PM, KOSAKI Motohiro wrote:
    > > >
    > > > > Now, vmscan pageout() is one of IO throuput degression source.
    > > > > Some IO workload makes very much order-0 allocation and reclaim
    > > > > and pageout's 4K IOs are making annoying lots seeks.
    > > > >
    > > > > At least, kswapd can avoid such pageout() because kswapd don't
    > > > > need to consider OOM-Killer situation. that's no risk.
    > > > >
    > > > > Signed-off-by: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
    > > >
    > > > What's your opinion on trying to cluster the writes done by pageout,
    > > > instead of not doing any paging out in kswapd?
    > > > Something along these lines:
    > >
    > > Interesting.
    > > So, I'd like to review your patch carefully. can you please give me one
    > > day? :)
    >
    > Hannes, if my remember is correct, you tried similar swap-cluster IO
    > long time ago. now I can't remember why we didn't merged such patch.
    > Do you remember anything?

    Oh, quite vividly in fact :) For a lot of swap loads the LRU order
    diverged heavily from swap slot order and readaround was a waste of
    time.

    Of course, the patch looked good, too, but it did not match reality
    that well.

    I guess 'how about this patch?' won't get us as far as 'how about
    those numbers/graphs of several real-life workloads? oh and here
    is the patch...'.

    > > > Cluster writes to disk due to memory pressure.
    > > >
    > > > Write out logically adjacent pages to the one we're paging out
    > > > so that we may get better IOs in these situations:
    > > > These pages are likely to be contiguous on disk to the one we're
    > > > writing out, so they should get merged into a single disk IO.
    > > >
    > > > Signed-off-by: Suleiman Souhlal <suleiman@google.com>

    For random IO, LRU order will have nothing to do with mapping/disk order.


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