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    SubjectRe: [PATCH] vmscan: remove wait_on_page_writeback() from pageout()
    On Wed, Jul 28, 2010 at 05:43:41PM +0800, KOSAKI Motohiro wrote:
    > > On Wed, Jul 28, 2010 at 04:46:54PM +0800, Wu Fengguang wrote:
    > > > The wait_on_page_writeback() call inside pageout() is virtually dead code.
    > > >
    > > > shrink_inactive_list()
    > > > shrink_page_list(PAGEOUT_IO_ASYNC)
    > > > pageout(PAGEOUT_IO_ASYNC)
    > > > shrink_page_list(PAGEOUT_IO_SYNC)
    > > > pageout(PAGEOUT_IO_SYNC)
    > > >
    > > > Because shrink_page_list/pageout(PAGEOUT_IO_SYNC) is always called after
    > > > a preceding shrink_page_list/pageout(PAGEOUT_IO_ASYNC), the first
    > > > pageout(ASYNC) converts dirty pages into writeback pages, the second
    > > > shrink_page_list(SYNC) waits on the clean of writeback pages before
    > > > calling pageout(SYNC). The second shrink_page_list(SYNC) can hardly run
    > > > into dirty pages for pageout(SYNC) unless in some race conditions.
    > > >
    > >
    > > It's possible for the second call to run into dirty pages as there is a
    > > congestion_wait() call between the first shrink_page_list() call and the
    > > second. That's a big window.
    > >
    > > > And the wait page-by-page behavior of pageout(SYNC) will lead to very
    > > > long stall time if running into some range of dirty pages.
    > >
    > > True, but this is also lumpy reclaim which is depending on a contiguous
    > > range of pages. It's better for it to wait on the selected range of pages
    > > which is known to contain at least one old page than excessively scan and
    > > reclaim newer pages.
    >
    > Today, I was successful to reproduce the Andres's issue. and I disagree this
    > opinion.
    > The root cause is, congestion_wait() mean "wait until clear io congestion". but
    > if the system have plenty dirty pages, flusher threads are issueing IO conteniously.
    > So, io congestion is not cleared long time. eventually, congestion_wait(BLK_RW_ASYNC, HZ/10)
    > become to equivalent to sleep(HZ/10).
    >
    > I would propose followint patch instead.
    >
    > And I've found synchronous lumpy reclaim have more serious problem. I woule like to
    > explain it as another mail.
    >
    > Thanks.
    >
    >
    >
    > >From 0266fb2c23aef659cd4e89fccfeb464f23257b74 Mon Sep 17 00:00:00 2001
    > From: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
    > Date: Tue, 27 Jul 2010 14:36:44 +0900
    > Subject: [PATCH] vmscan: synchronous lumpy reclaim don't call congestion_wait()
    >
    > congestion_wait() mean "waiting for number of requests in IO queue is
    > under congestion threshold".
    > That said, if the system have plenty dirty pages, flusher thread push
    > new request to IO queue conteniously. So, IO queue are not cleared
    > congestion status for a long time. thus, congestion_wait(HZ/10) is
    > almostly equivalent schedule_timeout(HZ/10).
    >
    > If the system 512MB memory, DEF_PRIORITY mean 128kB scan and 4096 times
    > shrink_inactive_list call. 4096 times 0.1sec stall makes crazy insane
    > long stall. That shouldn't.

    Good point. Maybe more clear to say: "It takes 4096 shrink_page_list()
    calls to scan 512MB memory."

    > In the other hand, this synchronous lumpy reclaim donesn't need this
    > congestion_wait() at all. shrink_page_list(PAGEOUT_IO_SYNC) cause to
    > call wait_on_page_writeback() and it provide sufficient waiting.

    Agreed.

    Reviewed-by: Wu Fengguang <fengguang.wu@intel.com>

    Thanks,
    Fengguang


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