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    SubjectRe: [PATCH] vmscan: remove wait_on_page_writeback() from pageout()
    On Wed, Jul 28, 2010 at 05:10:33PM +0800, Mel Gorman 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.

    OK there is a <=0.1s time window. Then what about the data set size?
    After first shrink_page_list(ASYNC), there will be hardly any pages
    left in the page_list except for the already under-writeback pages and
    other unreclaimable pages. So it still asks for some race conditions
    for hitting the second pageout(SYNC) -- some unreclaimable pages
    become reclaimable+dirty in the 0.1s time 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.
    >
    > > So it's bad
    > > idea anyway to call wait_on_page_writeback() inside pageout().
    > >
    >
    > I recognise that you are probably thinking of the stall-due-to-fork problem
    > but I'd expect the patch that raises the bar for <= PAGE_ALLOC_COSTLY_ORDER
    > to be sufficient. If not, I think it still makes sense to call
    > wait_on_page_writeback() for > PAGE_ALLOC_COSTLY_ORDER.

    The main intention of this patch is to remove semi-dead code.
    I'm less disturbed by the long stall time now with the previous patch ;)

    Thanks,
    Fengguang


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