Re: [PATCH] vmscan: remove wait_on_page_writeback() from pageout()

[Wu Fengguang <fengguang.wu@intel.com>]

On Wed, Jul 28, 2010 at 05:59:55PM +0800, KOSAKI Motohiro wrote:
> > On Wed, Jul 28, 2010 at 06:43:41PM +0900, 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.
> > 
> > Is Andres's issue not covered by the patch "vmscan: raise the bar to
> > PAGEOUT_IO_SYNC stalls" because wait_on_page_writeback() was the
> > main problem?
> 
> Well, "vmscan: raise the bar to PAGEOUT_IO_SYNC stalls" is completely bandaid and

No joking. The (DEF_PRIORITY-2) is obviously too permissive and shall be fixed.

> much IO under slow USB flash memory device still cause such problem even if the patch is applied.

As for this patch, raising the bar to PAGEOUT_IO_SYNC reduces both
calls to congestion_wait() and wait_on_page_writeback(). So it
absolutely helps by itself.

> But removing wait_on_page_writeback() doesn't solve the issue perfectly because current
> lumpy reclaim have multiple sick. again, I'm writing explaining mail.....

Let's submit the two known working fixes first?

Thanks,
Fengguang

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