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

[Mel Gorman]

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?

> 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@xxxxxxxxxxxxxx>
> 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.
> 
> 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.
> 
> Signed-off-by: KOSAKI Motohiro <kosaki.motohiro@xxxxxxxxxxxxxx>

I think the final paragraph makes a lot of sense. If a lumpy reclaimer is
going to get stalled on wait_on_page_writeback(), it should be a sufficient
throttling mechanism.

Will test.

> ---
>  mm/vmscan.c |    2 --
>  1 files changed, 0 insertions(+), 2 deletions(-)
> 
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index 97170eb..2aa16eb 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -1304,8 +1304,6 @@ shrink_inactive_list(unsigned long nr_to_scan, struct zone *zone,
>  	 */
>  	if (nr_reclaimed < nr_taken && !current_is_kswapd() &&
>  			sc->lumpy_reclaim_mode) {
> -		congestion_wait(BLK_RW_ASYNC, HZ/10);
> -
>  		/*
>  		 * The attempt at page out may have made some
>  		 * of the pages active, mark them inactive again.

-- 
Mel Gorman
Part-time Phd Student                          Linux Technology Center
University of Limerick                         IBM Dublin Software Lab
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