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

[KOSAKI Motohiro]

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

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.....



--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/