Re: [PATCH] mm: vmscan: count only dirty pages as congested

[Andrew Morton]

On Wed, 15 Oct 2014 16:07:42 -0700 Jamie Liu <jamieliu@xxxxxxxxxx> wrote:

> On Wed, Oct 15, 2014 at 1:05 PM, Andrew Morton
> <akpm@xxxxxxxxxxxxxxxxxxxx> wrote:
> > On Wed, 15 Oct 2014 12:58:35 -0700 Jamie Liu <jamieliu@xxxxxxxxxx> wrote:
> >
> >> shrink_page_list() counts all pages with a mapping, including clean
> >> pages, toward nr_congested if they're on a write-congested BDI.
> >> shrink_inactive_list() then sets ZONE_CONGESTED if nr_dirty ==
> >> nr_congested. Fix this apples-to-oranges comparison by only counting
> >> pages for nr_congested if they count for nr_dirty.
> >>
> >> ...
> >>
> >> --- a/mm/vmscan.c
> >> +++ b/mm/vmscan.c
> >> @@ -875,7 +875,8 @@ static unsigned long shrink_page_list(struct list_head *page_list,
> >>                * end of the LRU a second time.
> >>                */
> >>               mapping = page_mapping(page);
> >> -             if ((mapping && bdi_write_congested(mapping->backing_dev_info)) ||
> >> +             if (((dirty || writeback) && mapping &&
> >> +                  bdi_write_congested(mapping->backing_dev_info)) ||
> >>                   (writeback && PageReclaim(page)))
> >>                       nr_congested++;
> >
> > What are the observed runtime effects of this change?
>
> wait_iff_congested() only waits if ZONE_CONGESTED is set (and at least
> one BDI is still congested). Modulo concurrent changes to BDI
> congestion status:
> 
> After this change, the probability that a given shrink_inactive_list()
> sets ZONE_CONGESTED increases monotonically with the fraction of dirty
> pages on the LRU, to 100% if all dirty pages are backed by a
> write-congested BDI. This is in line with what appears to intended,
> judging by the comment:
> 
> /*
> * Tag a zone as congested if all the dirty pages scanned were
> * backed by a congested BDI and wait_iff_congested will stall.
> */
> if (nr_dirty && nr_dirty == nr_congested)
> set_bit(ZONE_CONGESTED, &zone->flags);
> 
> Before this change, the probability that a given
> shrink_inactive_list() sets ZONE_CONGESTED varies erratically. Because
> the ZONE_CONGESTED condition is nr_dirty && nr_dirty == nr_congested,
> the probability peaks when the fraction of dirty pages is equal to the
> fraction of file pages backed by congested BDIs. So under some
> circumstances, an increase in the fraction of dirty pages or in the
> fraction of congested pages can actually result in an *decreased*
> probability that reclaim will stall for writeback congestion, and vice
> versa; which is both counterintuitive and counterproductive.

(top-posting repaired.  Please don't do that!)

OK, I buy all that.  But has any runtime testing been performed to
confirm this and to quantify the effects?

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