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SubjectRe: [PATCH 2/3] Properly account for the number of page cache pages zone_reclaim() can reclaim
On Tue, Jun 09, 2009 at 05:19:41PM +0900, KOSAKI Motohiro wrote:
> Hi
>
> > On NUMA machines, the administrator can configure zone_relcaim_mode that
> > is a more targetted form of direct reclaim. On machines with large NUMA
> > distances for example, a zone_reclaim_mode defaults to 1 meaning that clean
> > unmapped pages will be reclaimed if the zone watermarks are not being met.
> >
> > There is a heuristic that determines if the scan is worthwhile but the
> > problem is that the heuristic is not being properly applied and is basically
> > assuming zone_reclaim_mode is 1 if it is enabled.
> >
> > This patch makes zone_reclaim() makes a better attempt at working out how
> > many pages it might be able to reclaim given the current reclaim_mode. If it
> > cannot clean pages, then NR_FILE_DIRTY number of pages are not candidates. If
> > it cannot swap, then NR_FILE_MAPPED are not. This indirectly addresses tmpfs
> > as those pages tend to be dirty as they are not cleaned by pdflush or sync.
> >
> > The ideal would be that the number of tmpfs pages would also be known
> > and account for like NR_FILE_MAPPED as swap is required to discard them.
> > A means of working this out quickly was not obvious but a comment is added
> > noting the problem.
> >
> > Signed-off-by: Mel Gorman <mel@csn.ul.ie>
> > ---
> > mm/vmscan.c | 18 ++++++++++++++++--
> > 1 files changed, 16 insertions(+), 2 deletions(-)
> >
> > diff --git a/mm/vmscan.c b/mm/vmscan.c
> > index ba211c1..ffe2f32 100644
> > --- a/mm/vmscan.c
> > +++ b/mm/vmscan.c
> > @@ -2380,6 +2380,21 @@ int zone_reclaim(struct zone *zone, gfp_t gfp_mask, unsigned int order)
> > {
> > int node_id;
> > int ret;
> > + int pagecache_reclaimable;
> > +
> > + /*
> > + * Work out how many page cache pages we can reclaim in this mode.
> > + *
> > + * NOTE: Ideally, tmpfs pages would be accounted as if they were
> > + * NR_FILE_MAPPED as swap is required to discard those
> > + * pages even when they are clean. However, there is no
> > + * way of quickly identifying the number of tmpfs pages
> > + */
>
> I think I and you tackle the same issue.
> Please see vmscan-change-the-number-of-the-unmapped-files-in-zone-reclaim.patch in -mm.
>

Awesome. This is why I posted the patches a bit earlier than I would
normally. Stuff like this is found :D

> My intension mean, tmpfs page and swapcache increased NR_FILE_PAGES.
> but they can't be reclaimed by zone_reclaim_mode==1.
>

Sounds familiar!

> Then, I decide to use following calculation.
>
> + nr_unmapped_file_pages = zone_page_state(zone, NR_INACTIVE_FILE) +
> + zone_page_state(zone, NR_ACTIVE_FILE) -
> + zone_page_state(zone, NR_FILE_MAPPED);
>

That should now be in a helper. If I use that calculation, it'll appear
in three different places. I'll do the shuffling.

>
> > + pagecache_reclaimable = zone_page_state(zone, NR_FILE_PAGES);
> > + if (!(zone_reclaim_mode & RECLAIM_WRITE))
> > + pagecache_reclaimable -= zone_page_state(zone, NR_FILE_DIRTY);
> > + if (!(zone_reclaim_mode & RECLAIM_SWAP))
> > + pagecache_reclaimable -= zone_page_state(zone, NR_FILE_MAPPED);
>
> if you hope to solve tmpfs issue, RECLAIM_WRITE/RECLAIM_SWAP are unrelated, I think.

For reclaim_zone() to do anything useful, the pages have to be cleaned
and swapped and that needs RECLAIM_WRITE and RECLAIM_SWAP. So, how are
they unrelated?

> Plus, Could you please see vmscan-zone_reclaim-use-may_swap.patch in -mm?
> it improve RECLAIM_SWAP by another way.
>

Looking now, I'm going to rebase this patchset on top of -mm where I can
take advantage of that patch.

Thanks a lot

>
>
> >
> > /*
> > * Zone reclaim reclaims unmapped file backed pages and
> > @@ -2391,8 +2406,7 @@ int zone_reclaim(struct zone *zone, gfp_t gfp_mask, unsigned int order)
> > * if less than a specified percentage of the zone is used by
> > * unmapped file backed pages.
> > */
> > - if (zone_page_state(zone, NR_FILE_PAGES) -
> > - zone_page_state(zone, NR_FILE_MAPPED) <= zone->min_unmapped_pages
> > + if (pagecache_reclaimable <= zone->min_unmapped_pages
> > && zone_page_state(zone, NR_SLAB_RECLAIMABLE)
> > <= zone->min_slab_pages)
> > return 0;

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


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