Re: [PATCH 1/3] Properly account for the number of page cachepages zone_reclaim() can reclaim

From: Mel Gorman
Date: Mon Jun 15 2009 - 06:05:58 EST


On Mon, Jun 15, 2009 at 01:51:16PM +0900, KOSAKI Motohiro wrote:
> > > > +/* Work out how many page cache pages we can reclaim in this reclaim_mode */
> > > > +static long zone_pagecache_reclaimable(struct zone *zone)
> > > > +{
> > > > + long nr_pagecache_reclaimable;
> > > > + long delta = 0;
> > > > +
> > > > + /*
> > > > + * If RECLAIM_SWAP is set, then all file pages are considered
> > > > + * potentially reclaimable. Otherwise, we have to worry about
> > > > + * pages like swapcache and zone_unmapped_file_pages() provides
> > > > + * a better estimate
> > > > + */
> > > > + if (zone_reclaim_mode & RECLAIM_SWAP)
> > > > + nr_pagecache_reclaimable = zone_page_state(zone, NR_FILE_PAGES);
> > > > + else
> > > > + nr_pagecache_reclaimable = zone_unmapped_file_pages(zone);
> > > > +
> > > > + /* If we can't clean pages, remove dirty pages from consideration */
> > > > + if (!(zone_reclaim_mode & RECLAIM_WRITE))
> > > > + delta += zone_page_state(zone, NR_FILE_DIRTY);
> > >
> > > no use delta?
> > >
> >
> > delta was used twice in an interim version when it was possible to overflow
> > the counter. I left it as is because if another counter is added that must
> > be subtracted from nr_pagecache_reclaimable, it'll be tidier to patch in if
> > delta was there. I can take it out if you prefer.
>
> Honestly, I'm confusing now.
>
> your last version have following usage of "delta"
>
> /* Beware of double accounting */
> if (delta < nr_pagecache_reclaimable)
> nr_pagecache_reclaimable -= delta;
>
> but current your patch don't have it.
> IOW, nobody use delta variable. I'm not sure about you forget to
> accurate to nr_pagecache_reclaimable or forget to remove
> "delta += zone_page_state(zone, NR_FILE_DIRTY);" line.
>
> Or, Am I missing anything?
> Now, I don't oppose this change. I only hope to clarify your intention.
>

You're not missing anything. I accidentally removed where delta was
being used. Sorry.

>
>
> > > > - 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);
> > > > -
> > > > - if (nr_unmapped_file_pages > zone->min_unmapped_pages) {
> > > > + if (zone_pagecache_reclaimable(zone) > zone->min_unmapped_pages) {
> > >
> > > Documentation/sysctl/vm.txt says
> > > =============================================================
> > >
> > > min_unmapped_ratio:
> > >
> > > This is available only on NUMA kernels.
> > >
> > > A percentage of the total pages in each zone. Zone reclaim will only
> > > occur if more than this percentage of pages are file backed and unmapped.
> > > This is to insure that a minimal amount of local pages is still available for
> > > file I/O even if the node is overallocated.
> > >
> > > The default is 1 percent.
> > >
> > > ==============================================================
> > >
> > > but your code condider more addional thing. Can you please change document too?
> > >
> >
> > How does this look?
> >
> > ==============================================================
> > min_unmapped_ratio:
> >
> > This is available only on NUMA kernels.
> >
> > This is a percentage of the total pages in each zone. Zone reclaim will only
> > occur if more than this percentage are in a state that zone_reclaim_mode
> > allows to be reclaimed.
> >
> > If zone_reclaim_mode has the value 4 OR'd, then the percentage is compared
> > against all file-backed unmapped pages including swapcache pages and tmpfs
> > files. Otherwise, only unmapped pages backed by normal files but not tmpfs
> > files and similar are considered.
> >
> > The default is 1 percent.
> > ==============================================================
>
> Great! thanks.
>

I'll prepare two patches after reviewing mmotm. The first will use delta
properly and the second will fix the documentation. Thanks

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