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SubjectRe: [RFC][PATCH 6/6] PM/Hibernate: Do not try to allocate too much memory too hard
On Sun, May 17, 2009 at 08:55:05PM +0800, Rafael J. Wysocki wrote:
> On Sunday 17 May 2009, Wu Fengguang wrote:

> > > +static unsigned long minimum_image_size(unsigned long saveable)
> > > +{
> > > + unsigned long size;
> > > +
> > > + /* Compute the number of saveable pages we can free. */
> > > + size = global_page_state(NR_SLAB_RECLAIMABLE)
> > > + + global_page_state(NR_ACTIVE_ANON)
> > > + + global_page_state(NR_INACTIVE_ANON)
> > > + + global_page_state(NR_ACTIVE_FILE)
> > > + + global_page_state(NR_INACTIVE_FILE);
> >
> > For example, we could drop the 1.25 ratio and calculate the above
> > reclaimable size with more meaningful constraints:
> >
> > /* slabs are not easy to reclaim */
> > size = global_page_state(NR_SLAB_RECLAIMABLE) / 2;
>
> Why 1/2?

Also a very coarse value:
- we don't want to stress icache/dcache too much
(unless they grow too large)
- my experience was that the icache/dcache are scanned in a slower
pace than lru pages.
- most importantly, inside the NR_SLAB_RECLAIMABLE pages, maybe half
of the pages are actually *in use* and cannot be freed:
% cat /proc/sys/fs/inode-nr
30450 16605
% cat /proc/sys/fs/dentry-state
41598 35731 45 0 0 0
See? More than half entries are in-use. Sure many of them will actually
become unused when dentries are freed, but in the mean time the internal
fragmentations in the slabs can go up.

> > /* keep NR_ACTIVE_ANON */
> > size += global_page_state(NR_INACTIVE_ANON);
>
> Why exactly did you omit ACTIVE_ANON?

To keep the "core working set" :)

> > /* keep mapped files */
> > size += global_page_state(NR_ACTIVE_FILE);
> > size += global_page_state(NR_INACTIVE_FILE);
> > size -= global_page_state(NR_FILE_MAPPED);
> >
> > That restores the hard core working set logic in the reverse way ;)
>
> I think the 1/2 factor for NR_SLAB_RECLAIMABLE may be too high in some cases,
> but I'm going to check that.

Yes, after updatedb. In that case simple magics numbers may not help.
In that case we should really first call shrink_slab() in a loop to
cut down the slab pages to a sane number.

Thanks,
Fengguang


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