Re: [PATCH v6 25/29] memcg/sl[au]b: shrink dead caches

Andrew Morton
2012-11-08T22:40:46Z

On Thu, 8 Nov 2012 23:31:16 +0100
Glauber Costa <glommer@parallels.com> wrote:

> On 11/08/2012 08:21 PM, Andrew Morton wrote:
> > On Thu, 8 Nov 2012 17:15:36 +0000
> > Christoph Lameter <cl@linux.com> wrote:
> >
> >> On Wed, 7 Nov 2012, Andrew Morton wrote:
> >>
> >>> What's up with kmem_cache_shrink? It's global and exported to modules
> >>> but its only external caller is some weird and hopelessly poorly
> >>> documented site down in drivers/acpi/osl.c. slab and slob implement
> >>> kmem_cache_shrink() *only* for acpi! wtf? Let's work out what acpi is
> >>> trying to actually do there, then do it properly, then killkillkill!
> >>
> >> kmem_cache_shrink is also used internally. Its simply releasing unused
> >> cached objects.
> >
> > Only in slub. It could be removed outright from the others and
> > simplified in slub.
> >
> >>> Secondly, as slab and slub (at least) have the ability to shed cached
> >>> memory, why aren't they hooked into the core cache-shinking machinery.
> >>> After all, it's called "shrink_slab"!
> >>
> >> Because the core cache shrinking needs the slab caches to free up memory
> >> from inodes and dentries. We could call kmem_cache_shrink at the end of
> >> the shrink passes in vmscan. The price would be that the caches would have
> >> to be repopulated when new allocations occur.
> >
> > Well, the shrinker shouldn't strips away all the cache. It will perform
> > a partial trim, the magnitude of which increases with perceived
> > external memory pressure.
> >
> > AFACIT, this is correct and desirable behaviour for shrinking
> > slab's internal caches.
> >
>
> I believe calling this from shrink_slab() is not a bad idea at all. If
> you're all in favour, I'll cook a patch for this soon

It sounds like a pretty big change but yes, well worth exploring.

I'd still like to give ACPI a thwap. That kmem_cache_shrink() in
drivers/acpi/osl.c was added unchangelogged in a megapatch
(73459f73e5d1602c59) so it's a mystery. Cc's optimistically added.