Re: [PATCH v2] mm: slowly shrink slabs with a relatively small number of objects

[Shakeel Butt <shakeelb@google.com>]

On Wed, Sep 5, 2018 at 2:23 PM Roman Gushchin <guro@fb.com> wrote:
>
> On Wed, Sep 05, 2018 at 01:51:52PM -0700, Andrew Morton wrote:
> > On Tue, 4 Sep 2018 15:47:07 -0700 Roman Gushchin <guro@fb.com> wrote:
> >
> > > Commit 9092c71bb724 ("mm: use sc->priority for slab shrink targets")
> > > changed the way how the target slab pressure is calculated and
> > > made it priority-based:
> > >
> > >     delta = freeable >> priority;
> > >     delta *= 4;
> > >     do_div(delta, shrinker->seeks);
> > >
> > > The problem is that on a default priority (which is 12) no pressure
> > > is applied at all, if the number of potentially reclaimable objects
> > > is less than 4096 (1<<12).
> > >
> > > This causes the last objects on slab caches of no longer used cgroups
> > > to never get reclaimed, resulting in dead cgroups staying around forever.
> >
> > But this problem pertains to all types of objects, not just the cgroup
> > cache, yes?
>
> Well, of course, but there is a dramatic difference in size.
>
> Most of these objects are taking few hundreds bytes (or less),
> while a memcg can take few hundred kilobytes on a modern multi-CPU
> machine. Mostly due to per-cpu stats and events counters.
>

Beside memcg, all of its kmem caches, most empty, are stuck in memory
as well. For SLAB even the memory overhead of an empty kmem cache is
not negligible.

Shakeel