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SubjectRe: [PATCH] mm: slowly shrink slabs with a relatively small number of objects
On Fri 31-08-18 14:31:41, Roman Gushchin wrote:
> On Fri, Aug 31, 2018 at 05:15:39PM -0400, Rik van Riel wrote:
> > On Fri, 2018-08-31 at 13:34 -0700, Roman Gushchin wrote:
> >
> > > diff --git a/mm/vmscan.c b/mm/vmscan.c
> > > index fa2c150ab7b9..c910cf6bf606 100644
> > > --- a/mm/vmscan.c
> > > +++ b/mm/vmscan.c
> > > @@ -476,6 +476,10 @@ static unsigned long do_shrink_slab(struct
> > > shrink_control *shrinkctl,
> > > delta = freeable >> priority;
> > > delta *= 4;
> > > do_div(delta, shrinker->seeks);
> > > +
> > > + if (delta == 0 && freeable > 0)
> > > + delta = min(freeable, batch_size);
> > > +
> > > total_scan += delta;
> > > if (total_scan < 0) {
> > > pr_err("shrink_slab: %pF negative objects to delete
> > > nr=%ld\n",
> >
> > I agree that we need to shrink slabs with fewer than
> > 4096 objects, but do we want to put more pressure on
> > a slab the moment it drops below 4096 than we applied
> > when it had just over 4096 objects on it?
> >
> > With this patch, a slab with 5000 objects on it will
> > get 1 item scanned, while a slab with 4000 objects on
> > it will see shrinker->batch or SHRINK_BATCH objects
> > scanned every time.
> >
> > I don't know if this would cause any issues, just
> > something to ponder.
>
> Hm, fair enough. So, basically we can always do
>
> delta = max(delta, min(freeable, batch_size));
>
> Does it look better?

Why don't you use the same heuristic we use for the normal LRU raclaim?

/*
* If the cgroup's already been deleted, make sure to
* scrape out the remaining cache.
*/
if (!scan && !mem_cgroup_online(memcg))
scan = min(size, SWAP_CLUSTER_MAX);

--
Michal Hocko
SUSE Labs

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