From: Michal Hocko <mhocko@kernel.org>
To: Roman Gushchin <guro@fb.com>
Cc: Rik van Riel <riel@surriel.com>,
linux-mm@kvack.org, linux-kernel@vger.kernel.org,
kernel-team@fb.com, Josef Bacik <jbacik@fb.com>,
Johannes Weiner <hannes@cmpxchg.org>,
Andrew Morton <akpm@linux-foundation.org>
Subject: Re: [PATCH] mm: slowly shrink slabs with a relatively small number of objects
Date: Tue, 4 Sep 2018 18:14:31 +0200 [thread overview]
Message-ID: <20180904161431.GP14951@dhcp22.suse.cz> (raw)
In-Reply-To: <20180904153445.GA22328@tower.DHCP.thefacebook.com>
On Tue 04-09-18 08:34:49, Roman Gushchin wrote:
> On Tue, Sep 04, 2018 at 09:00:05AM +0200, Michal Hocko wrote:
> > On Mon 03-09-18 13:28:06, Roman Gushchin wrote:
> > > On Mon, Sep 03, 2018 at 08:29:56PM +0200, Michal Hocko wrote:
> > > > 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?
> > >
> > > Because we do reparent kmem lru lists on offlining.
> > > Take a look at memcg_offline_kmem().
> >
> > Then I must be missing something. Why are we growing the number of dead
> > cgroups then?
>
> We do reparent LRU lists, but not objects. Objects (or, more precisely, pages)
> are still holding a reference to the memcg.
OK, this is what I missed. I thought that the reparenting includes all
the pages as well. Is there any strong reason that we cannot do that?
Performance/Locking/etc.?
Or maybe do not reparent at all and rely on the same reclaim heuristic
we do for normal pages?
I am not opposing your patch but I am trying to figure out whether that
is the best approach.
--
Michal Hocko
SUSE Labs
next prev parent reply other threads:[~2018-09-04 16:14 UTC|newest]
Thread overview: 13+ messages / expand[flat|nested] mbox.gz Atom feed top
2018-08-31 20:34 [PATCH] mm: slowly shrink slabs with a relatively small number of objects Roman Gushchin
2018-08-31 21:15 ` Rik van Riel
2018-08-31 21:31 ` Roman Gushchin
2018-09-01 1:27 ` Rik van Riel
2018-09-03 18:29 ` Michal Hocko
2018-09-03 20:28 ` Roman Gushchin
2018-09-04 7:00 ` Michal Hocko
2018-09-04 15:34 ` Roman Gushchin
2018-09-04 16:14 ` Michal Hocko [this message]
2018-09-04 17:52 ` Roman Gushchin
2018-09-04 18:06 ` Michal Hocko
2018-09-04 18:07 ` Michal Hocko
2018-09-04 20:34 ` Vladimir Davydov
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