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

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Date:   Mon, 3 Sep 2018 20:29:56 +0200
From:   Michal Hocko <mhocko@...nel.org>
To:     Roman Gushchin <guro@...com>
Cc:     Rik van Riel <riel@...riel.com>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, kernel-team@...com,
        Josef Bacik <jbacik@...com>,
        Johannes Weiner <hannes@...xchg.org>,
        Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [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