linux-kernel - Re: [PATCH -mm v2 8/8] slab: make dead memcg caches discard free slabs immediately

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Message-ID: <20140612065345.GD19918@js1304-P5Q-DELUXE>
Date:	Thu, 12 Jun 2014 15:53:45 +0900
From:	Joonsoo Kim <iamjoonsoo.kim@....com>
To:	Vladimir Davydov <vdavydov@...allels.com>
Cc:	Christoph Lameter <cl@...two.org>, akpm@...ux-foundation.org,
	rientjes@...gle.com, penberg@...nel.org, hannes@...xchg.org,
	mhocko@...e.cz, linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH -mm v2 8/8] slab: make dead memcg caches discard free
 slabs immediately

On Thu, Jun 12, 2014 at 01:24:34AM +0400, Vladimir Davydov wrote:
> On Tue, Jun 10, 2014 at 07:18:34PM +0400, Vladimir Davydov wrote:
> > On Tue, Jun 10, 2014 at 09:26:19AM -0500, Christoph Lameter wrote:
> > > On Tue, 10 Jun 2014, Vladimir Davydov wrote:
> > > 
> > > > Frankly, I incline to shrinking dead SLAB caches periodically from
> > > > cache_reap too, because it looks neater and less intrusive to me. Also
> > > > it has zero performance impact, which is nice.
> > > >
> > > > However, Christoph proposed to disable per cpu arrays for dead caches,
> > > > similarly to SLUB, and I decided to give it a try, just to see the end
> > > > code we'd have with it.
> > > >
> > > > I'm still not quite sure which way we should choose though...
> > > 
> > > Which one is cleaner?
> > 
> > To shrink dead caches aggressively, we only need to modify cache_reap
> > (see https://lkml.org/lkml/2014/5/30/271).
> 
> Hmm, reap_alien, which is called from cache_reap to shrink per node
> alien object arrays, only processes one node at a time. That means with
> the patch I gave a link to above it will take up to
> (REAPTIMEOUT_AC*nr_online_nodes) seconds to destroy a virtually empty
> dead cache, which may be quite long on large machines. Of course, we can
> make reap_alien walk over all alien caches of the current node, but that
> will probably hurt performance...

Hmm, maybe we have a few of objects on other node, doesn't it?

BTW, I have a question about cache_reap(). If there are many kmemcg
users, we would have a lot of slab caches and just to traverse slab
cache list could take some times. Is it no problem?

Thanks.

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