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

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Date:	Tue, 10 Jun 2014 19:18:34 +0400
From:	Vladimir Davydov <vdavydov@...allels.com>
To:	Christoph Lameter <cl@...two.org>
CC:	Joonsoo Kim <iamjoonsoo.kim@....com>, <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 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).

To zap object arrays for dead caches (this is what this patch does), we
have to:
 - set array_cache->limit to 0 for each per cpu, shared, and alien array
   caches on kmem_cache_shrink;
 - make cpu/node hotplug paths init new array cache sizes to 0;
 - make free paths (__cache_free, cache_free_alien) handle zero array
   cache size properly, because currently they doesn't.

So IMO the first one (reaping dead caches periodically) requires less
modifications and therefore is cleaner.

Thanks.
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