linux-kernel - Re: [PATCH -mm 8/8] slab: reap dead memcg caches aggressively

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Date:	Fri, 30 May 2014 10:01:26 -0500 (CDT)
From:	Christoph Lameter <cl@...two.org>
To:	Vladimir Davydov <vdavydov@...allels.com>
cc:	akpm@...ux-foundation.org, hannes@...xchg.org, mhocko@...e.cz,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH -mm 8/8] slab: reap dead memcg caches aggressively

On Fri, 30 May 2014, Vladimir Davydov wrote:

> There is no use in keeping free objects/slabs on dead memcg caches,
> because they will never be allocated. So let's make cache_reap() shrink
> as many free objects from such caches as possible.
>
> Note the difference between SLAB and SLUB handling of dead memcg caches.
> For SLUB, dead cache destruction is scheduled as soon as the last object
> is freed, because dead caches do not cache free objects. For SLAB, dead
> caches can keep some free objects on per cpu arrays, so that an empty
> dead cache will be hanging around until cache_reap() drains it.

Calling kmem_cache_shrink() should drain all caches though. Reduce the
size of the queues to zero or so before calling shrink so that no new
caches are build up?

> We don't disable free objects caching for SLAB, because it would force
> kfree to always take a spin lock, which would degrade performance
> significantly.

You can use a similar approach than in SLUB. Reduce the size of the per
cpu array objects to zero. Then SLAB will always fall back to its slow
path in cache_flusharray() where you may be able to do something with less
of an impact on performace.
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