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

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Date:	Sat, 31 May 2014 15:19:23 +0400
From:	Vladimir Davydov <vdavydov@...allels.com>
To:	Christoph Lameter <cl@...two.org>
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, May 30, 2014 at 10:01:26AM -0500, Christoph Lameter wrote:
> On Fri, 30 May 2014, Vladimir Davydov wrote:
> 
> > 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.

In contrast to SLUB, for SLAB this will slow down kfree significantly.
Fast path for SLAB is just putting an object to a per cpu array, while
the slow path requires taking a per node lock, which is much slower even
with no contention. There still can be lots of objects in a dead memcg
cache (e.g. hundreds of megabytes of dcache), so such performance
degradation is not acceptable, IMO.

OTOH, we already have cache_reap running periodically for each cache.
Making it drain all free objects in dead caches won't impact performance
at all, neither will it complicate the code. The only downside is a dead
cache won't be destroyed immediately after it becomes unused, but since
cache_reap runs pretty often (each several secs), it shouldn't result in
any problems, I guess.

Thanks.
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