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

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Date:	Mon, 2 Jun 2014 10:24:09 -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 Sat, 31 May 2014, Vladimir Davydov wrote:

> > 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.

But that is only when you want to destroy a cache. This is similar.

> 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.

I am not sure that there is such a stark difference to SLUB. SLUB also
takes the per node lock if necessary to handle freeing especially if you
zap the per cpu partial slab pages.
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