linux-kernel - Re: [PATCH v2 16/17] mm/slab: introduce new slab management type, OBJFREELIST

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Date:	Fri, 26 Feb 2016 10:21:24 -0600 (CST)
From:	Christoph Lameter <cl@...ux.com>
To:	js1304@...il.com
cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Pekka Enberg <penberg@...nel.org>,
	David Rientjes <rientjes@...gle.com>,
	Jesper Dangaard Brouer <brouer@...hat.com>,
	Vlastimil Babka <vbabka@...e.cz>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org, Joonsoo Kim <iamjoonsoo.kim@....com>
Subject: Re: [PATCH v2 16/17] mm/slab: introduce new slab management type,
 OBJFREELIST_SLAB

On Fri, 26 Feb 2016, js1304@...il.com wrote:

> Although this idea can apply to all caches whose size is larger than
> management array size, it isn't applied to caches which have a
> constructor.  If such cache's object is used for management array,
> constructor should be called for it before that object is returned to
> user.  I guess that overhead overwhelm benefit in that case so this idea
> doesn't applied to them at least now.

Caches which have a constructor (or are used with SLAB_RCU_FREE) have a
defined content even when they are free. Therefore they cannot be used
for the freelist.

> For summary, from now on, slab management type is determined by
> following logic.
>
> 1) if management array size is smaller than object size and no ctor, it
>    becomes OBJFREELIST_SLAB.

Also do not do this for RCU slabs.

> 2) if management array size is smaller than leftover, it becomes
>    NORMAL_SLAB which uses leftover as a array.
>
> 3) if OFF_SLAB help to save memory than way 4), it becomes OFF_SLAB.
>    It allocate a management array from the other cache so memory waste
>    happens.

Wonder how many of these ugly off slabs are left after what you did here.

> TOTAL = OBJFREELIST + NORMAL(leftover) + NORMAL + OFF
>
> /Before/
> 126 = 0 + 60 + 25 + 41
>
> /After/
> 126 = 97 + 12 + 15 + 2
>
> Result shows that number of caches that doesn't waste memory increase
> from 60 to 109.

Great results.

> v2: fix SLAB_DESTROTY_BY_RCU cache type handling

Ok how are they handled now? Do not see that dealt with in the patch.