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

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Date:	Fri, 29 Jan 2016 16:21:30 +0100
From:	Vlastimil Babka <vbabka@...e.cz>
To:	Joonsoo Kim <iamjoonsoo.kim@....com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Christoph Lameter <cl@...ux.com>,
	Pekka Enberg <penberg@...nel.org>,
	David Rientjes <rientjes@...gle.com>,
	Jesper Dangaard Brouer <brouer@...hat.com>,
	linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 16/16] mm/slab: introduce new slab management type,
 OBJFREELIST_SLAB

On 01/28/2016 05:51 AM, Joonsoo Kim wrote:
> On Wed, Jan 27, 2016 at 02:35:04PM +0100, Vlastimil Babka wrote:
>> On 01/14/2016 06:24 AM, Joonsoo Kim wrote:
>> > In fact, I tested another idea implementing OBJFREELIST_SLAB with
>> > extendable linked array through another freed object. It can remove
>> > memory waste completely but it causes more computational overhead
>> > in critical lock path and it seems that overhead outweigh benefit.
>> > So, this patch doesn't include it.
>> 
>> Can you elaborate? Do we actually need an extendable linked array? Why not just
>> store the pointer to the next free object into the object, NULL for the last
>> one? I.e. a singly-linked list. We should never need to actually traverse it?
> 
> As Christoph explained, it's the way SLUB manages freed objects. In SLAB
> case, it doesn't want to touch object itself. It's one of main difference
> between SLAB and SLUB. These objects are cache-cold now so touching object itself
> could cause more cache footprint.

Hm I see. Although I wouldn't bet on whether the now-freed object is more or
less cold than the freelist array itself (regardless of its placement) :)