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

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Date:	Thu, 28 Jan 2016 13:51:28 +0900
From:	Joonsoo Kim <iamjoonsoo.kim@....com>
To:	Vlastimil Babka <vbabka@...e.cz>
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 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.

> 
> freeing object obj:
> *obj = page->freelist;
> page->freelist = obj;
> 
> allocating object:
> obj = page->freelist;
> page->freelist = *obj;
> *obj = NULL;
> 
> That means two writes, but if we omit managing page->active, it's not an

It's not just matter of number of instructions as explained above. Touching
more cache line should also be avoided.

Thanks.