linux-kernel - Re: [patch 09/10] Remove the SLOB allocator for 2.6.23

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Date:	Tue, 10 Jul 2007 11:17:36 +0300 (EEST)
From:	Pekka J Enberg <penberg@...helsinki.fi>
To:	Christoph Lameter <clameter@....com>
cc:	Nick Piggin <nickpiggin@...oo.com.au>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Ingo Molnar <mingo@...e.hu>, linux-kernel@...r.kernel.org,
	linux-mm@...ck.org, suresh.b.siddha@...el.com,
	corey.d.gough@...el.com, Matt Mackall <mpm@...enic.com>,
	Denis Vlasenko <vda.linux@...glemail.com>,
	Erik Andersen <andersen@...epoet.org>
Subject: Re: [patch 09/10] Remove the SLOB allocator for 2.6.23

Hi Christoph,

On Mon, 9 Jul 2007, Pekka Enberg wrote:
> > I assume with "slab external fragmentation" you mean allocating a
> > whole page for a slab when there are not enough objects to fill the
> > whole thing thus wasting memory? We could try to combat that by
> > packing multiple variable-sized slabs within a single page. Also,
> > adding some non-power-of-two kmalloc caches might help with internal
> > fragmentation.

On Mon, 9 Jul 2007, Christoph Lameter wrote:
> Ther are already non-power-of-two kmalloc caches for 96 and 192 bytes 
> sizes.

I know that, but for my setup at least, there seems to be a need for a 
non-power of two cache between 512 and 1024. What I am seeing is average 
allocation size for kmalloc-512 being around 270-280 which wastes total 
of 10 KB of memory due to internal fragmentation. Might be a buggy caller 
that can be fixed with its own cache too.

On Mon, 9 Jul 2007, Pekka Enberg wrote:
> > In any case, SLUB needs some serious tuning for smaller machines
> > before we can get rid of SLOB.

On Mon, 9 Jul 2007, Christoph Lameter wrote:
> Switch off CONFIG_SLUB_DEBUG to get memory savings.

Curious, /proc/meminfo immediately after boot shows:

SLUB (debugging enabled):

(none):~# cat /proc/meminfo 
MemTotal:        30260 kB
MemFree:         22096 kB

SLUB (debugging disabled):

(none):~# cat /proc/meminfo 
MemTotal:        30276 kB
MemFree:         22244 kB

SLOB:

(none):~# cat /proc/meminfo 
MemTotal:        30280 kB
MemFree:         22004 kB

That's 92 KB advantage for SLUB with debugging enabled and 240 KB when 
debugging is disabled.

Nick, Matt, care to retest SLUB and SLOB for your setups?

				Pekka
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