linux-kernel - Re: [PATCH] SLUB use cmpxchg

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Message-ID: <Pine.LNX.4.64.0708211404290.3082@schroedinger.engr.sgi.com>
Date:	Tue, 21 Aug 2007 14:08:37 -0700 (PDT)
From:	Christoph Lameter <clameter@....com>
To:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>
cc:	akpm@...ux-foundation.org, linux-kernel@...r.kernel.org,
	mingo@...hat.com
Subject: Re: [PATCH] SLUB use cmpxchg_local

On Tue, 21 Aug 2007, Mathieu Desnoyers wrote:

> - Fixed an erroneous test in slab_free() (logic was flipped from the 
>   original code when testing for slow path. It explains the wrong 
>   numbers you have with big free).

If you look at the numbers that I posted earlier then you will see that 
even the measurements without free were not up to par.

> It applies on top of the 
> "SLUB Use cmpxchg() everywhere" patch.

Which one is that?

>              |     slab.git HEAD slub (min-max)    |  cmpxchg_local slub
> kmalloc(8)   |         190 - 201                   |         83
> kfree(8)     |         351 - 351                   |        363
> kmalloc(64)  |         224 - 245                   |        115
> kfree(64)    |         389 - 394                   |        397
> kmalloc(16384)|        713 - 741                   |        724
> kfree(16384) |         843 - 856                   |        843
> 
> Therefore, there seems to be a repeatable gain on the kmalloc fast path
> (more than twice faster). No significant performance hit for the kfree
> case, but no gain neither, same for large kmalloc, as expected.

There is a consistent loss on slab_free it seems. The 16k numbers are 
irrelevant since we do not use slab_alloc/slab_free due to the direct pass 
through patch but call the page allocator directly. That also explains 
that there is no loss there.

The kmalloc numbers look encouraging. I will check to see if I can 
reproduce it once I sort out the patches.
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