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

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Message-ID: <20070821234702.GE29691@Krystal>
Date:	Tue, 21 Aug 2007 19:47:02 -0400
From:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>
To:	Christoph Lameter <clameter@....com>
Cc:	akpm@...ux-foundation.org, linux-kernel@...r.kernel.org,
	mingo@...hat.com
Subject: Re: [PATCH] SLUB use cmpxchg_local

* Christoph Lameter (clameter@....com) wrote:
> On Tue, 21 Aug 2007, Mathieu Desnoyers wrote:
> 
> > Using cmpxchg_local vs cmpxchg has a clear impact on the fast paths, as
> > shown below: it saves about 60 to 70 cycles for kmalloc and 200 cycles
> > for the kmalloc/kfree pair (test 2).
> 
> Hmmmm.. I wonder if the AMD processors simply do the same in either 
> version.

No supposed to. I remember having posted numbers that show a
difference.

Are you running a UP or SMP kernel ? If you run a UP kernel, the
cmpxchg_local and cmpxchg are identical.

Oh, and if you run your tests at boot time, the alternatives code may
have removed the lock prefix, therefore making cmpxchg and cmpxchg_local
exactly the same.

Mathieu

-- 
Mathieu Desnoyers
Computer Engineering Ph.D. Student, Ecole Polytechnique de Montreal
OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F  BA06 3F25 A8FE 3BAE 9A68
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