linux-kernel - Re: x86 - cpu_relax

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Message-ID: <20100207180949.GA26388@8bytes.org>
Date:	Sun, 7 Feb 2010 19:09:49 +0100
From:	Joerg Roedel <joro@...tes.org>
To:	Michael Breuer <mbreuer@...jas.com>
Cc:	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: x86 - cpu_relax - why nop vs. pause?

On Sun, Feb 07, 2010 at 12:28:51PM -0500, Michael Breuer wrote:
> I did search and noticed some old discussions. Looking at both Intel and  
> AMD documentation, it would seem that PAUSE is the preferred instruction  
> within a spin lock. Further, both Intel and AMD specifications state  
> that the instruction is backward compatible with older x86 processors.

Its not the primary reason, but the hardware virtualization extensions
of x86 processors support an intercept after a configured amount of
pause instructions were executed. This is used to detect spinning vcpus
where the lock-holder is scheduled out.

> For fun, I changed nop to pause on my core i7 920 (smt enabled) and I'm  
> seeing about a 5-10% performance improvement on 2.6.33 rc7. Perf top  
> shows time spent in spin_lock under load drops from an average of around  
> 35% to about 25%.

What benchmarks have you used for your measurements?

	Joerg

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