Re: x86 - cpu_relax - why nop vs. pause?

Joerg Roedel
2010-02-07T18:09:52Z

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