x86 - cpu_relax - why nop vs. pause?

[Michael Breuer]

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.

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%.

Thoughts?
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