x86 - cpu_relax - why nop vs. pause?

Michael Breuer
2010-02-07T17:28:56Z

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?