Performance overhead of get_cycles_sync

Dor Laor
2007-12-11T13:11:53Z

Hi Ingo, Thomas,

In the latest kernel (2.6.24-rc3) I noticed a drastic performance
decrease for KVM networking.
The reason is many vmexit (exit reason is cpuid instruction) caused by
calls to gettimeofday that uses tsc sourceclock.
read_tsc calls get_cycles_sync which might call cpuid in order to
serialize the cpu.

Can you explain why the cpu needs to be serialized for every gettime call?
Do we need to be that accurate? (It will also slightly improve physical
hosts).
I believe you have a reason and the answer is yes. In that case can you
replace the serializing instruction
with an instruction that does not trigger vmexit? Maybe use 'ltr' for
example?

Regards,
Dor.