Messages in this thread | | | Date | Tue, 11 Dec 2007 14:37:38 +0100 | From | Ingo Molnar <> | Subject | Re: Performance overhead of get_cycles_sync |
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* Dor Laor <dor.laor@gmail.com> wrote:
> 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?
hm, where exactly does it call CPUID?
Ingo
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