lkml.org 
[lkml]   [2007]   [Dec]   [11]   [last100]   RSS Feed
Views: [more markup]  [less markup]  [headers]  [forward] 
 
Messages in this thread
/
DateTue, 11 Dec 2007 14:37:38 +0100
FromIngo Molnar <>
SubjectRe: Performance overhead of get_cycles_sync
* 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


\
 
 \ /
  Last update: 2007-12-11 14:41    [from the cache]
©2003-2008