Sorry for reposting it.
For example,
1 rdtsc() is invoked on CPU0
2 process is migrated to CPU1, and rdtsc() is invoked on CPU1
3 if TSC on CPU1 is slower than TSC on CPU0, can kernel guarantee
that the second rdtsc() doesn't return a value smaller than the one
returned by the first rdtsc()?
Thanks,
Forrest
On Fri, 2008-02-29 at 16:55 +0800, Zhao Forrest wrote:
> Sorry for reposting it.
>
> For example,
> 1 rdtsc() is invoked on CPU0
> 2 process is migrated to CPU1, and rdtsc() is invoked on CPU1
> 3 if TSC on CPU1 is slower than TSC on CPU0, can kernel guarantee
> that the second rdtsc() doesn't return a value smaller than the one
> returned by the first rdtsc()?
No, rdtsc() goes directly to the hardware. You need a (preferably cheap)
clock abstraction layer on top if you need this.
On 2/29/08, Peter Zijlstra <[email protected]> wrote:
>
> On Fri, 2008-02-29 at 16:55 +0800, Zhao Forrest wrote:
> > Sorry for reposting it.
> >
> > For example,
> > 1 rdtsc() is invoked on CPU0
> > 2 process is migrated to CPU1, and rdtsc() is invoked on CPU1
> > 3 if TSC on CPU1 is slower than TSC on CPU0, can kernel guarantee
> > that the second rdtsc() doesn't return a value smaller than the one
> > returned by the first rdtsc()?
>
> No, rdtsc() goes directly to the hardware. You need a (preferably cheap)
> clock abstraction layer on top if you need this.
Thank you for the clarification. I think gettimeofday() is such kind
of clock abstraction layer, am I right?
On Fri, 2008-02-29 at 22:20 +0800, Zhao Forrest wrote:
> On 2/29/08, Peter Zijlstra <[email protected]> wrote:
> >
> > On Fri, 2008-02-29 at 16:55 +0800, Zhao Forrest wrote:
> > > Sorry for reposting it.
> > >
> > > For example,
> > > 1 rdtsc() is invoked on CPU0
> > > 2 process is migrated to CPU1, and rdtsc() is invoked on CPU1
> > > 3 if TSC on CPU1 is slower than TSC on CPU0, can kernel guarantee
> > > that the second rdtsc() doesn't return a value smaller than the one
> > > returned by the first rdtsc()?
> >
> > No, rdtsc() goes directly to the hardware. You need a (preferably cheap)
> > clock abstraction layer on top if you need this.
>
> Thank you for the clarification. I think gettimeofday() is such kind
> of clock abstraction layer, am I right?
Yes, gtod is one such a layer, however it fails the 'cheap' test for
many definitions of cheap.