On Mon, Aug 26, 2002 at 11:45:36AM -0700, Martin J. Bligh wrote:
> >> And following your argument that these apps have been silenty broken
> >> since 1999, if there's no broken app out there, nobody will ever get the
> >> instruction fault. If there's any app broken out there we probably like
> >
> > No. rdtsc is still usefull if you are clever and statistically filter
> > out. Also rdtsc provides you number of cycles, so if you want to know
> > how many cycles mov %eax,%ebx takes, you can do that even on
> > speedstep. Anything that correlates rdtsc to real time is broken, however.
>
> It's not correlating it to real time that's the problem. It's getting resceduled
> inbetween calls that hurts. Take your example.
>
> rdtsc
> mov %eax,%ebx
> <- get rescheduled here
> rdtsc
>
> Broken. May even take negative "time".
you need to save %edx too, then it would be perfectly safe on a
synchronized TSC hardware (as far as the reschedule doesn't take more
than 2^64 ticks).
Andrea
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