Re: [PATCH] raise tsc clocksource rating

Ingo Molnar
2007-10-30T07:20:02Z

* Ian Pratt <Ian.Pratt@cl.cam.ac.uk> wrote:

> > > Sigh, I don't really want to have this fight again.
> >
> > i dont remember us having discussed this before, ever. If there's any
> > "fight" about monotonicity and SMP then it would be a pretty onesided
> > affair, with you being beaten up seriously ;-)
>
> Actually, it is possible, even for NUMA systems with CPUs running off
> completely different oscillators, and in the presence of CPU frequency
> changes, power management, and even in the presence of thermal
> throttling (though the latter introduces temporary inaccuracies it
> doesn't affect monotonicity or rate).
>
> Take a look at the Xen code to see how each physical CPU is
> independently calibrated on an ongoing basis, how movement of VCPUs
> between physical CPUs is tracked, and how shared variables are used to
> ensure montonicity if a guest requires it.

I think that's wishful thinking. Check out:

http://people.redhat.com/mingo/time-warp-test/time-warp-test.c

change TEST_TSC to 0, run it on an SMP guest (on a reasonably fast
machine) and let me know whether you can make SMP guests not come up
with monotonicity violations in the CLOCK tests.

Ingo