Re: [PATCH] raise tsc clocksource rating

Jeremy Fitzhardinge
2007-10-29T22:45:40Z

Zachary Amsden wrote:
> On Mon, 2007-10-29 at 20:10 -0300, Glauber de Oliveira Costa wrote:
>
>> From: Glauber de Oliveira Costa <glauber@t60.localdomain>
>>
>> tsc is very good time source (when it does not have drifts, does not
>> change it's frequency, i.e. when it works), so it should have its rating
>> raised to a value greater than, or equal 400.
>>
>> Since it's being a tendency among paravirt clocksources to use values
>> around 400, we should declare tsc as even better: So we use 500.
>>
>
> Why is the TSC better than a paravirt clocksource? In our case this is
> definitely inaccurate. Paravirt clocksources should be preferred to
> TSC, and both must be made available in hardware for platforms which do
> not support paravirt.
>
> Also, please cc all the paravirt developers on things related to
> paravirt, especially things with such broad effect. I think 400 is a
> good value for a perfect native clocksource. >400 should be reserved
> for super-real (i.e. paravirt) sources that should always be chosen over
> a hardware realistic implementation in a virtual environment.
>

Yes, agreed. The tsc is never the right thing to use if there's a
paravirt clocksource available.

What's wrong with rating it 300? What inferior clocksource does it lose
out to? Shouldn't that clocksource be lowered? (Why don't we just use
1 to 10?)

J