Re: [PATCH] raise tsc clocksource rating

[Jeremy Fitzhardinge <jeremy@goop.org>]

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