Zachary Amsden wrote:
>
> I'm not suggesting using the nominal value. I'm suggesting the
> measurement be done in the one and only place where there is perfect
> control of the system, the processor boot-strapping in the BIOS.
>
> Only the platform designers themselves know the speed of the oscillator
> which is modulating the clock and so only they should be calibrating the
> speed of the TSC.
>
No. *Noone*, including the manufacturers, know the speed of the
oscillator which is modulating the clock. What you have to do is
average over a timespan which is long enough that the SSM averages out
(a relatively small fraction of a second.)
As for trusting the BIOS on this, that's a total joke. Firmware vendors
can't get the most basic details right.
> If this modulation really does alter the frequency by +/- 2% (seems high
> to me, but hey, I don't design motherboards), using an LFO, then
> basically all the calibration done in Linux is broken and has been for
> some time. You can't calibrate only once, or risk being off by 2%, you
> can't calibrate repeatedly and take the fastest estimate, or you are off
> by 2%, and you can't calibrate repeatedly and take the average without
> risking SMI noise affecting the lowest clock speed measurement,
> contributing unknown error.
You have to calibrate over a sample interval long enough that the SSM
averages out.
> Hmm. Re-reading your e-mail, I see you are saying the nominal frequency
> may be off by 2% (and I easily believe that), not necessarily that the
> frequency modulation may be 2% (which I still think is high). Does
> anyone know what the actual bounds on spread spectrum modulation are or
> how fast the clock is modulated?
No, I'm saying the frequency modulation may be up to 2%. Typically it
is something like [-2%,+0%].
-hpa
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