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 _______________________________________________ Virtualization mailing list Virtualization@xxxxxxxxxxxxxxxxxxxxxxxxxx https://lists.linux-foundation.org/mailman/listinfo/virtualization
