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John Sigler wrote:

quoted

I'm using 2.6.20.7-rt8 on a P3.

(But some ntpd users have reported the problem with mainline kernels and 
different versions.)

quoted

I've noticed that the frequency offset of my system clock (computed 
either by ntpd, or by hand) changes drastically across reboots.

By drastically, I mean +/- 60 ppm every time I reboot.

Apparently this is caused by some imprecision in the frequency 
estimation done in calculate_cpu_khz() (arch/i386/kernel/tsc.c)

My brutally crude work-around was to skip the computation altogether 
and hard-code the "correct" value for cpu_khz.

http://linux.kernel.free.fr/cpu_khz.adlink

Has this issue been investigated more thoroughly by other users?

Is there, perhaps, more appropriate a forum to discuss such matters?

Regards.