[time-nuts] x86 CPU Timekeeping and clock generation

[Poul-Henning Kamp]

--------
Magnus Danielson writes:

> If the actual clock is more than 15 ppm off from the value in the drift
> file, it will never track in the frequency error. This is due to
> algorithm error in the NTPD. I have pointed out this problem, but there
> have been very little interest in fixing it.

You're thinking of the integrator-windup issue ?

Yeah, that's very old news.  I think I first documented it to Dave
Mills in 1994, but I never managed to convince him to take my fix
(Clamp I-term until D-term has the correct sign for the resiudal).

That's when I started writing my own PLLs.

In general, the driftfile should never be used if NTP has a network
connection, it's raison-d'être was primarily the machines syncing
time via telephone twice per day.


PS: The drift file also soak up rounding/truncation errors in the
    kernels timekeeping code.

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