sys_fuzzMime-Version: 1.0

[Achim Gratz]

Eric S. Raymond writes:
> Stiction in this context = "adjacent clock reads could get back the
> same value", is that right?  Suddenly a whole bunch of things, like
> the implications of only updating the clock on a scheduler interrupt,
> make sense.

Yes, either the same value or a value that has increased by a token
amount that doesn't correspond to the actual increment in time that has
passed between the two clock readings.

> Therefore I *deduce* that the PLL correction (the one NTP does, not
> the in-kernel one Hal tells us is associated with PPS) requires a
> monotonically increasing clock.  It's the simplest explanation for the
> way libntp/systime.c works, and it explains *everything* that has puzzled
> me about that code.

The thing the PLL (more specifically the loop filter) should care about
is that the error estimate it makes is unbiased and has a (relatively)
white spectrum.  That's exactly what doesn't happen when you have a
clock that jumps and you try to read it several times inbetween those
jumps.


Regards,
Achim.
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