[chrony-users] Specifying refclock accuracy

[James Clark <jjc@xxxxxxxxxx>]

I'm a bit unclear about how a refclock is supposed to specify how
accurate it is. There are the delay and precision options. I think I
understand what the docs say about delay. But I'm less clear about
precision. What are the semantics of that, how does it impact what
chrony does and how is it related to delay? The other thing I'm not
clear about is how the precision and delay relate to the figure shown
for the margin of error (after +/- in the last column of the chrony
sources output).

As a concrete example, for a PHC extpps source (with no link to
anything) connected to the PPS output of a GPS whose datasheet
specifies a 30ns RMS accuracy, what should I specify for delay and
precision and how do I get a sensible figure for margin of error? I've
tried various figures in the range 10ns-100ns for delay and precision,
but the margin of error remains in the 700ns-800ns range. The margin
of error figure is similar to the dispersion figure in the refclocks
log, so I'm guessing that's the main cause. Why is the dispersion so
large and is there anything that can be done to fix it?

At the other end of the scale, for a SOCK refclock for GPS messages
where there's say 0.1s variation in the time the message is received,
what is the right thing to put in refclock so that chrony won't treat
it as a false ticker? Would it be delay 0.2? Is precision relevant
here?

James

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