| Re: [chrony-dev] chrony after start/wake up |
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On Tue, 28 Nov 2017, Gerriet M. Denkmann wrote:
I noticed that it takes chrony about 20 - 40 minutes after start/wake up to settle down to a plausible value for frequency.
(The offsets are fine after about 10 minutes - and not too bad before this).
What is your poll range? The default for ntp servers over the net is a poll of
6-10 A poll of 6 means 2^6=64sec=1min. And it takes about 5 poll periods for
it to have a reasonable set of values to fit the offset and slope to.
A poll of 10 is 20 min roughly.
It helps if you a) do a burst right at the beginning, and b) tell it to do a
step rather than a drift slew to correct the initial offset.
What about this ( n = 20 to 40 minutes ):
if t = time since start/wake up < n and /var/lib/chrony/drift exists then
usedFrequency = [ (n-t) * frequency from /var/lib/chrony/drift + t * computed frequency from insuffizient data ] / n
The problem is that when you start your computer it is cold. The crystal
inside has a different rate than after it warms up when the computer has been
running for a while. The saved drift would be a hot drift rate. Plus the
computer recalibrates the clock every time it starts and that calibration
(seconds per crystal tick) is not the worlds greatest. So the remembered drift
is off by at least a few PPM from the true drift rate.
else
usedFrequency = computed frequency from no longer insuffizient data
end if
Gerriet.
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