| Re: [chrony-users] Possible bug in PPS support |
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Bill Unruh wrote:
Ok but rather than "only a few hours" I would like to see "only a few minutes".
But that would be totally rediculous. The offset of the local clock from UTC
after even a few hours is still far far better than that from the network, and
far better even than 20us. Remember what you want to know is how far the local
clock is from UTC, not whether or not the local clock has not heard from PPS
in the past few minutes.
You don't support my calculation that if the clock apparently wandered away 3400us
after 13 hours, it would take about 5 minutes to wander 20us?
I would think it is a best-case calculation as it assumes a linear drift in one direction.
I practice it will probably wobble, and take less than 5 minutes to wander 20us.
Please note we are talking MICROseconds here. Not MILLIseconds. I don't think
many standard systems will remain within 20us for several hours if left without sync.
(it would likely require some TCXO clock option)
The Span indicated by sourcestats is 79 for the PPS source now, and 103m for
the network sources.
Would that mean it drops the PPS after 79 seconds? That would be fine.
No. You really need to think through what you want and what the time on your
server machine delivers. After all if the computer clock in your local machine
was and exact track of UTC always to atto seconds, and you used the GPS only
to make determine the intial offset determination then it would be silly to
throw away that source just because the pps had not been heard from.
We are not interested in "time that is likely a good estimation". We require accurate time and if
we do not have it, or do not have certainty about it, we need to shutdown our application.
So we require some monitoring. Of course I can add monitoring of "sources" or "sourcestats"
to the monitoring of "tracking" that we currently do, and alert when "Reach" of the PPS
clock is zero. That is probably our quickest solution. However, I would have expected this
error condition (missing PPS pulses) to be somehow reflected in the "tracking" output.
Rob
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