| Re: [chrony-users] Possible bug in PPS support |
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....
Is it to be considered a bug, or is this just a design feature?
It's a feature, but there is apparently a bug which may make the
switch take much longer than it should.
However, we use this form of time synchronization because we need the clock
to be within about 20us
of real time. When the PPS sync is lost and only network sync is achieved,
that is not really attainable.
So we need some indication whenever there is no PPS sync.
Would it not be reasonable to indicate loss of PPS sync when the Reach value
becomes zero?
Ok, it could be that freewheeling keeps a more accurate time than syncing to
another source, but
at least the error condition should be monitored.
How could we work around that in this case?
Decreasing the maximum number of samples of the NTP source with the
maxsamples option should reduce the maximum span (as reported in
sourcestats) and also the time it will switch from unreachable
sources.
Increasing the maxclockerror would do that too if it was included in
the source selection. Even with the default value it would take only few
hours to switch in your case.
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.
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.
Rob
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