Re: [chrony-users] Strange "loss of PPS lock"

[Rob Janssen <chrony-users@xxxxxxxxx>]

On 1/19/23 10:16, Miroslav Lichvar wrote:
> On Thu, Jan 19, 2023 at 10:06:00AM +0100, Rob Janssen wrote:
>> On 1/19/23 09:51, Miroslav Lichvar wrote:
>>> Are they stratum 1 servers? What root delay and dispersion is reported
>>> in chronyc ntpdata?
>> They are in the region of 15-30us.  Indeed all those servers are
>> stratum 1, all running chrony and with a PPS via serial DCD.
> If those servers are in local network, the root distance might be
> smaller than the shifts in the PPS offset (e.g. due varying CPU load
> causing different delays in processing of the interrupt), which would
> cause the disagreement.
>
> Instead of the trust option I'd suggest to add "delay 2e-3" or some
> other suitable value to increase its interval and more reliably
> overlap with the NTP sources.
>
They are not in the local network.  It is a wide-area network consisting
of WiFi links, VPN over internet connection shared with others, etc.
There is no way I can "calibrate" systematic offsets as close as that.
I need the NTP servers only to provide time close enough for the PPS
to lock on the correct second.  If anything, I would be tempted to remove
all NTP sources and do a one-time NTP sync before starting chrony at boot.
However, having the NTP sources still provides some monitoring capability
(to watch the validity of the PPS which is already watched by others) and
also it could happen that the system reboots without network access
so the startup procedure would be complicated.

Rob

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