Re: [chrony-users] Multi-PPS+RTC Setup Questions

[Matt Corallo <ntp-lists@xxxxxxxxxxxxxxx>]

Relevant selectdata on 4.0 (which seems to be working great) is:

P RTC1                      N --TR- --TR-   22   1.0 -3326us +4679us (since changed to +/-4ms, not 2ms)
* GPS1                      N -P--- -P---   15   1.0   -22us -8175ns
+ GPS2                      N -P--- -P---    8   2.5 -9093ns -2063ns

Indeed, RTC1 is an external daemon that reads in from an RTC, checks that chrony isn't using it as its timesource and 
then roughly disciplines it according to chrony's time, all before feeding samples back into chrony over sock. Indeed, 
the two GPS refclocks are of different model, and do adjust slightly differently as their internal RTCs drift, but all 
within noise, or only slightly outside of it (at least once I compensate for them being persistently off by 30us, which, 
uhhh...wtf u-blox?).

With 3.4, I had to add "combinelimit 100" to get chrony to look at both refclocks instead of picking one and deciding 
that the other one was persistently off (from the first?) by just enough that it wasn't worth looking at (which does 
seem like a bug?), and its still in my chrony.conf, though I haven't tested whether I can remove it with 4.0.

Thanks,
Matt

On 9/29/20 4:29 AM, Miroslav Lichvar wrote:
> On Sun, Sep 27, 2020 at 12:48:02PM -0400, Matt Corallo wrote:
> > Ah! I'd hit send on this before reading the chrony 4.0 update list - initial
> > testing shows latest git HEAD to not have the issue of selecting the RTC.
>
> Interesting. I'm not sure which change in 4.0 would be related to
> that (assuming it's not related to the trust option). It might help if
> we could see the sources report, or selectdata in 4.0. I'd suspect
> there is an occasional disagreement between the two GPS refclocks,
> which would be fixed by setting a delay for them to reliably get an
> intersection.
>
> What exactly is the RTC refclock? Is it an external application
> feeding chronyd via SHM or SOCK?
>
> > > Finally, it would be nice to get a warning if "maxlockage" is not set
> > > when rate is >1 - if I'm understanding it correctly it doesn't make
> > > sense to have a rate > 1 if the lock source doesn't have a negative
> > > dpoll (which should roughly be the common setting for eg a GPS set  to
> > > > 1HZ PPS, as in my setup).
>
> Ok, I think I see what you mean. This could work only when locked to a
> polling refclock (i.e. not SOCK). I'll look into that after 4.0.

They are locked to ntpshm fed from gpsd, so, indeed, polling, if I understand correctly.

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