Re: [chrony-users] chrony not demoting broken PPS source

[Chris Spencer <spencercw@xxxxxxxxx>]

On Wed, 9 Sep 2020 at 11:10, Miroslav Lichvar <mlichvar@xxxxxxxxxx> wrote:
>
> On Wed, Sep 09, 2020 at 10:55:41AM +0100, Chris Spencer wrote:
> > Hi,
> >
> > I have come across some slightly odd behaviour from chrony. We have a
> > GPS time source with PPS. The device is currently indoors so the PPS
> > is dropping in and out, but NMEA timestamps are still coming through.
> > For some reason, chrony is still trying to use the broken PPS source
> > even though it's not providing any data:
> >
> > # chronyc sources
> > 210 Number of sources = 6
> > MS Name/IP address         Stratum Poll Reach LastRx Last sample
> > ===============================================================================
> > #- NMEA                          0   4   377    11    -77ms[  -77ms] +/-  100ms
> > #* GPS                           0   4     0  101m  +2315ns[+3536ns] +/-  141ns
> > ^- time.cloudflare.com           3   6   377   100  -1167us[-1167us] +/-   64ms
> > ^- ntp1.unix-solutions.be        2   6   136   359    -62us[  -62us] +/-   53ms
> > ^- ntp.devrandom.be              2   6   377   100   +352us[ +352us] +/-   24ms
> > ^- ntp2.unix-solutions.be        2   6   324   24m   -150us[ -150us] +/-   30ms
>
> The GPS time is much more accurate than the other time sources, so
> it may take a long time before chronyd decides it should switch to
> something else when it stops providing samples. It tries to minimize
> the error of the clock taking into account its estimated stability.
> You can speed up the reselection by increasing the maxclockerror
> value.

Ah that's interesting thanks. As long as it does eventually pick
another time source then that's probably fine for our purposes.

> > # cat /etc/chrony.conf
> > refclock SHM 0 offset 0.155 delay 0.2 refid NMEA
>
> The SHM refclock should have the noselect option if it's used only for
> locking the PPS. If the GPS has no fix, the NMEA time will probably
> drift away.

I did originally have 'noselect' turned on; however, it's possible for
us to get locked out of these devices if the clock is not vaguely in
the vicinity of the correct time, so I figured it would be safer to
let chrony use the NMEA time as a failsafe.

Thanks,
Chris

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