[chrony-users] Clock selection algorithm

[chrony-users@xxxxxxxxx]

On some systems (running Debian buster or bullseye and the distributor's chrony package)
I have a configuration that syncs to a PPS signal and has some external NTP servers
for the initial sync.  Configuration is like:

# PPS via /dev/pps0
refclock    PPS /dev/pps0 refid PPS prefer

# external servers
server        1.2.3.4 iburst
server        4.5.6.7 iburst

(usually there are like 4 external servers.  they are other servers in our local network
that run chrony as well and are also synced to PPS)

The command "ldattach 18 /dev/ttyS0" is run before chrony to create the /dev/pps0
device, with the PPS of the GPSDO attached to the DCD line of COM1.

Now is normally works very well.  The reported offsets in "chronyc sources" are very
small.  Low microseconds to high nanoseconds.

But sometimes, probably due to interrupt latency, the offset of the PPS refclock has
some spikes (e.g. spiking to 1 millisecond) and in this situation the PPS clock
is sometimes marked as an outlier (x) because all the NTP refclocks are closer to the
system time than this PPS sample is.  Chrony selects an NTP refclock and increases
the stratum.  It spends some time (like 5-10 minutes) tracking the NTP servers before
it reselects the PPS source.  This then alerts our monitoring system.

I wonder if there is some way to make it stick with the "prefer"ed refclock for a bit
longer, maybe not agressively apply the correction (the NTP refclocks, which are
sampled less often, probably are right).  It could disregard the outlier PPS samples,
but as the PPS is the preferred reference, it should stick to that until it becomes
unusable.

Is there some configuration trick to achieve this, or is this a wellknown issue that
is solved in some recent version?


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