Re: [chrony-users] Possible bug in PPS support

[Rob Janssen <rob@xxxxxxxxxxxxxxxxx>]

Miroslav Lichvar wrote:
> On Mon, Oct 23, 2017 at 10:24:54AM -0700, Bill Unruh wrote:
> > On Mon, 23 Oct 2017, Rob Janssen wrote:
> > > You don't support my calculation that if the clock apparently wandered
> > > away 3400us
> > Again, no evidence of that 3400 us.
> If I understand it correctly, 3.4ms was the offset of the NTP source
> 13 hours after the PPS stopped working. The stddev of the NTP source
> from sourcestats is ~50 microseconds, so if the offset was originally
> better than 1.6ms (there are 3 different sources in the original
> report with -0.1ms, 1.5ms, 1.5ms offsets), it drifted at least by
> ~1.8ms in that time.

Yes, I forgot that there was a systematic 1.4 ms offset at the time the PPS sync was
active so after it ran unsynced for 13h and had a 3.4 ms offset the drift was more like
2ms instead of 3.4ms.
However, that still is 2 orders of magnitude more than we can allow.  So we certainly
need to alert on this condition, we cannot just freewheel for 13 hours and assume the
time is still accurate enough.

I am now testing with the root delay/dispersion.   A couple of minutes after the PPS
has been removed, the root delay remains at 0.000000001 seconds but the root dispersion
now has increased to 0.000662125 seconds.  That certainly is a value that is immediately
affected by the lack of sync, however I need to determine a threshold value for the
monitoring alert.
The tracking also shows "System time     : 0.000000009 seconds fast of NTP time"
but I cannot believe the time is still that accurate.

I understand now that the 10ms value shown in "chronyc sources" is based on the 20ms
roundtriptime of the network towards the NTP source.  This time is quite constant as
indicated by the low Std Dev but the fixed RTT apparently makes chrony believe the
network is dodgy (as Bill expresses it).  The only thing dodgy about it is that for this
particular site there is a systematic offset in the propagation time from/towards the
site of 1.4 ms resulting in the 1.4ms offset observed when PPS is available, probably
caused by asymmetric routing.  Other than that, it is quite stable. It is a network
designed for distribution of audio and video to transmitter sites, well dimensioned with
guaranteed bandwidth and not overloaded at any time.

Rob


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