[chrony-dev] handling of unstable sources

[Frank Muzzulini <frank.muzzulini@xxxxxxxxxxxx>]

Hello,

I am currently evaluating alternatives to the classic ntpd. Chrony seems 
to the best one so far. Thanks for the good work :)

There was however one test that I did not like:

I configured two servers, both stratum 1, configured with minpoll 5 
maxpoll 8:
A was modern one, internally running ntpd on an embedded linux. Its 
measurements were very stable.
B was an old hardware solution. It provided time only with ms-resolution 
(thus measurements seem to deviate by +-500us) and in the long run it 
produces a saw-tooth effect (slightly drifting off, then jump 5ms to 
correct it, etc). Being a primary source without any complex logic it 
always reports its root dispersion as 0.

Now I observed the following behaviour:

1.
A steadily increased to poll 8
B always stuck to poll 5 or 6

Not really a problem, although I think that in the long run such 
"random" deviations might be better handled with long polling rates.

2.
Chronyd switched between "no majority" and selecting one of the servers 
almost every other minute, reporting local stratum in the "no majority" 
phases.

Considering the fact that without any meassurements it sticks to a 
server (and the related stratum) much longer, I wonder why it does not 
stick to one of the servers in that case.

3.
In the long run chronyd seemed to prefer server B.

Now this is really odd. Is this caused by the claimed low root 
dispersion or because it got more meassurements from it? What ever the 
reason is, it would be better to prefer the stable server.

Regards
Frank

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