[ntp:questions] strange behaviour of ntp peerstats entries.
unruh-spam at physics.ubc.ca
Fri Jan 25 18:03:43 UTC 2008
"David L. Mills" <mills at udel.edu> writes:
>You could have saved a lot of time sweating the code if you looked at
>the briefings. See especially the before and after time series and note
>the 10 dB improvement in S/N.
I am sorry, but looking at the code is far simpler than trying to find
something in the documentation. Code I can use grep on, your documents I
>You might not have noticed a couple of crucial issues in the clock
I did notice them all. Thus my caveate. However throwing away 80% of the
precious data you have seems excessive.
>1. The basic principle is to select the samples that have not been
>delayed in queues, leaving hopefully the ones delayed only by
>propagation. It's better to discard the others that can only result in
>less accurate measurements.
>2. The filter samples are correlated only if the total span of the
>filter is less than the Allan intercept, generally assumed in the order
>of 2000 s. For poll intervals above that, the samples become
>uncorrelated, so fewer samples are used.
>3. Never use an old sample, only new onese. It often happens that the
>last used minimum delay sample is older than the most recent sample. The
>result is that the filter can toss out up to seven samples before
>finding a new candidate. The clock discipline loop is specifically
>designed to deal with that, which is one reason the time constant is
>larger than you might like.
>> OK, having looked at the code, I see what it is doing. It essentially
>> takes the measurement with the shortest delay in the past 8 measurements.
>> If that measurement happens to be the current one, it is actually used
>> in the clock loop. (Yes, I know this characterisation of the clock filter is cruder
>> than reality).
>> This makes the smaller variance of chrony even more impressive, since in my
>> tests, evey single measurement of offset was used to calculate the variance
>> in the case of chrony, while
>> for ntp, only those "smallest" values as reported in peerstats were used. This also
>> explains why the roundtrip variance in ntp was so much better than chrony's.
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