[ntp:questions] Allan deviation survey

David L. Mills mills at udel.edu
Tue Sep 14 02:16:48 UTC 2010


I think we are talking right past each other. Both Chrony and NTP 
implement the clock discipline using a second-order feedback loop that 
can minimize error in both time and frequency, although each uses a 
different loop filter. Chrony uses a least-squares technique; NTP uses a 
traditional phase-lock loop. The response of these loops is 
characterized by risetime and overshoot, or alternatively time constant 
and damping factor. If Chrony were designed to have similar risetime and 
overshoot characteristics and equivalent time constant, when operated 
under the same conditions (trace 1) it will perform in a manner similar 
to NTP. That was and is my claim.

I read your message very carefully and conclude you have done something 
very similar to what I have. You generated phase noise from an 
exponential distribution and verified it has slope -0.5 on a 
variance-time plot, then generated random-walt frequency noise and 
verified it has slope near zero on a variance-time plot or used some 
other equivalent technique to verify the distributions. Using trial and 
error you found appropriate factors to combine the phase and frequency 
noise to produce an Allan variance characteristic similar to trace 1. 
All this is not hard using Matlab, but you might have used something else.

The interesting thing to me is how you used that information to develop 
the claim that Chrony is far better than NTP? To support your claim, you 
would have to confront both Chrony and NTP with samples drawn from the 
resulting distribution and compare statistics.  The cumulative 
probability distributions in Chapter 6 of my book were made using the 
NTP simulator included in the NTP distribution. I assume you have 
something similar.

It would be interesting to repeat the experiment with trace 3 and NTP 
operating at a poll interval of 16 s..


Miroslav Lichvar wrote:

>On Fri, Sep 10, 2010 at 10:10:08PM +0000, David L. Mills wrote:
>>A previous message implied that, once the Allan characteristic was
>>determined, it would show chrony to be better than ntpd. Be advised
>>the default time constant (at 64 s poll interval) was specifically
>>chosen to match trace 1 on the graph mentioned above.
>Wasn't that rather for 16s poll interval? From simulations is seems
>that the phase noise would have to be 10-30 times higher (or the
>frequency noise lower, but that's unrealistic) to ntpd perform
>well at 64s poll interval.
>>In other
>>words, it is in fact optimum for that characteristic and chrony can
>>do no better.
>Well, it does better. With phase noise and random-walk frequency
>corresponding to the trace 1 from your graph chrony is about 5 times
>better than ntpd. With 30 times higher phase noise the difference is
>only in order of tens of percent, but it's still better.

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