[ntp:questions] NTP vs chrony comparison (Was: oscillations in ntp clock synchronization)

David L. Mills mills at udel.edu
Tue Jan 22 15:50:56 UTC 2008


The basic clock discipline feedback loop has been unchanged since 1992, 
although minor changes have been made to improve behavior in very long 
poll intervals. The only radical change has been using a preliminary 
15-minute initial frequency computation when no frequency file is 
available. So, if you are comparing ntpd and chrony at initial startup 
and without a frequency file, expect to find wide differences in 
behavior. Starting ntpd with an intentially "bad" frequency file is not 
useful unless you can configure chrony in the same way.

If you really do want a definitive experiment, do what I suggested 
earlier: measure the transient response of both ntpd and chrony starting 
from the SAME initial conditions and with a frequency file containing 
zero PPM. Pay attention to the poll interval, whch should be the same in 
both cases. That will tell you the story, the whole story and nothing 
but the truth.


Unruh wrote:

> mayer at ntp.isc.org (Danny Mayer) writes:
>>Unruh wrote:
>>>All I say is that the experiments I have carried out show that ntp is slow
>>>to converge if it starts of badly, and leaves the offset scatter larger
>>>than chrony does. It does have a smaller scatter in the rate. 
>>But you are using an extremely old version of ntp and things have 
>>radically changed since that version was released. Try rerunning you 
>>experiments with ntp 4.2.4 and see what you get then. You also need to 
>>fix your calculations if you are going to get good results as I 
>>mentioned in a previous message.
> Most of the standard deviation results are with 4.2.4. Only the startup was
> with 4.2.0. Are you saying that "things have radically changed" in the
> handling of the startup? After I collect more data on steady state, I will
> rerun startups both with no drift file and a bad drift file to see how fast
> the convergence is with 4.2.4.

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