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

Unruh unruh-spam at physics.ubc.ca
Tue Jan 22 18:37:47 UTC 2008

"David L. Mills" <mills at udel.edu> writes:


>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.

Of course I can. And have done so.
As you say, the transient response of ntp is terrible-- 3000 sec is far too

>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.

Precisely what I have been doing. 


>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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