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

Unruh unruh-spam at physics.ubc.ca
Thu Jan 24 23:20:12 UTC 2008

Brian Utterback <brian.utterback at sun.com> writes:

>Unruh wrote:
>> situation, but have no reasons for that worry. The very worst case is if
>> the system runs for a while on very short poll intervals, and then suddenly
>> has very log poll intervals. The short period estimation of the drift is
>> not a good estimator of the long period drift. But I suspect that NTP would
>> have problemsi in that situation as well. 

>Perhaps. Perhaps not. The NTP reference code chooses its own poll
>interval based on the clock stability and the sample jitter. For
>a frequency correction to be valid, the clock offset must be greater
>than the sample jitter. As the frequency gets closer to the correct
>value the poll interval must get longer. See, NTP has a different
>design goal than chrony. The goal of NTP is not merely to keep the
>clock in sync, but to also discipline the frequency, while also
>providing a stable time synchronization network. If your goal is
>to keep the offset as low as possible, just keep the poll interval
>as short as possible. That doesn't take much work.

No, I am refering to the case where the network suddenly goes down for 2
days. YOur poll has gone from 2 min ( say on maxpoll 10) to 3 days. 
The design goal of ntp and chrony is the same as you outline for ntp. 
The question is "what algorithm accomplishes those goals including
minimizing the offset" Chrony is just as adaptive on poll intervals as is
ntp. It is just that sometimes the world hands garbage, and the question is
how does the system respond to the garbage.

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