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

David L. Mills mills at udel.edu
Sat Jan 26 21:22:45 UTC 2008


The default 900-s stepout interval was originally determined by the time 
an old Spectracom WWVB receiver took to regain synchronization after a 
leapsecond and should probably be reduced. It can of course be tinkere.

During the initial training period the time is not disciplined other 
than to amortize the initial offset. The bookeeping to do that and 
preserve an accurate frequency measuremen got too tedious and fragile. 
So, at the end of the training period the offset that built up during 
the interval is amortized. I didn't think this was much of a problem, 
since in practice the training is done only once.


Petri Kaukasoina wrote:

> David Woolley  <david at ex.djwhome.demon.co.uk.invalid> wrote:
>>Petri Kaukasoina wrote:
>>>Basically, it stepped time with ntpdate, slept 100 seconds and stepped time
>>>again with ntpdate. From the time adjustment, the script calculated the
>>>drift value and put that to the drift file. Again, the time offset always
>>>stays below 1 ms.
>>That has quite a lot of similarity with what ntpd itself does if it is 
>>cold started with iburst.  The only big difference is that it uses 900, 
>>rather than 100 seconds.  I don't know if that is the same 900 as 
>>controlled by "tinker stepout", but, even if it is, the side effect on 
>>stepout's would probably be undesirable.  To cold start you need to 
>>delete the drift file, or not configure it.
> Hmm, I can't see that. I put in only one good time source with iburst,
> deleted the drift file and started ntpd. The time offset just keeps growing
> and the frequency changes in very small steps. Now, after 30 minutes time is
> already 25 ms off and the frequency is only 1.5 ppm (the correct value would
> be about 25 ppm).

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