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

David L. Mills mills at udel.edu
Wed Jan 23 20:21:37 UTC 2008


I turn my machines off and on all the time and the clock is set from the 
server within 11 seconds after starting ntpd. If I didn't use burst 
mode, that would take four minutes. Golly.

Please understand the difference between impulse response and poll 
interval. It is true that it might take 3000 s to amortize the initial 
offset from the TOC chip at power-up. This is no different than if some 
server torqued your clock by that amount.


Maarten Wiltink wrote:
> "Unruh" <unruh-spam at physics.ubc.ca> wrote in message
> news:aCqlj.12849$vp3.7702 at edtnps90...
>>"David L. Mills" <mills at udel.edu> writes:
>>>There are lots of ways to measure the loop transient response. The
>>>easiest way is to set the clock some 50-100 ms off from some stable
>>>source (not necessarily accurate) and watch the loop converge. The
>>>response should cross zero in about 3000 s and overshoot about 6
>>3000 s is a HUGE time. For people who switch on their computers daily,
>>that means most of their time is spent with the computer unsynchronised
>>to best accuracy. The timescale of chrony is far faster. (I am not a
>>writer of chrony.I am a user who is trying to get the very best out of
>>the timekeeping.)
> But NTP is from a time when people didn't switch on their computers
> daily. When NTP was young, dinosaurs walked the machine room and
> _you_ did _not_ get to decide when the machine on the other end of
> your terminal was rebooted.
> NTP can, after weeks of training, teach a computer to keep time very,
> very well. As a result, it's less optimised for the other end of the
> spectrum.
> Features like iburst and the drift file can get your clock synchronised
> to within a few milliseconds in less than a minute. If you want better
> than that, or you want it faster... don't turn your computer off.
> Groetjes,
> Maarten Wiltink

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