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

Unruh unruh-spam at physics.ubc.ca
Wed Jan 23 21:58:12 UTC 2008


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

>Maarten,

>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.

When you say "the clock is set" what do you mean? With what accuracy is the
clock running 4 min after powerup in comparison with its accuracy after say
5 days. (let me define the accuracy as the offset ,not the jitter, but the
offset on each measurement from your best time source.)




>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.

So, if some server did torque your clock by 50ms as a one time event, or if
you stepped your system clock by 50ms, how long would it take ntp to settle
down (lets say you are running at maxpoll 7, minpoll 4). Let us assume that
in steady state your clock is controlled to 50usec. HOw long would it take
to regain that +- 50usec behaviour with ntp? Again, I mean by +- 50 usec
that the measurement offsets ( what is reported in the peerstats
file as "clock offset") are fluctuating by +-50usec?
You may not like that as a measure of the clock accuracy, but I want to be
clear that we are not talking about different things.





>Dave

>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
>>>>percent
>>>
>>>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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