[ntp:questions] NTP vs chrony comparison (Was: oscillations in ntp clock synchronization)
unruh-spam at physics.ubc.ca
Fri Jan 25 17:49:05 UTC 2008
David Woolley <david at ex.djwhome.demon.co.uk.invalid> writes:
>David L. Mills wrote:
>> The NTP discipline is basically a type-II feedback control system. Your
>> training should recall exactly how such a loop works and how it responds
>> to a 50-ms step. Eleven seconds after NTP comes up the mitigation
>You both have problems here.
>Dave Mills: your problem is that you haven't explained why one should
>continue to use a long time constant linear feedback system when a human
>observer can easily tell you how to get within 10 microseconds of the
>correct time after no more than about 3 samples.
>Bill Unruh: you haven't explained what real world situation this test
>is simulating; it is a standard doctrine that ntpd is not a substitute
>for good hardware and system software (e.g. you shouldn't use ntpd to
>get round lost clock interrupts).
The real world situation that the test is run on (not simulating) is having
a computer on a lan with another computer running ntp from a Garmin PPS
acting as the server. It is a "best case" scenario, I will completely agee.
I still get round trip times of msec rather than 150usec at times, the
oscillators on the machines have glitches in which teh clock rate changes
by 1-2PPS suddenly ( over less than 1/2 hr) and then long periods of
I have NOT tested the two in situations where there are longer paths,
through many routers. I have not tested it on the road to Mandalay, or
Indonesia. I have been looking at the real world response in a working
system but where the network delays are minimal.
Is my testing complete? Heavens no. It is one data point.
Do I expect chrony to fall over on the road to Mandalay? Looking at its
design, no, but experiments are the answer.
>> algorithms present that transient to the loop and what happens
>> afterwards conforms to the equations of control theory. Discussion about
>> what happens at any time after that is a matter of mathematics and ntpd
>> does conform to the mathematics as confirmed by observation and simulation.
>That's an indication that the equations are inappropriate in that context.
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