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

David Woolley david at ex.djwhome.demon.co.uk.invalid
Thu Jan 24 23:46:56 UTC 2008


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).

> 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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