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

Unruh unruh-spam at physics.ubc.ca
Sat Jan 26 21:35:29 UTC 2008


mayer at ntp.isc.org (Danny Mayer) writes:

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

>>> Reading your claims literally, chrony would have to slew the clock 
>>> considerably greater than the 500 PPM provided by the standard Unix 
>>> adjtime() system call. Please explain how it does that.
>> 
>> Using the Linux adjtimex system call which has the ability to change the
>> ticksize which gives much greater than 500PPM slew rate for the clocks.
>> ( Up to 100000PPM, although that is never used. ) And as I understand it,
>> your handling of leap seconds in ntp also uses far greater than 500PPM slew rates. 

>No, ntpd deliberately limits frequency changes to 500 PPM. That's hard 
>coded. You need to avoid using anything greater than that as Dave has 
>explained. That would be the reason why it taks ntpd longer to bring the 
>clock back to the right time.

Well, no to both. ntpd steps, which hardly obeys that limit, and the reason
ntp takes such a long time is that it has an intergration loop with such a
long time constant. If it put its mind to it and used the 500PPM to get rid
of a 50ms offset, it would only take 200 sec, not 3 hours.
It slowly jacks the PPM to 400 or so and then slowly drops it again below
the nominal. This is done to avoid trashing or instability.



dd>Danny




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