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

Danny Mayer mayer at ntp.isc.org
Sat Jan 26 04:32:07 UTC 2008


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.

Danny



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