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

David L. Mills mills at udel.edu
Sat Jan 26 20:52:21 UTC 2008


Unless the computer clock intrinsic frequency error is huge, the only 
time the 500-PPM kicks in is with a 100-ms step transient and poll 
interval 16 s. The loop still works if it hits the stops; it just can't 
drive the offset to zero.


Danny Mayer wrote:

> 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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