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

Unruh unruh-spam at physics.ubc.ca
Mon Jan 28 11:47:55 UTC 2008

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

>David L. Mills wrote:
>> Danny,
>> It doesn't stop working; it just clamps whatever it gets to +-500 PPM as 
>> appropriate. If the intrinsic error is greater than 500 PPM, the loop 
>> will do what it can with the residual it can't correct showing as a 
>> systematic time ofset.
>> Dave

>I didn't mean to suggest that ntpd stopped running. It was that the 
>clock was drifting steadily off into the sunset. I realize that if the 
>problem corrected itself ntpd would bring things back to normal.

But that suggests that the drift rate of your chip became bigger than
500PPM, which is huge. Maybe something altered the tick size
inappropriately. ntp should have hauled the offset back to zero -- just
taking a longer time ( 100msec at 500PPM takes about 200 sec to eliminate--
which is not that long.)


>> Danny Mayer wrote:
>>> David L. Mills wrote:
>>>> Danny,
>>>> 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.
>>>> Dave
>>> Yes, I found this out when my laptop stopped disciplined the clock and 
>>> was complaining about the frequency limits and I started digging into 
>>> the code to figure out why.
>>> Danny
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