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

Unruh unruh-spam at physics.ubc.ca
Fri Jan 25 17:57:04 UTC 2008

"Richard B. Gilbert" <rgilbert88 at comcast.net> writes:

>Unruh wrote:
>> "David L. Mills" <mills at udel.edu> writes:
>> I certainly have trouble with the time constant. It is long. I means that
>> ntp responds very slowly to changes. I understand why it was chosen. But it
>> is also true that building the system for a bumpy road to Malaysia requires
>> a different car than on the streets of New York.
>> Anyway, the question is not whether or not the design of ntp is reasonable,
>> it is. The question is whether or not it is "best". On the streets of
>> Vancouver, it is definitely not "best".


>You are certainly at liberty to write your own version that does it your 
>way.  The startup performance of ntpd seems a little clumsy to me and I 
>think that I could do it better by hand if I had knobs to tweak.  OTOH I 
>see it in ONE setting on ONE network and ntpd really has to work on the 
>streets of New York, Vancouver, and "Podunk".

>I use a GPS timing receiver as a reference clock and what I see when 
>ntpd starts up is that it makes a "mad dash" for zero offset, overshoots 
>and then "rings" for a while.  It gets where it's going eventually and I 
>normally run the machine 24x7 except during power outrages that last 
>longer than my UPS battery so it's not a big issue for me.

No, it is not just startup. Real oscillators have glitches. It is how the
system responds to those that is also important. As I point out, the
oscillators on my system have 2-10PPS glitches-- changes in the drift by
that amount in 10s of minutes. How does the system respond?

And in steady state, with real oscillators with changing drifts on the .1
to 1 PPM range, how well does the system perform?

>If I had the knobs to control it I would "tap the brakes" gently as it 
>approached zero offset with the objective of reaching zero offset and 
>the correct frequency simultaneously.  But I don't have the math I'd 
>need to tamper with the code intelligently nor do I have the experience 
>to make it work under all the conditions likely to be encountered.

>Dave and his crew DO have that knowledge and experience and I'm glad 
>that they are willing to share it with us.

More information about the questions mailing list