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

David Woolley david at ex.djwhome.demon.co.uk.invalid
Mon Jan 21 19:04:00 UTC 2008

In article <XP4lj.15282$yQ1.8066 at edtnps89>,
Bill Unruh <unruh at physics.ubc.ca> wrote:

> Offset error:
>    NTP: Mean=-3.1usec, Std Dev=63.1usec

If offset is the value reported by ntpq, please note that, when ntpd
is locked up, this is an indication of the instantaneous measurement
error, the actual error in the local time should be more stable (there may
be systematic error) by one or two orders of magnitude.

More generally though, Dave Mills really needs to get in here and defend
his clock discipline algorithm, and the Chrony developer needs to 
defend theirs.  Arguing the cases by proxy isn't particularly satisfactory.

Dave, please remember that what tends to concern people about the algorithm
is not the behaviour in response to gaussian phase noise, but its behaviour
in response to transients, in particular startup transients.  (Personally
I would say that lost clock tick transients should be fixed at source,
but Bill Unruh would also like it to tolerate those well.)

>   Chrony: Mean=-1.5 usec, Std Dev=20.1usec

Given the way that I understand it works, I think this is the actual
correction applied on that sample.

> Rate fluctuation:
>   NTP:Mean=25.32  Std Dev=.078 (PPM) 
>   Chrony: Mean=25.26 Std Dev=.091 (PPM)

The means depend on the hardware, and, as long as they are within the order
of one standard deviation of each other, they are as good as each other.

>From the point of view of another machine, chrony will have episodes where
the frequency changes much more, as it applies the phase correction.

> over the weekend, and chrony encompassed the weekdays when the grad
> students use the computer) the offset control by chrony was a factor of 3
> better than by ntp.  

If the figures are the actual correction for chrony and the sample error for
ntpd and Dave Mills is correct about the phase noise rejection of the ntpd 
filter being a couple of orders of magnitude, ntpd might actually be 30 
times better.

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