[ntp:questions] Loopstats jitter field mostly zero?

David Taylor david-taylor at blueyonder.co.uk.invalid
Mon Nov 26 07:45:41 UTC 2012

On 25/11/2012 21:37, Dave Hart wrote:
> Jitter is a measure of the variability of the phase offset across up
> to eight entries in the clock filter register of the selected peer,
> the "best" sample of which ends up as the third column of loopstats.
> For a LAN peer best is lowest delay.  For PPS, it's the most recent,
> as the delays are all zero and older entries accumulate more error
> budget.  I expect the Windows PC to have a somewhat higher jitter than
> similar hardware running FreeBSD or Linux, but the pattern of exactly
> zero jitter recorded by your ARM-powered Pi doesn't look credible to
> me.  It's happening right off the bat for the LAN source at startup,
> and then shortly after switching to PPS.
> I bet you don't see any exactly zero jitter in loopstats on your
> FreeBSD PPS server.  I wonder if x86/x64 Linux ntp-dev users see
> anything similar?
> I'd appreciate others taking a look at their loopstats on Linux
> systems and especially non-x86 Linux to see if 0.000000000 shows up in
> the jitter (fifth column).  I think we need more information to
> understand the problem.
> Cheers,
> Dave Hart


Many thanks for your reply.  I have just enabled loopstats logging on 
the FreeBSD server and, just as you expected, the values are not zero 
(averaging at just over 2 microseconds).

I would expect the ~700 MHz ARM systems to be a little worse than the 
FreeBSD box, which is an Intel single-core, hyperhtreaded 1.66 GHz Atom.
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