[ntp:questions] Garmin LVC 18x jitter problem

Michael Haardt michael at moria.de
Wed Aug 14 18:04:23 UTC 2019

Miroslav Lichvar <mlichvar at redhat.com> writes:
> On 2019-08-12, Michael Haardt <michael at moria.de> wrote:
> > I would appreciate if we could focus on the major issues first, like
> > why the modified jitter is not shown by ntpq.
> I think the explanation is that you are modifying jitter of individual
> samples, but ntpq -p is showing jitter between offsets. Print all
> variables of the peer with ntpq -c "rv $ASSID" and look for "jitter"
> and "filtdisp".

You are right:

associd=48733 status=961a conf, reach, sel_sys.peer, 1 event, sys_peer,
srcadr=SHM(0), srcport=123, dstadr=, dstport=123, leap=00,
stratum=0, precision=-20, rootdelay=0.000, rootdisp=0.000, refid=GPS,
reftime=e0fec9d6.89f0223c  Wed, Aug 14 2019 19:56:38.538,
rec=e0fec9d7.c9977670  Wed, Aug 14 2019 19:56:39.787, reach=377,
unreach=0, hmode=3, pmode=4, hpoll=4, ppoll=4, headway=0, flash=00 ok,
keyid=0, ttl=0, offset=-21.688, delay=0.000, dispersion=199.450,
filtdelay=     0.00    0.00    0.00    0.00    0.00    0.00    0.00    0.00,
filtoffset=  -21.69  -24.50  -21.05  -20.86  -24.06  -31.32  -18.34  -39.11,
filtdisp=    200.00  200.24  200.48  200.72  200.96  201.20  201.44  201.68

Indeed I used minjitter 0.2.

I can't say that I understand the difference, though.  Do I modify what
I should modify? So far I only ever checked the offset and noticed that
it does vary in a way that can't be estimated with a (short) filter,
and set minjitter in a way that would always include all offsets.

It looks like there is a fine difference between jitter and dispersion,
and while the code calls it jitter, I modified the dispersion.

Could you elaborate?


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