Re: Clarifications about PPS SHM content

[Gary E. Miller]

Yo Martin!

On Thu, 19 Mar 2020 13:44:31 -0400
Martin Boissonneault <address@hidden> wrote:

> Thanks for confirming my intuition. This thought came from watching 
> ntpshmmon. The answer caused the question ;-)

Good data does that.

> Follow-up question: I recall someone said that all time sources have
> an influence over the stability of the NTPd time, and if you had one 
> unstable time source connected (like the GNSS serial shm?), the whole 
> stability was affected even if you have a better, more precise clock 
> selected.

Well, sorta, sometimes.  NTP decision making is PhD level obfuxcation.

ntpd will only use info from a source is it is part of the best cluster.
So a bad pool server will usually be ignored, but it may, every once in
a while, yield a good time and be included in the cluster, then be quickly
thrown out.

Within the cluster, the bext chier is picked, and gets the "*".  That is
then filtered and then used to discipline the PLL that adjusts the system
clock.

IFF a bad chimer gets selected will it throw around the system clock.  This
can happen, during startup, when you have only a PPS and a GPS chimer, with
little jitter history.  ntpd flips a coin, and it it selects the GPS first,
less than optimal things happen.

This is why you put a short poll on the PPS, and put it first in the
ntp.conf, so it gets selected first, if possible.

> Considering my GNSS serial SHM has been (x) and is stratum
> 11, do you think it is negatively affecting my stability and should
> be commented out?

I am unaware how stratum plays into NTP selection algorithms.  Best to
ask that on their mailing list.

> I know my ntpviz graphs would look better without the serial shm ;-)

Easy: noselect.

RGDS
GARY
---------------------------------------------------------------------------
Gary E. Miller Rellim 109 NW Wilmington Ave., Suite E, Bend, OR 97703
        address@hidden  Tel:+1 541 382 8588

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    "If you can't measure it, you can't improve it." - Lord Kelvin

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