Problem with GPSD refclock

Gary E. Miller gem at
Mon Jan 30 23:32:59 UTC 2017

Yo Rich!

On Mon, 30 Jan 2017 14:37:17 -0800
Rich Wales <richw at> wrote:

> > Agreed, the trick is how to keep SHM(0) from being selected, except
> > as a last resort.  As I already said the best way to do that is in
> > dispute. So please experiment and report back.  
> Thanks.  I did some experimenting just now, and my conclusion is that
> I'm best off using *only* the PPS-synced SHM(1) and not including
> SHM(0) in my configuration at all.

I tink your tests were not long engouh to say.

> After several minutes, though,

It can take hours for ntpd to stabilize.  Do not even bother looking
at your data for less than an hour after startup.

Startup issues are a known problem with ntpd.

> Non-PPS-corrected SHM(0) is really bad news, even when fudged to make
> it "fairly close" to the correct time -- mostly, I think, because of
> its high jitter.

Especially when fudged 'failrly close'.  Not a recommended thing to do.

> Even with a high stratum (6), ntpsec still liked SHM(0) better than
> outside stratum-1 or -2 servers.  I did *not* mark SHM(0) as preferred
> in ntp.conf, but ntpsec seemed to prefer it anyway (possibly because
> it gives great weight to local refclocks regardless of stratum or
> jitter?).

Yeah, I suspect strtum not really part of chimer selection.

> If I were to lose my good GPS signal, my home network would probably
> sync to SHM(0) even if it had a high stratum, and even in the presence
> of multiple good external time sources.  So I'm probably best off not
> using SHM(0) at all -- either marking it as "noselect", or simply not
> having it in the configuration at all.

Or giving is a far off fudge, so it is used as a last resort.

> If I were to lose my good GPS signal *and* lose Internet connectivity
> at the same time, I would probably be better off just letting my home
> net run free for a while, until either GPS or Internet service was
> restored, rather than use SHM(0) even as a last resort.

For some definition of 'a while'.

> SHM(1) works just fine, even without SHM(0) in the configuration at
> all.

Of course.  Usually.

Gary E. Miller Rellim 109 NW Wilmington Ave., Suite E, Bend, OR 97703
	gem at  Tel:+1 541 382 8588

	    Veritas liberabit vos. -- Quid est veritas?
    "If you can’t measure it, you can’t improve it." - Lord Kelvin
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