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Re: [gpsd-users] Garmin 18X-5Hz

From: Gary E. Miller
Subject: Re: [gpsd-users] Garmin 18X-5Hz
Date: Mon, 15 Aug 2016 12:44:21 -0700

Yo Miroslav!

On Mon, 15 Aug 2016 15:43:48 +0200
Miroslav Lichvar <address@hidden> wrote:

> > Not my experience.  And not possible without patching your ntpd.  
> > For exammple:
> > 
> > You'll notice I'm also graphing temps, at the bottom.
> > 
> > I would encourage you to setup some graphs and prove your point.  
> I think your graphs demonstrate the problem very well. The periodic
> excursions of about 30 microseconds are exactly what I'm talking
> about.

Those are caused by the graphing solution I use.  A total 
replacement is underway.

Plus, the recent PPS sampling discussion shows up problems in how
SHM() is used.

Still, the +/- 2 microSec 50% of the time is pretty good for a toy

> When the cron job heats up the CPU, the loop is too slow to
> handle such a rapid change in the frequency of the clock and a large
> offset accumulates before it can catch up.

Can't be.  I set my clock to a fixed speed (performannce govenor), not

> Halving the polling interval (which sets the time constant of the
> loop) should halve the excursion.

Sadly, I've tested that, and exactly the opposite happens. ddrown was
just showing me how the SAMPLE() macro in ntpd works.  It takes very
sample it gets, linda sorta once a second, and averages them between
minpoll.  So a longer minpoll allos the bad reading to get sampled out.

> Ideally, the loop should be able to adjust the polling interval
> automatically.

Clearly nothing in ntpd is 'ideal'.  We got poor Eric backed up until
2018 with obvious fixes....

> FWIW, chrony works much better in such conditions.

Yes, and the gpsd time howto does recommend chronyd.  But my purpose
here is to get NTPsec fixed.


Are those on Raspberry Pi's?

I assume the later is chronyd?  Neither have a good a PPM as my ntpd's.

If I turn off on-box stats my numbers look like your 2nd one.

> While on the first graph (ntpd) the offset swings to about 12
> microseconds, on the second graph (chronyd) there are only barely
> visible blips of about 2 microseconds. You can also see one or two
> places where the GPS signal was lost for a minute. Note that this is
> the offset from the refclocks log, which is better for comparing with
> the ntpd's loopstats offset than the tracking offset.

I'm keeping all the stats ntpd throws out.

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

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