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

From: Gary E. Miller
Subject: Re: [gpsd-users] Garmin 18X-5Hz
Date: Tue, 16 Aug 2016 11:39:30 -0700

Yo Miroslav!

On Tue, 16 Aug 2016 09:32:37 +0200
Miroslav Lichvar <address@hidden> wrote:

> On Mon, Aug 15, 2016 at 12:44:21PM -0700, Gary E. Miller wrote:
> > On Mon, 15 Aug 2016 15:43:48 +0200
> > Miroslav Lichvar <address@hidden> wrote:  
> > > 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 ondemand.  
> I mean frequency of the oscillator on which is based the system clock.
> As the temperature increases the clock slows down and ntpd/chronyd has
> to tell the kernel to speed up the system clock to compensate for that
> change.

The temp is no gonna take a steap jump for 10 seconds.  That is the length
of my spike.

> > > 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.  
> Yes, longer minpoll increases the number of filtered samples per clock
> update, so the estimate of the offset should be more accurate. But if
> the loop is too slow to keep up with the rapid changes in frequency,
> increasing minpoll will only make it worse. In your configuration
> which gives you 30 us swings if minpoll was decreased by one, they
> should drop to about 15 us.

As I said, already tried that.  Made it worse.

> > >
> > >  
> > 
> > Are those on Raspberry Pi's?  
> No, it's an Intel machine with active cooling. On the graph you can
> see the frequency changes rapidly in both directions. On a Pi with no
> cooler the temperature goes up quickly, but it cools down slowly, so
> the offset swings only to one direction.

My take is totally different, the ints on the Pi are slow, so when the LF
goes up, the PPS pulses get delayed.  I can easily force this condition
and see it.  The time spike is way to short to have any relation to

> > If I turn off on-box stats my numbers look like your 2nd one.  
> Well, yes, that's the point I was making. You need to keep the
> temperature stable (e.g. not running any CPU-intensive tasks) if you
> want ntpd to work well. 

Once again, not related to temps.  The time constant of the temps is
much longer than 10 seconds.  If you notice the thermometer at the bottom
you can see that.  others have used the onboard Pi temp and got the same

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

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