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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 12:29:54 -0700 |
Yo Miroslav!
On Mon, 15 Aug 2016 13:55:53 +0200
Miroslav Lichvar <address@hidden> wrote:
> On Fri, Aug 12, 2016 at 11:29:48AM -0700, Gary E. Miller wrote:
> > On Fri, 12 Aug 2016 11:52:34 +0200
> > Miroslav Lichvar <address@hidden> wrote:
> > > As ntpd reads SHM only once per second,
> >
> > No. Minimmum read on an unpatched ntpd is 8 seconds. See
> > minpoll. I prefer to have it at 16 or 32 for better stability.
>
> The minpoll and maxpoll options control how often ntpd polls the
> refclock driver, not how often the driver polls its source. The SHM
> driver reads the SHM segment once per second. E.g. if minpoll and
> maxpoll are 3, there will be 8 samples collected and filtered between
> clock updates. If you increase the polling interval, more samples will
> be collected between updates and that can improve the stability of the
> filtered offset.
I just dug into the code with ddrown. I had no idea ntpd was
oversampling faster tham minpoll. A very bad design, with a conceptual
error similar to the one that was recently in ntpshmmon. This now
explains some of the defects I have seen for a long time in ntpd. Like
a bias to slow ticks over fast ticks. Asymetrical convergence and
osciilations in the convergence.
Nyquist is very clear, you can't sample a one second signal at one
second. The results in ntpshmmon were aweful. To see how awful, just
recompile ntpshmmon with a one second sample rate, then jiggle the sys
clock. This explains a lot why PPS on ntpd behaves as badly as it does.
I have been looking at other software, like Lady Heather, that gets far
better PPS results than ntpd does.
Sadly I don't see this on the NTPsec timeline for a while, so much cruft...
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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- Re: [gpsd-users] Garmin 18X-5Hz, (continued)
- Re: [gpsd-users] Garmin 18X-5Hz, Gary E. Miller, 2016/08/10
- Re: [gpsd-users] Garmin 18X-5Hz, Miroslav Lichvar, 2016/08/11
- Re: [gpsd-users] Garmin 18X-5Hz, Gary E. Miller, 2016/08/11
- Re: [gpsd-users] Garmin 18X-5Hz, Miroslav Lichvar, 2016/08/12
- Re: [gpsd-users] Garmin 18X-5Hz, Roger Oberholtzer, 2016/08/12
- Re: [gpsd-users] Garmin 18X-5Hz, Miroslav Lichvar, 2016/08/12
- Re: [gpsd-users] Garmin 18X-5Hz, Roger Oberholtzer, 2016/08/12
- Re: [gpsd-users] Garmin 18X-5Hz, Gary E. Miller, 2016/08/12
- Re: [gpsd-users] Garmin 18X-5Hz, Gary E. Miller, 2016/08/12
- Re: [gpsd-users] Garmin 18X-5Hz, Miroslav Lichvar, 2016/08/15
- Re: [gpsd-users] Garmin 18X-5Hz,
Gary E. Miller <=
- Re: [gpsd-users] Garmin 18X-5Hz, Gary E. Miller, 2016/08/12
- Re: [gpsd-users] Garmin 18X-5Hz, Roger Oberholtzer, 2016/08/16
- Re: [gpsd-users] Garmin 18X-5Hz, Doug Robinson, 2016/08/16
- Re: [gpsd-users] Garmin 18X-5Hz, David J Taylor, 2016/08/16
- Re: [gpsd-users] Garmin 18X-5Hz, Gary E. Miller, 2016/08/16
- Re: [gpsd-users] Garmin 18X-5Hz, David J Taylor, 2016/08/17
- Re: [gpsd-users] Garmin 18X-5Hz, Gary E. Miller, 2016/08/17
- Re: [gpsd-users] Garmin 18X-5Hz, Roger Oberholtzer, 2016/08/18
- Re: [gpsd-users] Garmin 18X-5Hz, Gary E. Miller, 2016/08/16
- Re: [gpsd-users] Garmin 18X-5Hz, Robin Schwab, 2016/08/16