[ntp:questions] A Christmas puzzler - intermittent offset oscillations with a PPS source

Mischanko, Edward T Edward.Mischanko at arcelormittal.com
Sat Dec 22 03:38:48 UTC 2012


> -----Original Message-----
> From: questions-
> bounces+edward.mischanko=arcelormittal.com at lists.ntp.org
> [mailto:questions-
> bounces+edward.mischanko=arcelormittal.com at lists.ntp.org] On
> Behalf Of A C
> Sent: Friday, December 21, 2012 3:56 PM
> To: questions at lists.ntp.org
> Subject: Re: [ntp:questions] A Christmas puzzler - intermittent
> offset oscillations with a PPS source
> 
> On 12/21/2012 11:51, Rick Jones wrote:
> > David Taylor <david-taylor at blueyonder.co.uk.invalid> wrote:
> >> Although the offset appears to have a 1.25 hour period from
> the MRTG
> >> graphs, examining the loopstats directly shows that the
> actual
> >> period is just over about 5/3 minutes - just over 100
> seconds.  I
> >> don't know what's happening.  I would have put it down to
> poor GPS
> >> strength, except that the effect lasts almost a whole day,
> and GPS
> >> missing usually contributes bigger offset spikes.  I would
> not
> >> expect it to be the navigation mode of the GPS as the
> excursions are
> >> larger than I would expect between navigation and timing
> modes, but
> >> I don't have a lot of experience in that area.  One
> difference in
> >> the configuration is that #1 - the one with the offsets -
> runs and
> >> uses gpsd for the coarse seconds, whereas #2 relies on the
> rest of
> >> the network.  This seems to causes a higher CPU usage in #1,
> shown
> >> in there graphs:
> >
> >>     http://www.satsignal.eu/mrtg/performance_raspi-1.php
> >>     http://www.satsignal.eu/mrtg/performance_raspi-2.php
> >
> > Does the gpsd do anything every 5/3 minutes?  Or put another
> way, can
> > you find a similar periodicity in the CPU utilization?  If it
> does do
> > something interesting at that frequency and it involves a
> system call,
> > while the act of tracing would perturb things, you might find
> it in a
> > (timestamped) system call trace (strace) of the gpsd.
> >
> > Perhaps the luck of process scheduling and the gpsd or some
> other
> > daemon holds-off the ntpd?  (Raspberry Pi's are single-core
> systems
> > right?)  Does the ntpd run at a higher (realtime?) priority
> than the
> > gpsd?
> >
> > Might there be any other background dameons consuming more CPU
> on the
> > one system than the other?
> >
> 
> I see something similar under NetBSD and a GlobalSat receiver
> using gpsd
> for coarse numbering and direct PPS via DCD into ntpd for the
> PPS sync
> (ATOM driver).  The period of instability in my case is much
> longer but
> regular at something like 100 hours (there's also a smaller
> spike every
> 24 hours due to logrotate and similar housekeeping).  I've never
> tracked
> down the 100 hour cycle, though.
> 
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[Mischanko, Edward T] 

Why use GPSD? I am running FreeBSD 8.2 and use the NMEA driver along with the ATOM driver.  If this is incorrect, how do I use GPSD?  I am very new to FreeBSD and UNIX in general, so your patience is appreciated.

Regards,
Ed


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