[ntp:questions] like a kid with a new toy (PPS jitter)

p.kennedy at fugro.com.au p.kennedy at fugro.com.au
Fri Feb 6 03:25:15 UTC 2009


On Feb 6, 7:23 am, Dave Hart <daveh... at gmail.com> wrote:
> On Feb 5, 9:33 pm, Terje Mathisen <"terje.mathisen at tmsw.no"> wrote:
>
>
>
> > Dave Hart wrote:
> > > Looking at the last 800 loopstats lines for the refclock, representing
> > > a bit more than 3 hours of 16s polls, the offsets in microseconds look
> > > like
>
> > > -79.497    min
> > > -26.961    mean
> > > 20.565     max
> > > 17.381     stddev
>
> > > The jitter (us)
>
> > > 1.907      min
> > > 7.974      mean
> > > 35.64      max
> > > 4.651      stddev
>
> > > How does that look to more jaded PPS eyes?
>
> > Pretty bad actually: After a day or so you should see 0-5 us offset and
> > jitter.
>
> I hope, but I'm not optimistic.  This is PPS on Windows without kernel
> support, so the timestamp comes after the OS has noticed the state
> change of the CD line and passed that news along to ntpd.  A spot
> check of recent peerstats lines looks consistent with the stats above.
>
> > > The refclock is a Garmin GPS 18x LVC connected to a dual PII 400 via
>
> > I have one of those! (A Dell?)
>
> Yes, PowerEdge 2300.  It uses 9G and 18G drives on a SCSI RAID
> controller.  A whole gig of RAM ;)  I had a dual-400 410 workstation
> as well, it's been long retired.
>
> > It might work even better in single-cpu mode if it is a dedicated ntp
> > server.
>
> It's running with a patch to lock the main and timer threads to the
> 2nd CPU using SetThreadAffinity.  I haven't tried locking the async I/
> O thread as well, but I will now.  It's also running with a patch to
> the interpolation code which is more selective about baseline counter/
> time pairs it uses.  There is a faster server on the same LAN I could
> try as well, but I figured slower is better for understanding how bad
> the user-mode PPS implementation is.
>
> Cheers,
> Dave Hart

Under windows OS, I believe working in the 10's of microsecod range is
pretty good, and a darn site better than flat NMEA.  Terje, can you
let me know what is bad about this?






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