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

Danny Mayer mayer at ntp.isc.org
Sat Feb 7 18:51:37 UTC 2009

Dave Hart 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.

You only set the thread affinity if it really needs it otherwise it
doesn't really matter which CPU it's using. Certainly the I/O thread
should not be set.

> Cheers,
> Dave Hart
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