[ntp:questions] Slow convergence of NTP with GPS/PPS

David McConnell davidm at pipstechnology.co.uk
Wed Oct 1 10:24:22 UTC 2008

Thanks for the responses.

We have tried -g and minpoll/maxpoll are by default 4 for the GPS reference
We even recompiled ntpd with source modified to allow poll at 2sec intervals
(minpoll=1) but this did not seem to make much difference.

We have also tried fiddling some of the "tinker" settings (step and stepout)
but this just seems to lead to instability.

Also, even if we set the time pretty much perfect (within 5ms offset), ntpd
appears to first *increase* the offset to well out of our spec, then correct
through zero offset - overshooting the other way (again well out of our
spec) and then typically crawls back in after which it is stable - and
ultimately wonderfully accurate and stable.

I was hoping that some of the other tinker parameters ("allan" or
"dispersion" for e.g.) might have an effect - but what are sensible values
to use?
I realise that this will compromise ntpd's performance in other ways, but,
we could tolerate worse final accuracey and jitter in exchange for getting
to within 5ms "quickly".

The driftfile also sometimes seems to do more harm than good - especially
after a reboot.

> -----Original Message-----
> From: questions-bounces+davidm=pipstechnology.co.uk at lists.ntp.org
> [mailto:questions-bounces+davidm=pipstechnology.co.uk at lists.ntp.org]On
> Behalf Of David McConnell
> Sent: 30 September 2008 14:04
> To: questions at lists.ntp.org; linuxpps at ml.enneenne.com
> Subject: [ntp:questions] Slow convergence of NTP with GPS/PPS
> Hi
> We are using Linux ntpd with GPS/PPS reference clock to
> discipline the time
> on our systems.
> Our application requires good time accuracy (better than 5ms)
> but it also
> needs to get there quickly (as quickly as possible, but
> ideally taking no
> more than about 15 minutes).
> (The Linux/ntpd is running on a remote embedded device that
> is frequently
> restarted - possibly once a day or so - so we cant wait hours for
> convergence).
> Currently ntpd can take hours to achieve the desired acuracy.
> So, the question is simple - is there any way to
> significantly speedup the
> convergence of ntpd (using GPS/PPS reference clock)?
> We would be prepared to compromise somewhat on accuracy and jitter.
> (Currently accuracy and jitter values are excellent with
> jitter as low as 1
> microsecond and accuracy better than 10 uS but it can take a
> day or two to
> get there).
> It does not seem unreasonable to expect that the ntpd could
> achieve the
> required accuracy within 15 minutes or so - but nothing we
> have tried seems
> to work.
> Have tried modifying some of the tinker values, but we dont really
> understand what they all do - and have not really had any success.
> So to summarise:
> 1) Is it possible to speedup ntpd convergence (using GPS/PPS reference
> clock)?
> 2) If so, how - and what are the tradeoffs?
> Any help appreciated
> David
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