[LinuxPPS] So far, so good

[Hal V. Engel]

On Monday 09 August 2010 09:49:44 am Paul wrote:
> On Mon, 2010-08-09 at 09:22 -0600, clemens at dwf.com wrote:
> > > On Sunday 08 August 2010 11:54:58 pm Paul wrote:
> > > > As the title suggests I am making encouraging progress with my Oncore
> > > > synchronised server. However I have a couple of questions.
> > > >
> > > > 1. I am using the 'site survey' mode as I do not know my accurate
> > > > position. I gather that greater accuracy is obtained by setting the
> > > > site position. Does the oncore output somewhere where it thinks it
> > > > is? I can't find it in the logs, but of course I may not be
> > > > understanding what I see.
> >
> > If you ask for a site survey, the ONCORE driver will average the next
> > 10000 positions, taken at 1s intervals, which takes about 3hours.
> >
> > It puts the result in the clockstats file (I accumulate this in
> > /var/log/ntpstats/clockstats) and in the messages log, with log-level
> > LOG_NOTICE.  The default is for messages with log-level of LOG_NOTICE and
> > above to get logged, so the messages should be there too.
> >
> > In either case, grep for the string 'ONCORE_SS_DONE' .  That will be
> > followed by a message that says 'Surveyed posn:'  and the position as
> > lat/lon/ht.
> >
> > Good luck.
> 
> Thanks for this and from Hal. But still how can I see drift of less than
> 1 microsecond? The peers command in ntpq is only measuring microseconds
> - what else do you use?

You are correct the ntpq only shows offset down to microseconds ntptime should 
show you nanosecond offsets.  My system once it has settled down will typically 
show offsets in the +-250 nano seconds range when I query it with ntptime.  If 
I increase the system load (by running a large build for example) the offsets 
will increase and then settle down again once ntp has adjusted for the higher 
latency of the interrupt handler.

Also the offset is not the drift of the clock (drift means by how much the 
system clock is running fast or slow and this is usually expressed in parts 
per million or parts per billion).  Rather the offset is a snap shot of how far 
off from the actual time ntp thinks your system clock is at that moment.

Hal