[ntp:questions] How to determine hardware latency for PPS offset given simple tools.

Charles Elliott elliott.ch at verizon.net
Sat Jun 22 01:16:28 UTC 2013


You may be making a mistake using stratum 1 servers; especially the
government servers are way over used, and there can be large, variable, and
indeterminate delays just pushing a packet thru them.  Stick with stratum 2
servers; few people know about them apparently.

And this is not off-topic.  You efforts may be aided by having one computer
connected to 9 or 10 external servers (using the huff puff option) and to
your two local clocks (using the noselect option).  That way you can tell
which local clock is closer to the truth and how much they differ.  For the
computer connected to the external servers, don't run anything else on it
while you are experimenting.  A high load, shouldn't, but does affect how
fast NTPD can adjust to changing conditions.  Also, watch the offsets and
jitter.  When they become high, the router your ISP uses is jammed up with
movie downloads or commercial data.  You should wait until the Internet is
quiescent before observing or taking measurements.

Best of luck!

Charles Elliott

> -----Original Message-----
> From: questions-bounces+elliott.ch=verizon.net at lists.ntp.org
> [mailto:questions-bounces+elliott.ch=verizon.net at lists.ntp.org] On
> Behalf Of Paul G
> Sent: Friday, June 21, 2013 10:16 AM
> To: questions at lists.ntp.org
> Subject: Re: [ntp:questions] How to determin hardware latency for PPS
> offset given simple tools.
> 
> On Thursday, June 20, 2013 5:51:50 PM UTC-4, E-Mail Sent to this
> address will be added to the BlackLists wrote:
> >
> >
> >
> > enable stats [etc. etc.]
> 
> As noted earlier I've done that or it's not applicable.  E.g. I only
> use the
> PPS driver and my seconds are numbered by an appliance that doesn't run
> ntpd.
> 
> >
> >  On one system at a time, ...
> >   have several other NTP servers configured {I usually shoot for 6 to
> 10}
> 
> I don't have six to ten stratum one servers (but maybe I should) and it
> doesn't
> seem useful to compare my << 500 microsecond offsets to non-local
> clocks.
> 
> >    with all involved systems continuously running for more than one
> day;
> >     take an average ...
> 
> In another thread (and some here) I explain how I've done that and I
> don't
> really like the e.g. 124 microsecond time1 I derived.  However it does
> result
> in O(1) microsec. offsets between some of my clocks.
> 
> >  perhaps the PPS signal is inverted?
> 
> It's not.
> 
> > It seemed like David Taylor covered that on may 25th.
> 
> Yes.  While I appreciate the suggestions and the good will behind them
> they don't seem informed by my question/problem description.
> 
> My key point is that ntpq appears to be telling me odd things.  E.g. my
> network
> is low latency, symmetric and consistent but some of my offsets are one
> or two
> orders of magnitude beyond other offsets.
> 
> So my question is how to find what I hope is hardware latency using the
> tools
> at hand or the coverse given multiple S1 clocks with O(10) microsecond
> offsets
> which one is right.
> 
> I expect I will move a set of them to a 10/100 switch and see if that
> makes a
> difference.
> 
> Ideally all my clocks pairs would look like this (both have time1 0):
> 
> localhost oPPS(0) .PPS. ... 377    0.000    0.001   0.001
> black     +aster  .PPS. ... 377    0.065   -0.003   0.004
> 
> localhost oPPS(0) .PPS. ... 377    0.000    0.000   0.004
> aster     +black  .PPS. ... 377    0.066    0.007   0.010
> 
> But maybe some of them are just not up to the task:
> aster     *ntp1   .PPS. ... 377    0.526    0.129   0.166
> 
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