minpoll=maxpoll from 1 to 7

Gary E. Miller gem at rellim.com
Thu Sep 22 04:45:18 UTC 2016

Yo Hal!

On Wed, 21 Sep 2016 19:24:31 -0700
Hal Murray <hmurray at megapathdsl.net> wrote:

> Nice work.  Thanks.

I wish it brought more clarity...

> Would you please remind us about what the PPS setups on those systems
> are. (I think you said the Xeon has a USB setup.)


Notes on the system config, and links to the ntp.conf are on those pages.

> And what do the column headings really mean?

Those match the headings on the ntpviz pages, links just above.

> Do you have a recipe for generating that data?  I expect it comes
> from ntpviz, but what do I type?  Is there anything interesting on
> the graphs? ...

I just copy the numbers off the graphs.  I think I'll need to automate
it some more.

> For things like this, there is usually a tradeoff.  If you sample too
> close together (short poll) the errors are dominated by the accuracy
> of the measurement.  If you sample far apart, the underlying clock is
> changing while you are measuring it.  That leads to the classic V
> shape for ADEV graphs. The bottom of the V is usually the best place
> to measure.

Yeah, I'm just surprised at how different the Pi3 and the Xeon are.

> For a good PPS (GPIO or real serial port) that works out to be 1
> second or less.

SO far I'm seeing 4s as the best on the Pi3.

> For PPS over USB things get messy because the error may not be
> Gaussian.  If you don't hit one of the ugly hanging-bridge cases,
> more measurements help to average out the noise.

Yeah, not even started to look at that yet.  What I see is the
USB phase locks to the PPS and the results are 'too good'.

Gary E. Miller Rellim 109 NW Wilmington Ave., Suite E, Bend, OR 97703
	gem at rellim.com  Tel:+1 541 382 8588
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