LKM Timemark Driver
Gary E. Miller
gem at rellim.com
Tue Aug 28 21:28:30 UTC 2018
On Tue, 28 Aug 2018 22:56:37 +0200
Achim Gratz via devel <devel at ntpsec.org> wrote:
> Gary E. Miller via devel writes:
> >> So do I, in fact I run between 140ns to 240ns of "average jitter"
> >> pretty consistently across five machines (three different rasPi and
> >> two TinkerBoard).
> > I was talking Standard Deviation, a bit better measure of the
> > jiter.
> As you should know, the jitter distribution is not normal,
Yes, which is why ntpviz provides skewness and kurtosis so you
can tell how the distribution differs from a standard distribution.
ntpviz also supplies a number of other bits about the data set:
mean, average, min, max, etc.
> so strictly
> speaking the standard deviation you talk about doesn't exist.
Nope. Standard Deviation applies to ALL data sets.
"In statistics, the standard deviation (SD, also represented by the
Greek letter sigma σ or the Latin letter s) is a measure that is
used to quantify the amount of variation or dispersion of a set of
People are taught in college how Standard Deviation looks for a
"normal distrubution', so they assume that one denotes the other.
> You can
> however obtain a similar measure of distribution width that converges
> to the standard deviation for normal distributions under fairly loose
Which is useless for us, because, as you rightly noted, we have nothing
at all that looks like a normal distribution.
The skewness and kurtosis was added to ntpviz to make this clear.
> > Can you post your ntpvix URL?
> No, it's an internal network and I won't change that.
Not a requirement to post ntpviz data. Many people upload their
ntpd from their stratum one to another server to do the computations.
> Besides, as you
> well know, I don't run ntpviz.
Thus my request that you do.
> I'm pretty sure I posted my plots
> before however, I can check later this week to point you at the
Which would be OLD data. I'm looking for LIVE data.
> > Yes, studies have shown the consumer GPS can be stable in the 10 to
> > 20 ns range, but the offset from "true" GPS can be much worse.
> Again, I wasn't talking about that number. The GPS module's internal
> time representation, for all we know, is the ground truth towards
> we're supposed to coverge.
No, we are trying to converge on UTC(USNO).
> > They are all we have. If you can create some more meaningful
> > numbers then please send patches to ntpviz.
> You need some independent measurement to get there. The ntpviz plots
> will never show an independent view.
Yup, which is why we are working to get the PPS-out patch into the
kernel. That coupled with my TAPR-TICC and Rb will give us that
So hopefully 'never' is this year.
> > For now we can use our numbers as arbitrary goodness numbers to
> > compare two configurations.
> Yes, but you keep making statements that go far beyond that confidence
We'll have to disagree on that. My data is open to all to see, in real
time. I'm open to suggestions on how to improve it more.
> > Yup, got that. I think my Rb is 10e-14. At least that is what the
> > calibration papers said two months ago.
> Read it again. You're not getting that performance at tau=1s, not
> even close.
I did dig it out, my mistake. I'm "only" getting 1.5e-11 at tau=1s. I
guess I'll have to settle for that.
> In fact you're unlikely to get that performance even for
> larger tau unless you discipline it via GPS (in which case you get
> there maybe at around 2…4 weeks).
At which time you are measuring the GPS, not the Rb. And a time scale
that is useless for measuring the short term jitter we are discussing.
And, to answer the next question. Yes, I am discipling my Rb against a
GPS. The additional accuracy is very small. Maybe another 10x.
Gary E. Miller Rellim 109 NW Wilmington Ave., Suite E, Bend, OR 97703
gem at rellim.com Tel:+1 541 382 8588
Veritas liberabit vos. -- Quid est veritas?
"If you can’t measure it, you can’t improve it." - Lord Kelvin
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