LKM Timemark Driver
Stromeko at nexgo.de
Tue Aug 28 20:56:37 UTC 2018
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, so strictly
speaking the standard deviation you talk about doesn't exist. You can
however obtain a similar measure of distribution width that converges to
the standard deviation for normal distributions under fairly loose
conditions. That's what is called a "robust estimator" in statistics.
The "average jitter" is one such estimator (in proper statistical terms
I've used the mean absolute deviation from the mean across a full week
> Can you post your ntpvix URL?
No, it's an internal network and I won't change that. Besides, as you
well know, I don't run ntpviz. I'm pretty sure I posted my plots before
however, I can check later this week to point you at the archives.
> 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. So "zero" really means no offset to the GPS module
time, not GPS ensemble time and certainly not UTC (that latter link is
only established some weeks later anyway).
> That will need a multi-frequency GPS receiver. Even a multi-channel
> GPS receiver has difficutly getting under 10 ns before post proecssing.
> The GPS signal itself is just not that accurate.
>> Stop deluding yourself with meaningless numbers.
> 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.
> 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
> 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. 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).
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