libsodium mess

[Gary E. Miller]

Yo Kurt!

On Fri, 20 Jan 2017 19:53:00 +0100
Kurt Roeckx <kurt at roeckx.be> wrote:

> > Nope.  ntpd clearly tells me that my jitter is 100 micro Seconds.
> > I get the same results using chronyd.  
> 
> Yes, but i think you're saying your resolution is 1 ms.

No, my resolution is 1 nano Second.  That is the LSB on the system
clock.  Note I do not claim accuracy near that.

> > No, there is jitter on that of 1 mS to 0.1 mS.  
> 
> I have no idea why you always write S or Seconds. The correct SI
> unit is s, not S, and it's also just second. Is this some German
> capitalization thing?

Being a native born USA citizen, with USA citizenship going way back,
it is unlikelly to be a German thing.

I am well aware of SI units.  You will note that we in the USA join
with the great countries Libya and Myanmar it not using SI units.  Unlike
the rest of the world.

I choose to converse with people, of many languages and cultures, in
ways that cause the least confusion.  I find I get the least requests
for clarification when I spell out my units, as in: nano Seconds.

That said, you know what I meant, you just have a hard time groking it.


> > > First, even though the PPS signal is only once per second, you're
> > > really measuring the pulse, and you could say you're measuring
> > > that with a resolution of 1 ms.  
> > 
> > No, the resolutin of my measurement is the resolution of the system
> > clock.  
> 
> But you only check about every 1 to 2 ms as I understand it. I was
> under the impression that you talked about that as your
> resolution. Normally you would call that the sample period, but
> we're measuring time here which always confuses things somewhat.

Certinaly not a sample period.  To me a sample period implies and
average over time, that is NOT the case here.

> In this case jitter and precision really are the same thing. But
> if what you're measuring isn't time they're different things.

Sadly we keep running into word definition conflicts.  'NTP precision'
is not at all related to 'measurement precision' and we keep flipping
back and forth conflating those terms.

In the NTP context, 'NTP precision' is just 'NTP jitter' smoothed and
bounded on a longer time scale.  I wish I could explain it better, you'll
just have to look at the code.

> But I have no good other term other than the sample period /
> frequency. It will clearly limit your accuracy, but it's not the
> accuracy itself. NTP seems to be calling this precision.

Just forget 'sampler period', it has zero relevance here.

> > > You would instead need to modify the output of the PPS
> > > signal to add random jitter to it, so that it doesn't always
> > > happen every 1024 polls.  
> > 
> > That would be many mS of jitter.  Also not good.  
> 
> This might be counter intuitive, but adding noise can improve your
> results, but it needs to be white noise. But there are usually
> other ways to improve things to, like having a higher sample rate
> in this case.

It can sometimes, and it will not other times.  My direct observation
validates that statement.  Depends on many things.

> > The precision can be 10e6 worse than that sys_fuzz.
> > 
> > Once again, we really need an NTP specific dictionary because NTP
> > defines things in unusual ways.  
> 
> Oh, I do agree that NTP is confusing the terms, and I hope we can
> at least agree on how they are used now.

Well, first we have to figure it out ourselves.  So much of the detail
is mushes into long forgotten code, patched by many people that did
not understand.

> ntp returns for me: precision=-23

Which precision?  NTP has a sysclock precision and a precision for
each peer, erver and refclock.

In almost all those case it is calculated from that clock's jitter which
is likely around 0.1 micro Second.

> That means it can read my system clock at a rate of 2^23 times per
> second,

Uh, no.  Totally unrelated.  The rate NTP can read the system clock is
sys_fuzz.  Calculated from totally different underlying data using totally
unrelated algorithms.  For where/how sys_fuzzz is calculated see other
email o nthis list in the last day.

RGDS
GARY
---------------------------------------------------------------------------
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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