✘sys_fuzz * ntp_random()

[Gary E. Miller]

Yo Achim!

On Tue, 24 Jan 2017 22:38:50 +0100
Achim Gratz <Stromeko at nexgo.de> wrote:

> Gary E. Miller writes:
> > Last week we had a discussion on sys_fuzz and the value of adding
> > random noise to some measurements.  The code defi2nes sys_fuzz asL
> >
> >     "* The sys_fuzz variable measures the minimum time to read the
> > system
> >      * clock, regardless of its precision."
> >
> > Rondomness of half the sys_fuzz is then added to some values, like
> > this:
> >
> >     fuzz = ntp_random() * 2. / FRAC * sys_fuzz
> >
> > Makes no sense to me.  Adding randomness helps when you have
> > hysteresis, stiction, friction, lash and some other things, but
> > none of those apply to NTP.  
> 
> https://en.wikipedia.org/wiki/Dither
> 
> You might have enough other noise in any particular system so the
> sys_fuzz (which whitens the quantization noise on the clock reading)
> doesn't make a difference or you might not.  But presumably Dave Mills
> didn't put it in there just because he was trying to add useless code.

Dr. Mills did not put it there.  This was added in 2011 by Dave hart.
See my previous email that probably crossed by yours in flight.

And as Eric's LOC counts show, there was a TON of useless code in there.
Still is, we just do not know which parts yet.

> > So I took two identical RasPi's and forced sys_fuzz to zero on one
> > of them.  Four days later I can see no difference in the results
> > from those two.  
> 
> Which proves exactly nothing.

It proves that sys_fuzz does nothing on a RasPi.  At least undetectable
over 4 days.  I give you it proves nothing about other systems.

I'll once again raise the challenge to someone else to prove it
actually does something, somewhere.  No point involving deities or
more bike shedding, give me data!

Now knowing more about bug 2037 I'm also confident that the
characteristics of the RasPi clock where not those being fixed in bug
2037.

It seems the main case was to fix an OS (like Windows) that interpolated
time (badly) from a really slow (64kHz) clock chip.  And in that case
the sys_fuzz was not really part of the solution, it just got sucked in
as an after thought.

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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