Apparent protocol-machine bug, new top priority

[Gary E. Miller]

Yo Fred!

On Mon, 25 Sep 2017 16:55:30 -0700 (PDT)
Fred Wright via devel <devel at ntpsec.org> wrote:

> PPS(2) is the counter-capture PPS source, and is the primary timing
> reference.  

Can you explain a bit more about this source?  How does this differ
from the KPPS or PPS(TIOCMIWAIT) sources?

> SHM(1) is the combined NMEA/PPS source from GPSD, which is
> configured to use the interrupt-based PPS driver, and hence
> illustrates the offset in the interrupt-based capture.

Your SHM(1), on both your hosts, seem to not be very good.

Are you using KPPS?  Or just TIOCMWAIT?


> Again, PPS(2) is the main timing reference, though it's listed as
> 127.127.22.2 due to the lame partial translation table in ntpviz.

What do you think ntpviz should do better?  Just convert 127.127.22.C
to PPS(X) ?  That would be ann easy patch.

> The actual time offsets are
> visible in the loopstats graph and in the PPS(2) peer offset graph,
> and are substantially smaller than the offsets in SHM(1).  QED.

Well, your much poorer than normal SHM(1) Standard Deviation casts
doubt on your QED.

> > Uh, that is not my experience.  And I have more control over my
> > temperature than I have over my interrupt latency.  
> 
> See above.  And note that you can at least make the latency (as well
> as the variation in latency) as small as possible by running the CPU
> as fast as possible, rather than slowing it down for "thermal
> stability".

I always run my ntpd's with the perfomance governor.  So not an issue
for me.

I get 'thermal stability' by controlling an external heater and use that
to stabilize my test enclosure.  Then for fine temp control I vary the
CPU workload to stabilize the CPU temp.

Prolly several reasons why my SHM(1) seems to have 40x less jitter than
yours.

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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