Re: [gpsd-dev] ntpshmmon

[Gary E. Miller]

Yo Nuno!

On Thu, 16 Jun 2016 23:23:07 +0100
Nuno Gonçalves <address@hidden> wrote:

> Hi,
> 
> Thanks. This works fine for me. Just a question: the usleep value is a
> tradeoff between activation frequency and print latency. Since this is
> a monitoring tool, do we need to activate it a 1KHz?

Not exactly, this is more a test tool than a monitoring tool.  No one
is gonna look at ntpshmmon for more than a few minutes.  

As I noted in a code comment, the Garmin GPS-18x may output PPS at
5Hz, with a minimum pulse width of 20 milliSec.  Some GPS are much
faster, even 20 Hz.  We don't want to miss any bad behavior, and bad
behaviour is not where you expect it.

A simple wake-up and check is not much CPU load.  On the Pi3 ntpshmmon
is 2.6% of 1 CPU, out of 4 CPUs.  So less than 1% of the Pi3's power.

As a diagnostic, not production, tool that is very acceptable.  With some
effort, if anyone cared, that could be improved a lot.

> I wouldn't see a
> problem of runing it at 10Hz or even 1Hz.

Think Nyquist.  Just to see 5Hz square wave you gotta run at 10 Hz.  And
we are not talking square waves.  A 20 Hz PPS needs at least 40 Hz, much
shorter due to the pulse nature of it.

Remember, this is trying to catch glitches that are hard for humans to see.

OTOH, if we could figure out how fast ntpd polls the SHms, we could use
that as a guide to how fast ntpshmmon needs to check.  But we would still
want to be at least 2x that, because of our friend Nyquist and because this
is a debug tool.

And people keep reseting the cycle to 1Hz, and rediscovering why it does
not work.  that is why the comments are in the code, and will shout
louder and louder everytime somone ignores them.

> For the user it doesn't
> really matter if the results are printed with 100ms or 1s delay...

True, but when we miss a pulse, we got a big problem.  We are trying to
get PPS goo dto better than 1 microSec!

> What are the weird data you are looking? Cycle misses? Delays above a
> threshold? Delays that are statistically inconsistent?

PPS that is one second early or late.  I'm not really sure since I need
a good tool to find it.  I see on 3 well instrument hosts that ntpd
goes weird by 1 second now and then, but I don't see why.

RGDS
GARY
---------------------------------------------------------------------------
Gary E. Miller Rellim 109 NW Wilmington Ave., Suite E, Bend, OR 97703
        address@hidden  Tel:+1 541 382 8588

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