Re: Clarifications about PPS SHM content

[Martin Boissonneault]

Hi Gary, Hi everyone!

    On 2020-03-22 21:27, Gary E. Miller wrote:
> Yo Martin!
>
> On Sat, 21 Mar 2020 01:39:41 -0400
> Martin Boissonneault <address@hidden> wrote:
>
> > I hope this pic answers your question :
> >       
> The stat that matters: Standard Deviation 280 ns.
>
> You should be able to cut that by more than half.
>
> > If you think I can improve those figures, ping me privately so I can
> > start a thread on the ntpsec mailing list.
> >       
> Many people do better.  gpsd-users is the place to discuss.  Check
> the archives.
>     
    Good!
> Look closely at the graph of PPS jitter over time.  Compare it to your
> cron jobs, and other jobs.
>     
    I don't have much experience with Debian/Raspbian/Linux optimization
    and process tracking. I struggled to find the cause of the time
    error spike that was caused by systemd's timesyncd service.

The primary purpose of that Raspberry Pi 3B+ (running Raspbian Buster) is airplane tracking with dump1090-fa and dump978-fa software-defined radio decoders, which receive their input over USB from two FlightAware RTL-SDR FlightSticks. It is then redistributed in 6 ways over RPi's Ethernet. This software-defined radio decoding is USB and CPU intensive with a CPU utilization average of 37%. That's not great for latency!

In order to help, I recompiled the kernel with the following modifications from the original Raspbian kernel .config:

CONFIG_HZ_PERIODIC=y 	(was CONFIG_NO_HZ_COMMON=y)
CONFIG_HZ_250=y 	(was CONFIG_HZ_100=y)
CONFIG_HZ=250 		(was CONFIG_HZ=100)

I forgot to set CONFIG_CPU_FREQ_DEFAULT_GOV_PERFORMANCE=y but this governor is set somewhere else (and confirmed operating) in my Pi configurations.

> Chart your CPU and room temps.  See if you can stabilize your temps.
>
> Send us the link to your ntpviz page.
>     
    Here are the ntpviz graphs, where the LM0 temperature is the
    temperature inside the case as read by the DS3231 RTC:
      http://ve2mrx.dyndns.info:10180/ntp/day/
      http://ve2mrx.dyndns.info:10180/ntp/week/

Here, you have various aircraft-tracking-centric graphs from https://github.com/wiedehopf/graphs1090 with various OS metrics:
http://ve2mrx.dyndns.info:10180/graphs1090/

Last but not least, (http : // ve2mrx.dyndns.info:10180/gpsd/gpsd.php).

My ham-radio call sign gives me little privacy here, but everyone be warned: Any unexpected visitor will receive a lick in the face who may or may not be COVID-19 infected! So, advise before dropping-in! ;-) More seriously, self-isolate! We have at least 3 more weeks to self-isolate, with most businesses closing in Ontario, Quebec and other places. Yes, that's where we are now! SELF-ISOLATE! SOCIAL DISTANCING IS (mostly) OUTDATED!

Now, the rPi is in a closed plastic case on a keyboard drawer. The temperature changes are usually slow as the airflow is limited in the keyboard drawer, but the room has a large window to the south. You can see in the graphs when it's sunny as the sun increases the temperature of the room. And there are spikes due to CPU usage.

If I had the budget, the Pi would be in a double-box, where the inner one would be a sealed metal box, and the plastic outer one, actively temperature-stabilized with fans. I have the electronics skills, I have the time, just not the budget... Hmm, maybe a paint can in a concrete bucket could work? You need a non-thermally conductive thermal mass... :-)

If you have any tips, I'll try them!

Thanks, have a good morning/day/night,
Martin,
alone at home with a cat, getting bored because everything is closed, so I started playing games again...

> RGDS
> GARY
> ---------------------------------------------------------------------------
> Gary E. Miller Rellim 109 NW Wilmington Ave., Suite E, Bend, OR 97703
> 	address@hidden  Tel:+1 541 382 8588
>
> 	    Veritas liberabit vos. -- Quid est veritas?
>     "If you can't measure it, you can't improve it." - Lord Kelvin
>