Re: Clarifications about PPS SHM content

[Gary E. Miller]

Yo Martin!

On Mon, 23 Mar 2020 21:49:23 -0400
Martin Boissonneault <address@hidden> wrote:

> > 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.

Well, systemd(umb) says it all.  "Just Say No" (tm)

> 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.

They are much bigger than the newer SDRs and I doubt their marketing
that their LNA is better then the newer generic ones.

> 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!

Yeah.  That may be your limiting factor Be sure you gpsd and ntpd are
running at high priority.

> 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)

Can you quantify how much that helped?

> 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.

Yeah, setting it as the kernel default is pointless.  Almost every
distro overrides that.

> > 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/>
> http://ve2mrx.dyndns.info:10180/ntp/week/ 
> <http://ve2mrx.dyndns.info:10180/ntp/week/>

Pretty good, but not perfect, temp control.  Prolly no winto improve
that.  Something swung your frequency hard at 23 Mar 12:00Z

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

Nice.  Your disk I/O is a bit high for best timing.  Maybe buffer that
more?

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

Only showing a 2D fix, not optimal for timing.  Prolly good enough
considering your CPU and I/O issues.

> SELF-ISOLATE! SOCIAL DISTANCING IS (mostly) OUTDATED!

Sounds like a normal month to me.  I've been working from home for
years.

> 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... :-)

I just put the RasPi in a cardboard box with a small light bulb.  Then
toggle the light bulb on and off.  Pretty cheap.

Your temp change is so small that cpuheat may be enough to get it
solid.  No cost to that.

> alone at home with a cat, getting bored because everything is closed,
> so I started playing games again...

Sadly my dog died last year, so not even a pet here.  gpsd is my video
game of choice.

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

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