Kernel PPS processing

[Gary E. Miller]

Yo Achim!

On Sun, 03 Jul 2016 12:10:44 +0200
Achim Gratz <Stromeko at nexgo.de> wrote:

> Eric S. Raymond writes:
> > Gary E. Miller <gem at rellim.com>:  
> >> Anyone got a guess what the equivalent RasPi setting to turn off
> >> power saving would be?  
> >
> > turbo=1 in /boot/config.txt, I think. See also  
> 
> No.  That used to be "force_turbo=1", but is not needed anymore.

As of what kernel?  I notice RasPi's have all sorts of weird kernel
versions and patchsets.  My Gentoo RasPi is at 4.4.14-v7,
but wheezy is at 4.1.19-v7.  Worse my Odroid is at 3.10!

>  You
> simply define the maximum (and perhaps minimum) frequencies in
> /boot/config.txt and chose the proper CPUfreq governor.

Do I need to do this, or is 'performance' enough?  What would you put in 
config.txt to set the performance governor?

>  Default is
> ondemand, you will either want powersave (to keep at the low
> frequency) or performance (force maximum frequency).  So far I
> haven't seen a difference between those when it comes to NTP
> performance, but I'm running two isolated DCF77 clocks only at the
> moment.

How are you measuring?  Eyeballing 'ntpq -p' or running a week of
gnuplots?

> I can't find any obvious benefit
> from using nohz=off on the kernel line either, but then I'm not yet
> using a GPS receiver for the PPS.

Seemed to help about 5% for me.  I need a few hours of data to see the
difference.  A good test takes days, but eventually I'll loop back and
retest things.

My biggest problem is that restarting ntpd puts huge spikes into my
timing data, making it hard to see small changes.

> Keep in mind that the PPS
> timestamping is actually done by the VC4 and not the ARM in the BCM
> SoC.

Very interesting.  That is a new twist.  Got a citation for that?

RGDS
GARY
---------------------------------------------------------------------------
Gary E. Miller Rellim 109 NW Wilmington Ave., Suite E, Bend, OR 97703
	gem at rellim.com  Tel:+1 541 382 8588
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