Using the following command to check cpu frequency on RPi model 2B:

sudo watch -n 1 cat /sys/devices/system/cpu/cpu*/cpufreq/cpuinfo_cur_freq

I found that RPi model 2B does frequency scaling by default (nothing
modified in /boot/config.txt). It switches between 600 and 900 MHz.
This doesn't really give substantial power saving so I wanted to
replace 'ondemand' governor with 'performance' by setting
'force_turbo=1' in config.txt. But this caused all sorts of
instabilities, at least in combination with Pd, and I had to disable
turbo.

So there's two things going on which Pd doesn't like: frequency
scaling and core switching. Both happen on my core2duo laptop as well,
where Pd doesn't have a problem. Anyway, pd's '-nosleep' option saves
the day.

Now for the good news: performance gain. I compared model B+ with 2B
by running instances of Martin Brinkmann's chaosmonster1.pd
(http://www.martin-brinkmann.de/pd-patches.html) at default samplerate
and with 10 ms buffer. Model B+ can run one chaosmonster without
xruns, but not two. Model 2B can run five instances of
chaosmonster1.pd without xruns, while the GUI remains fully
responsive. This is not poor Pi as we know it, it's a different beast.
I must say that puredata 0.46-2 was compiled from Raspbian jessie
source on the Pi itself in both cases. This process uses GNU autotools
which may have set platform specific optimizations. In any case,
performance gain is much more substantial than I would expect.
Hopefully I'm not dreaming.

Katja

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