THE GOAL:
I want my Raspberry Pi 2 to automatically start up the Jack server with realtime scheduling, and subsequently start Pure Data with realtime scheduling, load a patch &c. without any user intervention from a login shell.
As a performance artist working primarily with psychodrama (the technology is definitely NOT the important part here), fiddling around at a terminal right before or during a performance is kind of... psychically inconvenient. I need a box that I can plug in, give the audio output to the sound guy, and be ready to go.
PREREQUISITES:
I use Raspbian with a Linux kernel compiled with realtime goodness. I have hand-compiled Jack2 and Pure Data with realtime support in order to take advantage of this. Running a process with realtime priority requires the proper PAM directives set in /etc/security/limits.conf and related places, but that is beyond the scope of this little write-up.
Also somewhat relevant: I use a M-Audio MobilePre USB soundcard (sounds pretty awful by today's standards, but it's an extremely USEFUL box and sounds good enough for the work I do). For full-duplex sound, this requires the RasPi's USB to be set to single speed. In this configuration, I can get just under 2.9ms latency with good CPU overhead for Pure Data to run a few of my 64-voice wavetable and delay line granulators. Yeah!
THE PROBLEM:
Purely by happenstance, I had given the jackd command in my startup script the option “-s” which allows the server to ignore overruns and so on. So things seemed to be working as expected, but I noticed a lot more glitches than when I manually started up Jack and Pd from the terminal without the “-s” option. Upon removing it from my startup script, everything failed! WAH.
So I started piping STDERR and STDOUT to text files so I could read what either Jack or Pd were complaining about. As it turns out, Jack was unable to start with realtime priority due to a permissions problem. (I assume this is one of the things the “-s” options allows jackd to ignore, and thus start up with non-realtime priority. Problem is that Pure Data can’t connect to a non-realtime Jack server when its “-rt” option specified.)
Now, I had already been through the whole rigamarole of setting proper memory and priority limits for the “audio” group, to which the user “pi” belongs. So I thought, okay, I have to execute these commands as “pi”, and while simulating a login shell because the security limits in question are only set during login.
So I did this:
su -l pi -c "/usr/local/bin/jackd -R -dalsa -dhw:1,0 -p128 -n3 -r44100 -S >> /home/pi/jackd.log 2>&1 &"
This says “login as user ‘pi’ and then run the jackd command with these options, piping the outputs to this log file and run it in the background”. Well, I still got all the same errors about not being able to set realtime priority. WHYYYYYYYYY?
THE SOLUTION:
I hunted and hunted and hunted on a Very Popular Search Engine til I decided to try searching “security limits not loaded with su -l” and found this.
(Makes me think of that Talking Heads lyric, “Isn’t it weird / Looks too obscure to me”.)
So by uncommenting the line # session required pam_limits.so in /etc/pam.d/su everything started working as expected.
CONCLUSION:
I now know a LOT MORE about PAM and how important it is to keep in mind when and in what order scripts and other little subsystems are executed; but also that sometimes the problem is EXTREMELY OBSCURE and is to be found in some seemingly far-flung config file.
I hope this helps anybody out there working with Pure Data and the RasPi. The second generation board really packs quite a punch and can run several hundred audio grains (run by vline~ and enveloped by vline~ and cos~) simultaneously without a problem. And I'm pretty sure this is just using ONE of the 4 cores!
I'm by no means an expert Linux sysadmin, so if you have any other suggestions or corrections, please let me know! I wouldn't have been able to get this far without all the generous and helpful writeups everybody else has contributed, both within the RasPi and Pure Data communities. If you have any questions about anything I glossed over here, I'll do my best to answer them.
