[LAU] Trying to get rid of xruns

[Matthew Smith]

Hi Folks

After a long (15 years) break in music making/recording, I have just set 
myself up with a new virtual studio - bit of a culture shock when all my 
previous experience was analogue plus Notator on an Atari.

I'm running Gentoo Linux (x86_64) on a dual-core ThinkPad with 1.5Gb 
RAM.  To save unnecessary processing overhead and because I like a 
'clean' desktop, I am using the Fluxbox window manager.

My current studio consists of:

* Jack
* Rosegarden
* Zynaddsubfx
* Xsynth
* Fluidsynth
* Hydrogen
* Samson C01U USB microphone
* Behringer UCA202 audio adapter
* Roland/Edirol PC-300 MIDI controller

In an attempt to fix any latency issues, I have used the RT-modified 
ketchup to get myself a kernel 2.6.25.8-rt7.  RT profile is selected, 
timer at 1kHz.  I have stripped much of my day-to-day stuff out of the 
kernel configuration so that I have a dual-boot configuration, one for 
'normal' work, the other just for audio.  I have also created a separate 
profile for the RT kernel so that only essential services are started.

Whilst going from the 'low-latency desktop' of my standard kernel to the 
RT kernel has made a huge difference in the number of xruns that I am 
seeing on Jack, it hasn't cured them completely.

My test configuration routes my MIDI keyboard through Jack, to 
Rosegarden to Jack, to Zynaddsubfx to Jack, to ALSA out.  I find that 
whilst playing - even when pushing down an unreasonable number of keys 
very quickly - I don't get any clicks or pops.  I do, however, get the 
occasional xrun.  Weird thing is that sometimes I get them when I'm 
hardly doing anything at all.

Jack connection parameters are priority=9, Frames/Period=512, Sample 
Rate=44100, Periods/Buffer=3.

At the end of dmesg, there seem to be a lot of errors or warnings - 
don't know if they're significant.  I've put them here:
<http://www.smiffysplace.com/files/dmesg_rt_20080731>
(For some reason, the top bit of dmesg got lost - hope this doesn't 
matter.  Can re-create if required.)

Kernel config is here:
<http://www.smiffysplace.com/files/kconfig_rt_20080731>

PAM is configured for the audio group (yes I am in it!):
@audio  -  rtprio 99
@audio  -  memlock 250000
@audio  -  nice -10

I've now pretty well exhausted the results of my Googling but am still 
not sure what I'm doing wrong or if these xruns may be safely ignored 
(I'm assuming they can't be).  Any suggestions would be greatly appreciated!

Cheers

M

-- 
Matthew Smith
Smiffytech - Technology Consulting & Web Application Development
Business: http://www.smiffytech.com/
Personal: http://www.smiffysplace.com/
LinkedIn: http://www.linkedin.com/in/smiffy