[linux-audio-user] Realtime Audio + Networking

Joshua Boyd jdboyd at jdboyd.net
Tue Jun 6 11:10:23 EDT 2006

I have a realtime audio/video system that almost works fine.  I'm not
too terribly concered with latency, so I'm using 400 samples of
buffering both in and out.  For the most part it all works.  System load
is about 1.5 according to top, and the 3 processes of my system list
approximately 75% combined CPU usage. 

However, I need the system to be network connected.  I need it to have
sshd, snmpd, and some web server (not yet decided upon) running.  When I
turn them on, any network traffic to either service causes glitches in
the audio, and possibly lost video frames.

I am using Linux 2.6.12 plus bigphysarea patch.  I'm using a custom
non-Alsa sound card driver (when I started, I needed all of the AES
metadata, but I no longer do.  At this point it is too late for me to
convert to using alsa though).  I can provide my kernel .config file if
that would help.

I have CONFIG_PREEMPT and CONFIG_PREEMPT_BKL on.  My application runs
with SCHED_FIFO and has an elevated priority, but not realtime
priority.  When I tried setting it to realtime priority it grinds the
networking, control panel, and VGA display applications to a halt.

If I set the audio driver to loop back, the network never interupts it.
It would be difficult to move the entire audio portion of my system into
a kernel module though.

I've been watching for awhile and haven't seen to much about how to fix
this.  Meanwhile, I've been tangling with this for months.

Joshua D. Boyd
jdboyd at jdboyd.net

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