Realtime Kernel in Ubuntu 9.04

Real-Time Support

After you've got the kernel by installing linux-rt, linux-headers-rt, you still need to set up real-time access for your applications.

All you have to do for this is give your audio group permissions to access the rtprio, nice, and memlock limits. To do this, you just need to run these commands, which will add some lines to the file /etc/security/limits.conf:

sudo su -c 'echo @audio - rtprio 99 >> /etc/security/limits.conf'
sudo su -c 'echo @audio - nice -19 >> /etc/security/limits.conf'
sudo su -c 'echo @audio - memlock unlimited >> /etc/security/limits.conf'

These value are suggested by http://jackaudio.org/faq. The memlock line determines how much of your memory can be locked by audio processes. Some recommend setting this as half of your total memory (RAM, in KB). See Florian Paul Schmidt's page.

In Intrepid and Januty Beta, you have to create a user group named "audio" and add your user name (and other users of the workstation if needed) to this group. You can do that very simply in "System / Administration / User Group".

Restart the workstation and it is ok.

reference here: https://help.ubuntu.com/community/UbuntuStudioPreparation