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Dapper preemptive kernel and rtlimits February 20, 2006

Posted by oktyabr in audio, debian, linux, realtime, ubuntu.
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Discovered that the Dapper 6.04 “flight 3” kernels, including the one on the iso report the following:

#cat /boot/config-2.6.15-12-386 | grep PREEMPT

#CONFIG_PREEMPT=y

This is something that has been missing from the current “breezy” kernels and seems to suggest that Ubuntu will be offering a true preemptive kernel that should improve desktop latency for realtime apps like jackd and clients!

Abandoning “realtime-lsm” for “rtlimits”, which I first read about at Jonathan Woithe’s Home page has made configuring realtime access on a user/group level basis as well as per invidual application very easy and most importantly, FAST!

Refer to his page on “set_rtlimits” for a handly little app that works quite well to ease this process: http://tapas.affenbande.org/?page_id=22 *updated package at his home page*

This has allowed me to get much better performance than when using ‘realtime-lsm’ on a breezy kernel… 5.8ms in KDE and 2.9ms in a conservative fluxbox or even xfce4 enviroment under load with zero xruns and cpu-dsp usage (as reported by qjackctl and ardour) usually no higher than 10-11%!

Very nice! Ubuntu as a music creation enviroment. /me is in bliss

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Comments»

1. jon - August 17, 2006

Great tip!

Heres a little more detail for anyone who wants to try this (worked fine for me on Dapper):

Download the tarfile from:
http://www.physics.adelaide.edu.au/~jwoithe/set_rlimits-1.2.0.tgz

Untar it, make and install it (from the directory containing the .tgz file):
$ tar -xvf set_rlimits-1.2.0.tgz
$ cd set_rlimits-1.*
$ make
$ sudo make install

Once its installed:
$ sudo gedit /etc/set_rtlimits.conf
Add the following 2 lines to the bottom of the file, then save it and quit:
ALL /usr/bin/jackd -1 80
ALL /usr/bin/ardour -1 80

This will allow jack and ardour to be run realtime with a maximum priority of 80 (you can add more apps to this file if you wish).

Create a wrapper script to run jack in realtime:
$ sudo gedit /usr/bin/jackd_rt

Add the following 2 lines to this new file, save it and exit:
#!/bin/bash
set_rtlimits -r=30 /usr/bin/jackd $*

Set execute permissions on our jack realtime wrapper:
$ chmod +x /usr/bin/jackd_rt

Bring up qjackctl. Click “setup”, and in the settings tab, uncheck “realtime”. Set “server path” to “/usr/bin/jackd_rt” (without the quotes). In the misc tab, ensure that “Start JACK audio server on startup” is selected.

If you stop and restart qjackctl jack will now be running as realtime (at priority 30 out of 100, running higher doesn’t seem to have any effect).

start ardour with the command:
$ set_rtlimits -r=30 /usr/bin/ardour

This will run ardour realtime. You should now be enjoying latency under 6ms.

Many thanks for the tip!

2. matthew - April 17, 2007

Hi, I really want to find some resources that tell me more about how to get realtime stuff working with ubuntu. I’m using 6.06 and it seems pretty realtime already. For example when I run jackd with -R it does make a difference. However, I’m not able to compile or insert the realtime module.

Any clues?

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