[linux-audio-user] kernel - using rtlimits, realtime_lsm

Hector Centeno-Garcia h.centeno at sympatico.ca
Thu Dec 22 01:29:08 EST 2005

Josh Lawrence wrote:

>Hello all,
>I'm a bit over my head here, so please be gentle :)
>I've been working with Ubuntu 5.10 and trying to use it as an audio
>machine.  I have been learning about different approaches to getting a
>multimedia kernel (low latency, etc.).  In reading around the web, I
>see that there are three general approaches:
>1.  Build your own kernel from scratch - too complicated for me at this time
>2.  Use the realtime_lsm module on an existing kernel - I've tried
>this, but I read it's no longer supported in the kernel because of
>3.  Use rtlimits, which is already a part of the default kernel.
>Last night I downloaded the set_rtlimits program and was able to get
>jackd running with real time capabilities.  I haven't done any
>real-world work yet, so I don't know if my efforts were truly
>successful.  Here are my questions:
>1.  Is rtlimits all that I need for audio work?  It seems way too
>simple.  I remember that people using 2.4 kernels had at least two
>patches that had to be applied before the kernel was suitable for
>audio work.  If I use rtlimits, do I still need to build a kernel
>with, for instance, the ck patches for a preemptable kernel?
>2.  Is anyone using rtlimits instead of realtime_lsm?  Is there any
>benefit to using one over the other, in terms of real-world audio
>3.  Should I just break down and install DeMudi?  :)
>I hope my questions make sense.  Please feel free to correct any
>errors in my logic here, that is exactly what I am looking for - to
>learn how to do things myself the right way.
>Josh Lawrence

This thread comes really handy to me. These last days I've been 
comparing the performance of Ubuntu Breezy and Agnula 1.3.0-RC1. I've 
been trying to get the grate support and user friendliness of Ubuntu 
with an audio performance as good as an audio specialized distro.  I 
just finished compiling a 2.6.14 kernel in Ubuntu with the rt patch and 
lsm module (this last one was quite easy to install using the 
module-assistant and the realtime-lsm package from the Ubuntu 
repositories). For some reason I found that the kernel in agnula 
performs better than the one I compiled in Ubuntu even though both are 
the same version, have complete preemption and use jack in realtime. In 
agnula I can get very stable 2ms but in Ubuntu trying the same latency 
fails with clicks in the audio related to the video activity (i.e. 
maximizing and minimizing windows, or very active GUIs). In both I'm 
using the latest nvidia drivers.

Until now I've been using Ubuntu with the stock and the lsm 
module and I'm quite surprised with the performance compared with the 
previous version of Ubuntu (named Warty Warthog), I can get latencies as 
low as 5ms almost completely stable but for Csound I have to set it to 
11ms if I don't want to risk getting a sporadic click while recording a 
performance in realtime. For my needs until now this has been enough.

Have you compiled the realtime-lsm module in Ubuntu using the stock 
kernel? One aspect that I haven't understood completely is the setting 
of priorities both in Jack and the IRQs. I've tried following the 
instructions at the tapas web site using | chrt -f -p 99 `pidof |but I 
can't notice any difference in the performance (using a complete 
preemtion kernel).

If you want to keep trying with Ubuntu in the forums and wiki you can 
find excellent how-tos about building kernels. I can post some links if 
you are interested.

Good luck!


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