[LAT] Fwd: [LAU] Are RT-patches needed anymore? (Was Re: >= 2.6.27 RT ETA?)

Lars-Erik Helander lehswe at gmail.com
Mon Feb 2 14:35:52 EST 2009

My response only got sent to Fernando. Here it is for all of you to see :)


---------- Forwarded message ----------
From: Lars-Erik Helander <lehswe at gmail.com>
Date: 2009/2/2
Subject: Re: [LAT] [LAU] Are RT-patches needed anymore? (Was Re: >=
2.6.27 RT  ETA?)
To: Fernando Lopez-Lezcano <nando at ccrma.stanford.edu>

Fernando your elaborate response is highly appreciated, thanks!!

>>   - Will "rtirq" make a difference on a non-RT-patched kernel system?
> No AFAIK. Non-rt patched kernels do not have irq processes running as
> separate SCHED_FIFO threads (which is what is tuned by rtirq).
This was my thought to.

>>   - Do I need to assign rtprios explicitly to all my processes
>> (qjackctl, qsynth, ...) or is that "automagic"?
> Automagic. Jackd gives the proper priority to the clients (to the thread
> of the client that handles audio i/o)
I use "lmms" every now and then. I have never been able to get "lmms"
to work well with jack so I use it with ALSA as the audio driver. This
works fine though I can not use jack application simultaneously. So in
this case do I have to explicitly give "lmms" SCHED_FIFO and rtprio?
How (using chrt?) ?

>> I have close to 30 years experience in developing realtime systems and
>> has been using Linux for audio in various distribution the last two
>> years. Further I have built RT-kernel for couple of distributions
>> (this weekend I successfully built and ran a 2.6.28-rt kernel from the
>> new git tree :) ).
> Ah, I have not tried this yet! How did that work out? Any errors or
> problems on boot? How exactly did you go about building the whole thing?
About a week ago building from the git produced a kernel that did not
boot properly, actually it did not even build properly and my own
remedy for the build problem resulted in a kernel that was not able to
boot up properly. However building from the git this weekend produced
a well working kernel. Only problem found is the softirq.c bug that
has been around for a while (it makes USB MIDI keyboards etc not to
work - they are discovered but no MIDI events are received) so I had
to apply a patch for that problem.
Build procedure was very straightforward using the git url I cloned
the git and got myself a complete (already "patched" kernel source
tree) and it was just to do the normal "make oldconfig", "make
menuconfig" ... procedures.

>> However I have no experience in using newer non-RT
>> kernels but I am very interested to find out how well you can have
>> them work, especially since whenever you find a nice distro you can
>> not use it because the hazzle of building an RT-kernel for that
>> particular distro is so big :(.
> Planet CCRMA (http://ccrma.stanford.edu/planetccrma/software/) has rt
> patched kernels of the* flavor for fc9 (in the "testing"
> repository) and fc10, and should be easy to install (not an objective
> assessment, I created and maintain Planet CCRMA :-).

I have also sucsessfully built and used with the Slitaz
(www.slitaz.org) distribution (the git kernel also works with that
In order to use the kernel with my USB MIDI devices I had to apply the
above mentioned softirq.c patch.


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