[linux-audio-dev] 2.6.15.7-rt

[Florian Paul Schmidt]

On Sun, 13 Aug 2006 12:46:35 +0200
Dominique Michel <dominique.michel at citycable.ch> wrote:

> A rt kernel will be of almost no use if you don't fix the priorities.
> You can use PAM-rlimits or the realtime-lsm module for that, and you
> have to be in the audio group.
> 
> The audio group will have a higher priority and this is the right way
> to do that.
> 
> You can take a lock at
> http://demudi.agnula.org/wiki/Low-latencyKernelBuildingHowto#Realtimescheduling
> 
> PAM-rlimits is the new way of doing that, but if you want an very
> low latency for intensive audio work as synthesis or filter, the
> realtime.lsm will be better.

There's no difference wrt to achievable latencies regarding lsm and pam.
Both are just ways to grant non root users SCHED_FIFO priorities for
their tasks.

> A very important issue with a rt kernel is at you must not have shared
> IRQ with a such kernel. You can check it with
> # cat /proc/interrupts
> It is specially important for the sound and the video card.
> http://ardour.org/system_requirements

This is also not true stated like that. It is very well possible to have
shared IRQ's in a -rt system. Usually one doesn't want to have one's
soundcard IRQ shared though, because then the other device driver [and
irq handler] runs at the same prio potentially causing delays for out
soundcard.

> 
> If you are new with rt-kernel, I would recommend you to install an
> audio distribution as Demudi, planet CCRMA, jacklab, musix, or the
> gentoo audio pro overlay. They all have that rt stuff trimmed for you.
> 
> With usb audio card, the IRQ period of the USB bus is 1 msec. That
> mean at you must use something as 48KHz 3 periods with jack (That will
> be a multiple of that irq period) in order to get a low
> latency with such sound cards.

> And you MUST NOT be root in all cases with a rt-kernel.

This is also not true. Of course it is possible to run your stuff as
root with a -rt kernel. This also saves you the hassle of setting up
either the realtime lsm or pam. But it is not recommended as running
user stuff as root is often a security risk.

Flo

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