13 Aug
2006
13 Aug
'06
9:52 a.m.
On Sun, 13 Aug 2006 12:46:35 +0200
Dominique Michel <dominique.michel(a)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#Realtimeschedu…
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
--
Palimm Palimm!
http://tapas.affenbande.org