[linux-audio-user] Problems appling realtime preempt patch to 2.6.12 (general)

Fernando Lopez-Lezcano nando at ccrma.Stanford.EDU
Tue Jul 12 16:46:13 EDT 2005

On Tue, 2005-07-12 at 12:56, Iain Duncan wrote:
> >>I've just joined the list and it's pretty interesting.
> >>Can someone tell me something about the differences between the
> >>preempt patch from Ingo and the "Realtime Linux Security Module"?
> >>I currently use the latter one and I'm quite satisfied as jack works
> >>really fast. Perhaps one should try this way instead of the vanilla
> >>kernel patch. There have to be pros and cons I'm interested in. Can
> >>someone point them out? 
> > 
> > 
> > The LSM and Ingo's patches are addressing 2 separate issues. Ingo's
> > patch is working to minimize sources of latency in various code paths in
> > the kernel. The rtlimits and the realtime-lsm are addressing the issue
> > of providing a security model for realtime applications to get the
> > capabilities they need to run in real-time. Ordinarilly these
> > capabilities (CAP_SET_PCAP, mlockall(), and SCHED_FIFO()) are only
> > accessible as root. rtlimits and the realtime-lsm provide infrastructure
> > for allowing these capabilities to be granted to only a specified set of
> > users/processes.
> So am I correct in believing that the preempt patch is the one that 
> makes a difference if I intend to run as root anyway?

It makes (should make?) a difference independent of whether you run as
root or not. If you run as root you can access everything. If you don't
run as root (highly recommended :-) you need some additional stuff to
get the same performance.

> ( Yeah I know I'm 
> not supposed to, but IMO my machine is just a synthesizer in disguise as 
> a computer and I'll unplug the ethernet cable if I need to. )

2.6.12 already includes the rlimits patch that will allow you to run
well as a non-root user. But you will need a patch to pam (if your
distro uses pam) or a special user program to access realtime scheduling
as a non-root user (was posted not long ago to this list).

-- Fernando

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