[LAD] [ANNOUNCE] Safe real-time on the desktop by default; Desktop/audio RT developers, read this!

Fernando Lopez-Lezcano nando at ccrma.Stanford.EDU
Wed Jun 24 03:25:35 UTC 2009

On Mon, 2009-06-22 at 15:14 +0200, Lennart Poettering wrote:
> On Sun, 21.06.09 20:58, Fernando Lopez-Lezcano (nando at ccrma.Stanford.EDU) wrote:
> > > > The question is relevant, I think, as the kernels that I use (Planet
> > > > CCRMA) are the rt patched kernels, currently limited to (I
> > > > think Thomas and the rt gang are working on 2.6.30, I imagine 2.6.31
> > > > support is still far in the future). 
> > > 
> > > Dunno. I disagree. The primary objective for rtkit is to be able to
> > > run media applications as RT by default, out-of-the-box. I doubt that
> > > this feature matters for legacy distros/kernels.
> > 
> > I'm talking about the rt kernels specifically. In that context users
> > don't have a real choice as to what they get to run and the meaning of
> > legacy is different (I understand you may not care because of the target
> > audience of PA, but then again the RealtimeKit is being proposed as a
> > generic solution for access to rt scheduling). 
> > 
> > If the rt patch does not catch up fast enough then I may need to run a <
> > 2.6.31 kernel (whatever is available at the time) in Fedora 12 when it
> > is released, hardly a legacy distro :-) 
> > 
> > (maybe unlikely, but it would not be the first time rt development
> > stalls for a looong time and you can't run the latest and greatest - not
> > complaining, just a fact)
> Again, the worst thing that happens is that you need to bump
> RLIMIT_RTPRIO for your user, as you always did. rtkit doesn't take
> that away. 
> Summary:
> Kernel that lacks SCHED_RESET_ON_FORK: RLIMIT_RTPRIO is what matters.
> Kernel that has SCHED_RESET_ON_FORK: RLIMIT_RTPRIO still matters when
> it is set, rtkit is used as fallback.

I understand. 

> Also note that supporting rtkit often enough doesn't add a build-time
> dep to you application and the runtime dependency is very soft too: if
> it isn't found it's not used, that's all.

Let's assume for a moment that when rtkit becomes part of Fedora the rt
kernel patch has not caught up to 2.6.31, would you consider changing
the behavior so that if the kernel does not have SCHED_RESET_ON_FORK
(ie: it is older than 2.6.31) rtkit would still grant rt if the other
conditions are met? Or add a boot time configuration to be able to do

That would mean that _temporarily_ older rt kernels (and jack users that
prefer or need to use them) would be able to use the same out of the box
rt granting mechanism as the rest of Fedora. 

-- Fernando

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