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

Paul Davis paul at linuxaudiosystems.com
Tue Jun 23 22:44:24 UTC 2009


On Tue, Jun 23, 2009 at 6:19 PM, Fernando
Lopez-Lezcano<nando at ccrma.stanford.edu> wrote:

> see here for an interesting entry:
>  https://bugzilla.redhat.com/show_bug.cgi?id=442959

that is hilarious :)

> and
>  http://bugzilla.kernel.org/show_bug.cgi?id=10361
>  (referenced inside the previous ticket)
> this is when it was apparently added to the Fedora kernels:
> --------
> * Fri Apr 18 2008 Kyle McMartin <kmcmartin at redhat.com> -
> Enable CONFIG_RT_GROUP_SCHED (#442959)
> --------
>
> There's something else I'm missing because I'm still able to run rt as a
> non-root user even though I have all this stuff. Is Fedora configuring
> this somewhere else? Is this overriden by something else?

this seems deeply odd. i am also in that same situation. it seems that
something *must* be overriding it, but what?

>> the API looks entirely usable to me, in the sense that i could easily
>> imagine a GUI control panel for this that would be comprehensible to
>> most users.
>
> I think the idea is no control panel :-)

yeah, i note that in the second bug report there, peter z. makes some
comments about RT scheduling that are not entirely accurate for our
use case.
we are aiming at deterministic behaviour by each client in a JACK
graph, but we don't actually expect determinism across time (e.g. as
clients are started and stopped). we're happy to eat 95% of the CPU
time, even if we generally expect the typical figure to be more like
10%. this tends  to force JACK towards a cgroup that has a perhaps
unreasonably large allocation, and a misleading one. AFAICT, the
Sum(all cgroup runtime) cannot exceed the rt sched period. So you
cannot set things up to allow "JACK" to get 95% and <other RT stuff>
to get 95% and just have stuff break if you try to do both. this is
good and bad - its an accurate reflection of resource reservation
policy, but it doesn't actually reflect what a regular user may want -
running two RT subsystems at different times without reconfiguring
their cgroups. of course, i have no idea what those two RT subsystems
might be, so its a bit moot.



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