RT_GROUP_SCHED kernel option makes JACK unusable in Ubuntu Natty

Tim Gardner tim.gardner at canonical.com
Tue Dec 14 14:57:08 GMT 2010

On 12/13/2010 09:34 PM, Ronan Jouchet wrote:
> Hi Ubuntustudio-devel, Ubuntu-kernel, JACK-devel, Alessio,
> After talking with adi in #ffado and las in #jack, it appears that
> Natty's kernel needs adjustment to be usable for audio work. Currently
> the kernel flavors shipped by Ubuntu have the RT_GROUP_SCHED option
> activated, which makes JACK unusable (las put up
> http://jackaudio.org/linux_group_sched to sum up the situation).
> *Until this problem is solved, JACK is unusable under Natty*. Las
> mentioned several solutions:
>     0. Disabling RT_GROUP_SCHED for all kernels
>     1. Disabling RT_GROUP_SCHED for the kernel targeting audio users
> (-lowlatency, currently living in abogani's PPA)
>     2. Set up a cgroup and invoke JACK and every client from a shell
> script that joins the cgroup first
>     3. Use cgrules.conf to at least put processes into the right group by
> default
>     4. Use Ubuntu's specific mechanism to put user tasks into a
> RT-enabled cgroup
> Quoting las: "(1) is the desirable option; (2) would be next and (3)
> would be hard, because it would enforce 'application<foobar>  always run
> in the<bazbar>  cgroup' which is not really very useful or correct". (0)
> may also be considered since this kernel option is apparently very
> specialized for embedded systems and doesn't make sense for a desktop
> distro.
> Which path should be followed is beyond my knowledge. I just hope this
> discussion will pave the way to a usable audio stack in Natty.
> Resources:
>     LP bug: https://bugs.launchpad.net/ubuntu/+source/linux/+bug/690010
>     IRC : #jack, #ubuntustudio,  #ubuntustudio-devel
>     IRC log of my discussion with las: http://pastebin.com/2yw7aPhr
> All the best,

I'm not opposed to disabling RT_GROUP_SCHED. After all, it _is_ marked 
EXPERIMENTAL. Have you proven that a kernel with RT_GROUP_SCHED disabled 
actually fixes the issues that you're seeing with JACK ?

Tim Gardner tim.gardner at canonical.com

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