Re: [Jack-Devel] Real Time extensions for JACK

On Fri, 2011-04-15 at 14:56 -0400, Paul Davis wrote:
> On Fri, Apr 15, 2011 at 2:49 PM, Bearcat M. Sandor
> <[hidden]> wrote:
> > On 4/14/2011 7:55 AM, John Rigg wrote:
> >>
> >> JACK requires realtime scheduling privileges, which the standard Linux
> >> kernel has been able to provide for a long time now.
> >>
> >> The -rt patch for the kernel provides realtime preemption, which is
> >> not required by JACK.
> >>
> >> John
> >
> > Can someone explain what the difference between realtime scheduling and
> > realtime preempting is?  I know what the words mean, but what  does a system
> > do differently in each case?
> 
> simple, short answer limited to SCHED_FIFO realtime scheduling:
> 
>    * a thread that is set to run with SCHED_FIFO will run forever, or
> until it blocks
>         (e.g. it goes to sleep or does file i/o). it will also be
> temporarily interrupted
>         by an interrupt handler, for as long as the handler runs.
> 
> this is totally unlike normal scheduling, where threads will be
> stopped so that something else can run for a while.
> 
> that's realtime scheduling, and access to it is "controlled" on linux
> (not on OS X)
> 
> realtime preemption is a bad name, but what is generally being
> referred to is how interrupts are handled. in a regular kernel, when a
> device
> interrupts the CPU, the handler will run right away and will continue
> to run until its finished. in kernels with threaded interrupt handlers
> (the most important part of "realtime preemption"), only a tiny part
> of the handler will run, and the rest of it takes place in a thread
> whose priority can be controlled. this means that bulk of the work
> done by an interrupt handler for, say, a video device or a keyboard or
> network interface can be explicitly prioritized to be "less important"
> than a SCHED_FIFO thread, meaning that even interrupts from these
> devices will have very little impact on the execution of the
> SCHED_FIFO thread's code. there are other aspects to "realtime
> preemption", but its not really worth getting into them here.

Thank you Paul!  I had to read it twice, but that was a very clear
explanation.  

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