Re: [ck] [PATCH][RFC] sched: Isochronous class for unprivileged soft rt scheduling

[Lee Revell <rlrevell@joe-job.com>]

On Tue, 2005-01-18 at 10:17 -0600, Jack O'Quin wrote:
> Cal <hihone@bigpond.net.au> writes:
> 
> > There's a collection of test summaries from jack_test3.2 runs at
> > <http://www.graggrag.com/ck-tests/ck-tests-0501182249.txt>
> >
> > Tests were run with iso_cpu at 70, 90, 99, 100, each test was run
> > twice. The discrepancies between consecutive runs (with same
> > parameters) is puzzling.  Also recorded were tests with SCHED_FIFO and
> > SCHED_RR.
> 
> It's probably suffering from some of the same problems of thread
> granularity we saw running nice --20.  It looks like you used
> schedtool to start jackd.  IIUC, that will cause all jackd processes
> to run in the specified scheduling class.  JACK is carefully written
> not to do that.  Did you also use schedtool to start all the clients?
> 
> I think your puzzling discrepancies are probably due to interference
> from non-realtime JACK threads running at elevated priority.

Isn't this going to be a showstopper?  If I understand the scheduler
correctly, a nice -20 task is not guaranteed to preempt a nice -19 task,
if the scheduler decides that one is more CPU bound than the other and
lowers its dynamic priority.  The design of JACK, however, requires the
higher priority threads to *always* preempt the lower ones.

Lee