quoted

main audio thread:
while running:
  snd_pcm_read  // blocks until data is delivered from ALSA
  wake helper(s)
  do work()
  sync_with_helper(s)
  snd_pcm_write // blocks until data is delivered to ALSA

alsa is basically driven by the hardware, which delivers/requests data
every ~1.3ms.

quoted

distributing the work is a little tricky: to avoid excessive scheduling
overhead, the worker threads are typically woken up (after snd_pcm_read)
and the result is collected where sync_with_helper() typically boils
down to busy waiting. in this case, the workers as rate-monotonic in a
similar manner as the main audio thread.

When you say "rate-monitonic", do you mean that the worker threads with
smaller period (high rate, so) have a higher fixed-priority? If so, what
are the other periods?

the period will be the same period: i wake up the workers once per
period of the main thread, distribute the work and collect it at the
end. so "period" is basically the same, "runtime" a little lower.

as sync_with_helper() does a busy-wait, the "main audio thread" has a
relative lower priority than the worker threads, right?

typically all threads have the same priority and there will be at most N
threads for N physical CPU cores. so each thread will have a CPU to run
on and no thread will be preempted.

So, your main wish here is to avoid having a watchdog thread to
"throttle" misbehaving workload/setup?

mainly ... and nerdy research if SCHED_DEADLINE is useful for audio
applications ... ;)

For curiosity, what are the "time-constraints" setup used on mach?

it's pretty similar to the SCHED_DEADLINE constraints:

from:
https://opensource.apple.com/source/xnu/xnu-124.7/osfmk/mach/thread_policy.h.auto.html

/*
 * THREAD_TIME_CONSTRAINT_POLICY:
 *
 * This scheduling mode is for threads which have real time
 * constraints on their execution.
 *
 * Parameters:
 *
 * period: This is the nominal amount of time between separate
 * processing arrivals, specified in absolute time units.  A
 * value of 0 indicates that there is no inherent periodicity in
 * the computation.
 *
 * computation: This is the nominal amount of computation
 * time needed during a separate processing arrival, specified
 * in absolute time units.
 *
 * constraint: This is the maximum amount of real time that
 * may elapse from the start of a separate processing arrival
 * to the end of computation for logically correct functioning,
 * specified in absolute time units.  Must be (>= computation).
 * Note that latency = (constraint - computation).
 *
 * preemptible: This indicates that the computation may be
 * interrupted, subject to the constraint specified above.
 */

cheers,
tim