>> 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. >> 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
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