On 08/20/2018 01:54 PM, Tim Blechmann wrote: >> In your case - assuming you could already use DEADLINE for !root - do >> you think you would need to use the same interface for the multiple >> threads you are distributing work among? Or would you be fine with >> assigning runtime/deadline for each one of those? > > that's a tricky question and depends on the use-case: for me it's "good > enough" to use something like: > > 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. Hi Tim! > > 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? as sync_with_helper() does a busy-wait, the "main audio thread" has a relative lower priority than the worker threads, right? > one could also use lock-free queues with semaphores to wake only as many > threads as needed for the graph topology (which can depend on user > input). in this case SCHED_FIFO sounds more suitable. > -- > > from a practical point of view: i'm not targeting a safety critical > system. one advantage i'm seeing of DEADLINE over FIFO/RR is that it's > easier to prevent lockups (e.g. when a user overloads the system). in > the linux audio world this is typically done by a watchdog thread. the > other part of the unix world (mach) is using time-constraint threads by > default for audio use cases. so i'd assume that DEADLINE would free me > from the need to spawn the watchdog thread ... So, your main wish here is to avoid having a watchdog thread to "throttle" misbehaving workload/setup? For curiosity, what are the "time-constraints" setup used on mach? -- Daniel
