[Jack-Devel] Helper threads with lower priority?

Johannes Lorenz j.spam at lorenz-ho.me
Sat Jun 29 14:53:45 CEST 2019


we run a DAW (LMMS) with jack and would like to exploit multiple cores. 
Currently, we have one jack process (using `jack_set_process_callback`) 
and multiple other processes that share all the work. The problem is 
that only the jack process has RT prio. Imagine the following example:

* 4 threads, of which one is the jack thread, 3 have lower priority
* 4 tasks (e.g. 4 instruments play one note each)
* the realtime process is finished quickly, the other threads get 
preempted by the kernel

These are situations where using such non-realtime helper threads can 
threaten realtime behaviour. What is the best practice?

* Is it possible/good to use multiple rtprio threads controlled by jack? 
(multiple clients, or multiple threads in one client)
* Should we keep using our non-rt helper threads?
* Should we be single-threaded, to have a (probably) longer average 
time, but a better worst-case time?
* Should we start with helper threads and only the last few jobs will 
only be run by the realtime thread (to avoid problems like in the 

Many thanks on advance!


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