[Jack-Devel] JACK 2 is not real time safe on posix?

Johannes Lorenz johannes89 at mailueberfall.de
Sun Apr 19 11:59:15 CEST 2015


it is often being said that JACK 2 is real time safe, for example in the JACK faq. I wondered how JACK 2 suspends a thread, so I checked what happens after the JackClient finished CallProcessCallback():

  -> JackClient::CycleWaitAux()
  -> JackClient::WaitSync()
  -> JackGraphManager::SuspendRefNum()
  -> JackConnectionManager::SuspendRefNum()
  -> virtual JackSynchro::TimedWait()

However, I found only two classes in the `posix' folder that implement this virtual function:

  * JackPosixSemaphore: uses `sem_timedwait()' to block
  * JackFifo: uses `read()' to block

However, both `sem_timedwait()' and `read()' are syscalls, so they contain context switch, which means they are non realtime. Actually, the JACK documentation warns to use function like these if you pass a process() callback to jack [1].

So how can JACK claim it would be realtime? (No objections, I just don't understand it)


[1] http://jackaudio.org/files/docs/html/group__ClientCallbacks.html#gafb5ec9fb4b736606d676c135fb97888b

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