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

[Lars Luthman]

On Sun, 2015-04-19 at 11:59 +0200, Johannes Lorenz wrote: 
> Hello,
> 
> 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::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)

The next JACK client in the graph, and the JACK server, run in different
processes from the currently running client (except for internal clients
and multiple clients per process, but those are special cases) so there
has to be a context switch if you want to pass control to them.

It's realtime safe as long as you know that those system calls pass
control to the thread you want them to pass control to and not some
other random thread that has nothing to do with JACK. As long as the
semaphores are posted and waited on in the right order, and the
scheduler is properly configured, and all the threads have the right
priority, it will work. You could in theory pass control to other
threads in your process callback in a realtime safe way if everything
was designed correctly, but there isn't much point in doing so and it's
difficult to get right. Most of the time, for most of the uses, I/O
functions, lock functions etc are not realtime safe.


--ll
-------------- next part --------------
A non-text attachment was scrubbed...
Name: signature.asc
Type: application/pgp-signature
Size: 801 bytes
Desc: This is a digitally signed message part
URL: <https://lists.linuxaudio.org/archives/jackaudio/attachments/20150419/b8b4e7d3/attachment.sig>