[linux-audio-dev] NPTL and real-time sched semantics

[Kai Vehmanen]

Has anyone done any low-latency audio testing with the new native pthread
implementation (comes with rh9.0 at least)...?

I was just reading through [1] and noticed the following (in section 8):

""Realtime support is mostly missing from the library implementation. The
system calls to select scheduling parameters are available but they have
no effects. The reason for this is that large parts of the kernel do not
follow the rules for realtime scheduling. Waking one of the threads
waiting for a futex is not done by looking at the priorities of the
waiters.""

If I've understood right, SCHED_FIFO semantics do not have any meaning
between threads of one process if NPTL is used! Hopefully this is right,
as it would cause quite a lot of problems (GUIs and disk i/o threads can
freely block audio processing even though using SCHED_FIFO). :(

[1] http://people.redhat.com/drepper/nptl-design.pdf

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