Re: [linux-audio-dev] Re: desktop and multimedia as an afterthought?

[hui]

On Tue, Jul 13, 2004 at 01:09:28PM +0100, Martijn Sipkema wrote:
> [...]
> > Please double-check that there are no priority inversion problems and that
> > the application is correctly setting the scheduling policy and that it is
> > mlocking everything appropriately.
> 
> I don't think it is currently possible to have cooperating threads with
> different priorities without priority inversion when using a mutex to
> serialize access to shared data; and using a mutex is in fact the only portable
> way to do that...
> 
> Thus, the fact that Linux does not support protocols to prevent priority
> inversion (please correct me if I am wrong) kind of suggests that supporting
> realtime applications is not considered very important.

Any use of an explicit or implied blocking mutex across threads with differing
priorities can results in priority inversion problems. The real problem, however,
is contention. If you get rid of the contention in a certain critical section,
you then also get rid of latency in the system. They are one and the same problem.

> It is often heard in the Linux audio community that mutexes are not realtime
> safe and a lock-free ringbuffer should be used instead. Using such a lock-free
> ringbuffer requires non-standard atomic integer operations and does not
> guarantee memory synchronization (and should probably not perform
> significantly better than a decent mutex implementation) and is thus not
> portable.

It's to decouple the system from various time related problems with jitter.
It's critical to use this since the nature of Linus is so temporally coarse
that these techniques must be used to "smooth" over latency problems in the
Linux kernel.

I personally would love to see these audio applications run on a first-class
basis under Linux. Unfortunately, that won't happen until it gets near real
time support prevasively through the kernel just like in SGI's IRIX. Multimedia
applications really need to be under a hard real time system with special
scheduler support so that CPU resources, IO channels can be throttled.

The techniques Linux media folks are using now are basically a coarse hack
to get things basically working. This won't change unless some fundamental
concurrency issues (moving to a preemptive kernel with interrupt threads, etc..)
change in Linux. Scattering preemption points manually over 2.6 is starting to
look unmanable from all of the stack traces I've been reading in these latency
related threads.

That's all. :)

bill

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