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Paul Coccoli wrote:

quoted text
> On Fri, Oct 17, 2008 at 8:46 AM, Paul Davis  wrote:

>> On Fri, 2008-10-17 at 14:13 +0200, Olivier Guilyardi wrote:

>>

>>> Okay, I wrote such a test. It fails with Jack's ringbuffer (jack1 r3004) but

>>> succeeds with Portaudio's one (r1240).

>> Nice work. Nobody ask why we didn't do this 5 years ago!

Thanks Paul :)
>>

quoted text
>>> The Portaudio code looks more and more robust to me. It's also surprisingly

>>> short. Maybe that the best would be to replace jack's ringbuffer with it? I

>>> think it should be possible to keep the jack_ringbuffer api unchanged.

>> I'd rather add the memory barriers to the JACK code, but this could be a

>> race to see who does what first. A memory barrier is typically single

>> instruction. The complication tends to be defining them in a

>> sufficiently portable way.

>>

> Why do you suspect you need memory barriers?  My concern with

> ringbuffer.c is the non-atomic ops on the read and write pointers.

> They're marked volatile, but what I think you really want is make all

> ops on those fields atomic.  Stuff like this:

[...]
As I said, the Portaudio ringbuffer passes the test even with memory barriers

deactivated. But the lack of memory barriers in jack code was the reason I wrote

this test, please read the whole thread for links to relevant documents.
Yeah, at PulseAudio they say that the way jack ringbuffer uses volatile is wrong.
Anyway, I've read a lot of different (and very opposed) point of views on these

matters, and IMO there's nothing like thorough testing to sort this out.
To figure whether memory barriers are needed or not, we need to run the test on

various architectures.
For this purpose, I've set up a small svn repo with everything to run the test

easily. There's no dependency, you just need pthread, gcc and sh.
Here's how to run the test:
svn co http://svn.samalyse.com/misc/rbtest

cd rbtest

make test
It tests the ringbuffers from (all included) jack, portaudio and portaudio

without barriers. That can take up to 6 minutes. It has to be a bit long due to

the probabilistic nature of failures.
LAUsers could you please run this test on various architectures? Especially

PowerPC single-cpu and SMP, x86 single-cpu, SMP, in various flavors, etc...
Cheers,
--

  Olivier Guilyardi / Samalyse

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