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Olivier Guilyardi wrote:

quoted text
> Paul Coccoli wrote:

>> 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 :)

>

>>>> 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,

> 

vendor_id       : GenuineIntel

cpu family      : 6

model           : 15

model name      : Intel(R) Core(TM)2 CPU         T7200  @ 2.00GHz

stepping        : 6
Starting ringbuffer tests (buffer size: 512)
=== Jack ringbuffer test ===

starting ringbuffer stress test (2 minutes max)

buffer size (bytes): 512

array size (bytes): 256

reader started on cpu 0

writer started on cpu: 1

48128 != 48000 at offset 0

failure in chunk 971504
=== Portaudio ringbuffer test ===

starting ringbuffer stress test (2 minutes max)

buffer size (bytes): 512

array size (bytes): 256

reader started on cpu 1

writer started on cpu: 0

Success
=== Portaudio ringbuffer test (without memory barriers) ===

starting ringbuffer stress test (2 minutes max)

buffer size (bytes): 512

array size (bytes): 256

reader started on cpu 1

writer started on cpu: 0

Success
It would be great if this got fixed since we (FFADO) exercised the GPL

on the jack ringbuffer code...
Greets,
Pieter

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