[LAU] Realtime latency kernel testing

[Robin Gareus]

On 01/07/2011 01:32 PM, torbenh wrote:
> On Sun, Dec 12, 2010 at 07:22:28PM +0100, Jeremy Jongepier wrote:
>> On 12/12/2010 06:42 PM, Ronald Stewart wrote:
>>> Hi,
>>>
>>> I would go with what Robin said.  That being said, Robin's tweaks on
>>> Transmission last year on 89 set the pace for our build (brilliant!).  Since
>>> then tuning jack2 (jackdmp) Rui's rtirq, plus tuning for specific chips /
>>> computer hardware makes a difference.  If you want something now that truly
>>> stands up and has had some of the best Linux developers touch the project,
>>> go with Transmission 4.2.  I know Paul will jump in and tune us all up with
>>> our thoughts (go Paul!) but it should be stated again we are getting lights
>>> out performance without RT on our new multi-touch Tablets for Pro Audio with
>>> 2.6.35, Meego/AtomN450.
>>
>> So with 2.6.35 rtirq also works with a non real-time kernel?
> 
> i am not aware of normal kernels having threaded irq handlers.

No it they do not.

> additionally jack2 does not mlockall clients.

If the machine as enough RAM or no SWAP partition this won't be a problem.

The ability to distribute audio load over multiple CPU cores is a big
pro. (tschak was not packaged when building the Transmission disto and
we were somewhat conservative, as well).

> so basically i would say, that this configuration works is pretty much
> luck.

Vanilla kernels > 2.6.33 do offer great overall performance: smooth
Destkop and acceptable audio performance. Peak performance is for sure
better than with the latest RT kernel (2.6.33.7.2-rt30).

However that it works _reliable_ is indeed luck.

> maybe robin can clarify this.

I don't know the details for the new Indamixx tablets, maybe the
sound-card (and USB ports for external audio interfaces) are on a
dedicated IRQ (they were not on the first generation Indamixx netbooks:
the audio IRQ was shared with the graphics card and WiFi; a RT kernel
and rtirq was pretty much a requirement)

I don't think reliable low latency is a major goal for Indamixx. Most
use-cases are quite fine with high latency that can be compensated for.

Also see an article I'm just writing with Luis:
http://wiki.linuxaudio.org/wiki/jack_latency_tests#does_latency_really_really_matter

An occasional rare x-run is probably sth. Indamixx users can live with.
After all it is a portable studio, not something super-pro-high-end to
be mounted in a studio or used on-stage. Besides overall performance is
for sure better than on comparable windows products even without RT kernel.

FWIW: Thomas Gleixer has announced that he's working on a RT patch for
2.6.37 but there's no ETA.

ciao,
robin

>> Best,
>>
>> Jeremy
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