Re: future of -rt kernels for realtime audio

[Pedro Ribeiro <pedrib@gmail.com>]

On 6 July 2010 20:53, Philipp Überbacher <hollunder@lavabit.com> wrote:
> Excerpts from Pedro Ribeiro's message of 2010-07-06 13:19:31 +0200:
>> Hi,
>>
>> I've been using -rt kernels since 2.6.29 because I do realtime audio
>> on my laptop.
>>
>> The audio stability has been steadily improving since, and now I find
>> that I can use 2.6.34 without the -rt patch and achieve the same
>> stability as 2.6.33-rt - well, my latency requirements aren't that
>> high, I just need to maintain 8.9ms completely stable, however before
>> .34 it would be impossible without the -rt patch.
>>
>> So out of curiosity, what changed for .34? According to [1], on .33
>> Raw Spinlock Annotation was introduced in the mainline kernel.
>> However, as said above, I can't get the same performance than with
>> .34.
>>
>> I remember that I read somewhere that the one the biggest problems
>> with latency requirements was the use of the BKL. Do you think there
>> will be a significant improvement of latency (in specific cases of
>> course) with the scheduled removal of BKL for 2.6.36?
>>
>> Thanks for the help,
>> Pedro
>>
>> [1] http://www.osadl.org/Realtime-Linux.projects-realtime-linux.0.html
>
> Ah, nice to hear that the BKL removal is scheduled. Pretty much all I
> know is that Linus set the BKL removal as precondition for preempt-rt to
> be merged with mainline.
> I guess this means that preempt-rt could disappear in some post-.36
> version. I won't hold my breath.
> Anyway, the stock kernel has been good for a while, but there are some
> cases where I still experience big differences. Some, mostly audio
> unrelated, actions or kinds of load cause lots of xruns with the stock
> kernel and none with -rt. Prime example for me is xrandr, enable an
> external monitor -> xruns, not so with -rt. There are a couple of other
> cases. It's mostly about this kind of stuff, the achievable latency is
> pretty much the same in my experience (which is only with an USB
> interface).
> --
> Regards,
> Philipp
>
> --

I agree with you that there are some corner cases where you can
trigger xruns. However, I noticed a very big difference with 2.6.34.

It was easy to trigger xruns, even with .33-rt in the following situations:
- physical memory full, swap at 20%
- change virtual desktop (yes strange but it always triggered it)
- running cpu intensive apps
- lots of disk i/o happening

With vanilla .34, ALL of the above disappeared except the disk i/o
thing. I can safely say that .34 is more stable than .33-rt with the
latency I need (6 to 8 ms).

I'm using USB audio snd-usb-caiaq, and barely anything changed between
the two kernel versions. I still would not use it for live
performance, but then again, I don't do them :)

Regards,
Pedro
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