Re: future of -rt kernels for realtime audio

["Philipp Überbacher" <hollunder@lavabit.com>]

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

--
"Wir stehen selbst enttäuscht und sehn betroffen / Den Vorhang zu und alle Fragen offen." Bertolt Brecht, Der gute Mensch von Sezuan

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