[LAU] Are RT-patches needed anymore? (Was Re: >= 2.6.27 RT ETA?)

[torbenh at gmx.de]

On Thu, Jan 29, 2009 at 03:41:48PM -0800, Fernando Lopez-Lezcano wrote:
> On Tue, 2009-01-27 at 14:06 +0100, Peder Hedlund wrote:
> > Quoting Ken Restivo <ken at restivo.org>:
> > 
> > > And here is the next installment in the saga of trying to get Ingo   
> > > RT going on my Asus EEE.
> > >
> > > I successfully built and ran the 2.6.26.8-rt12 with the alsa_seq   
> > > patch. It ran.
> > >
> > > The problem is that neither the Ethernet (atl1e) or wireless   
> > > (rt2860sta) work. So I pretty much had to reboot back out of it   
> > > immediately.
> > 
> > I've been running the standard kernel from openSUSE 11.0 on my Athlon  
> > 2000+ and can get down to at least 5.3ms latency on an Audiophile 2496  
> > using the limits.conf "trick".
> > 
> > Do people really need lower latencies for music purposes or are we  
> > just thinking "well, I needed the RT patch three years ago; I ain't  
> > stopping now" ?
> 
> It depends on your usage (this question seems to come up every couple of
> months lately). The current kernels are much better in low latency
> applications than three years ago. They are usable if you don't require
> "low" latencies (64 or 128 x 2). What you get also strongly depends on
> the hardware mix you have. 
> 
> If you want to use 64 or 128 frame periods (or less) you probably will
> need at rt patched kernel in most cases. Then again if an occasional
> xrun is not a problem then you would be fine with the stock kernel. 

i am running with -p64 -n3 on an intel-hda with 2.6.28
of course internal cards have the greatest potential for lowlatencies.
so this might be unfair, compared to pci.

and i havent really seen xruns which i could not relate to some
programm which wasnt RT-safe, and i am compiling stuff most of the day...
though perhaps i am not pushing the DSP load hard enough.

i did not even turn preemptible RCU on.
the latency measurement instrumentation is also in 2.6.28 btw.

> 
> While going down to, say, 1 or 2 mSecs of latency might be thought of as
> unnecessarily low, if your system can work at those levels then you are
> going to be more likely to never get an xrun when running at 5 mSecs
> latency. And if you are performing in a concert situation you _don't_
> want an xrun, not even one. Linux, even rt patched, does not have a hard
> realtime scheduler with deadline guarantees, etc. 
> 
> -- Fernando
> 
> 
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-- 
torben Hohn
http://galan.sourceforge.net -- The graphical Audio language