[Fedora-music-list] Low Latency vs. Real Time Kernel

[Fernando Lopez-Lezcano]

On 02/14/2012 11:31 AM, Brian Monroe wrote:
> I've been spending a lot of time on the #opensourcemusicians channel
> talking to Ubuntu Studio users about their kernel and latency times
> they're getting. Seems like most of them are using g a stock kernel with
> the preemptive option enabled and they are getting great latency results
> (2ms)while utilizing the @audio group on their user. I ended up
> compiling my own low latency kernel and I haven't had any issues with it
> yet. If this is what we are missing for the spin I'd be happy to
> maintain packaging for the kernel.  I know ccrma has been behind a few
> kernel releases.

The latest I have, current in testing is 3.2.2 + rt11 (for Fedora 15 and 
16). I am currently trying to build 3.2.6 + rt12.

The current rt not in testing is a 3.0.x based release (fc15/16). I have 
not seen a big interest on being up to date - I just try to keep up with 
the latest rt patch set. If there is more interest I could try to keep 
up (but keeping up with _what_?, for a bit I was testing a 3.2 based rt 
patched kernel and that was still not available for fc16 as an official 
release).

> I saw the instructions for adding the real time patch for a tick less
> kernel and from what I can tell it wouldn't be hard to get that rolling
> as well.
>
> I'm not entirely sure what ccrma does differently with their kernels
> compared to other Linux users,

"compared to other Linux users"? I don't follow.

>and I'm still a bit of a noob so I could
> be off base with this, but I would reason that we should be able to just
> utilize the same settings to archive similar performance enhancements.
>
> I thought I read that ccrma uses a unique scheduler, but if we could get
> a 2ms latency time without it, the point may be moot.

Nope, no unique scheduler or other stuff. Where did you read that? 
(links please?)

The Planet CCRMA rt patched kernels are based on recent Fedora source 
packages (usually from Koji) that are the closest I can find to the 
kernel releases for which rt patches are available. To that source 
package I add the rt patch, drop Fedora patches that are already 
included or conflict, and built that. I use pretty much the stock Fedora 
kernel configuration files except for whatever tweaks are necessary to 
enable the rt patch for full preemption. That's about it.

As work in the rt patches has progressed the stock Fedora kernel (which 
is basically upstream plus a few patches that have not been merge yet) 
has become more and more usable for "normal" music work. For low latency 
work (in a word, using your computer as a musical instrument), an rt 
patched kernel still has an edge. Whether that really makes a difference 
depends on your usage, your tolerance to occasional xruns and even the 
exact hardware you are running on.

-- Fernando