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

[Fernando Lopez-Lezcano]

On 02/14/2012 08:55 PM, Brian Monroe wrote:
> On Tue, Feb 14, 2012 at 12:21 PM, Fernando Lopez-Lezcano
> <nando at ccrma.stanford.edu <mailto:nando at ccrma.stanford.edu>> wrote:
>
>     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.
>
> Ah, that may have been an error on my part, I thought last I checked,
> which admittedly was a week and a half ago there was only a F15 kernel
> on the CCRMA website.

The web site needs updating (and I just released 3.2.6 rt12 for fc16 
testing)... Pretty much everything that is available for fc15 is 
available for fc16 (I have been using the mailing lists to announce 
stuff and never have time to update the web site). Install 
planetccrma-repo and that should get you going.

>     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.
>
> Namely the Ubuntu Studio folks. Most users in #opensourcemusicians seem
> to use Ubuntu. Why? I don't know.

Probably part of the reason is that not being a US based distribution it 
does not care about some stuff that makes Fedora harder to use (the 
codec patent minefield). Fedora also has a reputation of being bleeding 
edge, more so than Ubuntu. As a technology testing ground it has in the 
past made decisions that advanced the state of the art (although some 
users question whether what happened was an "advance") and broken 
important audio stuff as a side effect, sometimes for a long time. That 
chases users away. I can atest to that.

>         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?)
>
> To be honest I'm not sure where I read this, but I do remember having
> conversations about it in one of the channels. Part of the reason I
> wanted to email the list was to hear what's what from the source, so
> thanks for clearing that up for me.
>
>     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.
>
> Is there any help needed for testing/ect? I'm just trying to figure out
> what I can do to start contributing.

If you are a Fedora user and you are interested in low latency kernels 
you can of course try the Planet CCRMA kernel-rt and kernel-rtPAE 
packages I maintain (plus rtirq and other goodies, install 
"planetccrma-core" or "planetccrma-core-PAE"). BTW, it is only recently 
that 3.0 and 3.2 kernels have evolved to a state where they behave as 
well or better than good'old 2.6.33.x, at leaste IMO.

As for an audio oriented spin that is _really_ good, the only way that 
can happen officially in Fedora (AFAIK) is for the upstream kernel to 
become good enough for that without the rt patches. It is probably 
getting there in terms of source code, but for the Fedora kernel to 
become better than average it would need to be rebuilt with more 
appropriate configuration options, namely threaded irq's.

-- Fernando