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

[Brian Monroe]

On Tue, Feb 14, 2012 at 12:21 PM, Fernando Lopez-Lezcano <
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 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.


>
>
>  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.


>
> 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
>
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