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

[Brendan Jones]

On 13/11/14 22:39, Be Ing wrote:
>
>
> One of the reasons I'd like to see a realtime (and/or low-latency) patched version of Linux in Fedora is to keep it more up-to-date than Planet CCRMA. The latest release in the CCRMA repo at the moment is 3.14 whereas the current stable version of Linux in Fedora Updates is 3.16 and 3.17 is in testing. I don't think being behind a week or two in updates would be a big deal, but being behind 3 releases of Linux is.

The latest realtime kernel _is_ 3.14.23. I think CCRMA has been doing a 
great job [1].

Some good points have been raised in this thread by you and others so 
I'll just vent ...

If you want a little bit of background into what goes in to creating the 
patch set read this article [2].

A lot of what was provided for earlier realtime kernels has been merged 
into the mainline kernel and for the majority of users this is more than 
sufficient (turn on thread irqs as a kernel parameter).

If you are suffering from latency problems, make sure to look to other 
sources such as wifi (blacklist it / remove from your initramfs if not 
required), GPU (eg. set your nvidia card to GPU/PowerMizer/Preferred 
Mode = MAx Performance) and do some research about the appropriate 
settings for your audio interface.

I can absolutely guarantee that only members of the kernel team would 
end up maintaining a realtime kernel for Fedora. That is why they are 
asking for empirical test data on why they should consider adopting one. 
Those problems/race conditions you've noticed will surely end up as bug 
reports that they have to action.

You could perform your own testing of an RT kernel starting with using 
CCRMA's kernels and compare them with the latest Fedora kernels (or even 
better grab the latest rawhide-no-debug kernel), and provide a link to 
the SRPM so that they can build the kernels themselves to verify.

For them to accept your data at all, you would need to do these tests 
without any third party drivers or software (ie. only software from 
Fedora mainline repositories). In terms of the test data they will be 
asking for, you need to show a marked improvement between the fedora 
kernels and realtime one.

If you want to learn what's involved I'd suggest downloading CCRMA's 
source RPM's and building a kernel yourself, and also follow the 
instructions here [3]. Its not as hard as it looks at first glance. Just 
time consuming.

[1] https://www.kernel.org/pub/linux/kernel/projects/rt/3.14/
[2] http://lwn.net/Articles/617140/
[3] https://fedoraproject.org/wiki/Building_a_custom_kernel