6.04 preview kernel: now with realtime module or not?

[Artemio]

Hey guys,

this is very important for me.

On the flight 2 preview changelog page I read: "The desktop kernel provides a smoother and more pleasant user experience by turning on preemptive multitasking and other features meant to allow user processes (your running applications like Firefox and Rhythmbox) to run without ever skipping a beat."

1. So does this mean the "capability" module is enabled by default?
2. Is there a pre-compiled "realtime" module which is what needed for serious audio works?

Thanks!

[incinerator]

Re: 6.04 preview kernel: now with realtime module or not?

So-called "capabilities" have nothing to do with realtime. They are a security feature afaik.

You are getting some terminology wrong here. Linux has always been a pre-emptive multitasking system. User-space processes can be interrupted during execution in order to let other processes do some work, thereby improving response time.
The only area the kernel has been lacking was when it came to interrupt things in the kernel itself. Certain functions in the kernel did not like to be interrupted and other processes had to wait until the kernel was finished with whatever it was doing, thereby increasing response times for userspace applications. That's what the "pre-emptible" kernel configuration option is all about, since 2.6 all vanilla kernels have had this option available.

I interpret the announcement like they finally have enabled that "pre-emptible" kernel option mentioned above. They may have added some patches to improve latency behaviour, as well. However, according to my experience a pristine 2.6 kernel already is quite good at that.

The standard linux kernel as used by all "normal" (=non-embedded) distros does not provide proper "hard realtime" (not even -ck patches do). If you want hard realtime for linux, check out montavista, windriver etc.

For gapless audio playback, other things are also very important. Choice of filesystem is one. Ext3 seems ok, whereas reiserfs seems to be really bad for that purpose, at least according to my personal experience. You need to good audio framework software like mad, jack or gstreamer, as well (arts sucks). Last but not least, the actual audio player software has to play along.

Btw, one should not get the impression that all the kernel code was not preemptible, that's not exactly true. Only certain areas in the code are dependend on not being interruptable by cpu-time demands from userspace. One can always optimize for low response time or high throughput, goals which do not necessarily coincide in the world of computing. I've been using GNU/Linux since 1998. Except for the reiserfs's ppp regarding latency I've always been quite satisfied with my gapless audio playback. Things might be different if you want to use Linux for professional audio processing/mixing, but not everyone is doing that.

[Artemio]

Re: 6.04 preview kernel: now with realtime module or not?

I indeed do not know much details, but I know that many audio-production applications (synths, sequencers, etc.) make use of the "realtime-lsm" module which allows for precise audio generation/playback no matter what else you are doing with the system.

I really wish (K)ubuntu had this module pre-built, would be another great plus for the distro.

And I'd like to know what exactly they did include in the 6.04 kernel.

[incinerator]

Re: 6.04 preview kernel: now with realtime module or not?

Ah, now you're talking. I wasn't aware of this realtime-lsm stuff and simply realtime didn't trigger my google fingers.

Anyways, checking what modules are built for the kernel is simple:
dpkg -L linux-image-2.6.15-8-386

All modules are inside the /lib/modules/kernelversion/kernel directory.

On other architectures you will have to substitute the 386 with somethin else, e.g.
linux-image-2.6.15-8-amd64-generic on amd64 installs. If you have installed an optimised kernel, you will need to use that.

It seems that module is not included in the standard ubuntu kernels. However, it would not be as simple as that. You would still need to define an appropriate user/group infrastructure and of course the applications would have to support it.

Also, one should not be fooled by the term realtime here. True, the Linux kernel knows something like "realtime priority" it can give processes. However, that simply means this process will have the highest priority, it doesn't necessarily mean you'll get hard realtime stuff.

Btw, that realtime-lsm project might indeed be a good idea to add to a desktop linux distribution. Check bugzilla.ubuntu.com and if you don't find anything related (search for realtime-lsm e.g.) then feel free to file a bug report. Iirc you can file "wishlist" bug reports there, that's exactly what you should do.

If you want to know more about real-time computing check out these to wikipedia pages:
http://en.wikipedia.org/wiki/Realtime
http://en.wikipedia.org/wiki/Real-time_computing

[Artemio]

Re: 6.04 preview kernel: now with realtime module or not?

Thanks for your reply!

Don't worry, I know what realtime means, I used to work with RTLinux a bit and I'm the author of RTLinux Installation HOWTO.

Audio apps, Hydrogen in particular, make use of the "realtime" LSM kernel module when it's available. The difference is drastic, without this module you'll have glitches in audio every time you switch a desktop or call some sub-window in the application.

I don't think it's that hard to include it in the base (K)ubuntu kernel, this will be a good point for many people to migrate.

I looked up the BugZilla like you suggested, and after finding to entry for this, filed a bug: http://bugzilla.ubuntu.com/show_bug.cgi?id=21206

Thanks again for your reply.


Artemiy.

[incinerator]

Re: 6.04 preview kernel: now with realtime module or not?

I had a look at your bug report. It seems there is still one misunderstanding remaining. realtime-lsm is a third party project. It is NOT part of the official Linux kernels as you can get them at kernel.org

Btw, the realtime-lsm has its own website at http://sourceforge.net/projects/realtime-lsm/

Therefore, two alternatives remain:
1.) Advocate the inclusion of the module into the main Linux kernel. Doing that here would be useless.
2.) Advocate the inclusion of the 3rd party module to be added as a patch to the Ubuntu kernels. That's best done via bugzilla or the Ubuntu Kernel Developers Mailing List.