Ubuntu 8.10: rt kernel improved?

[raboof]

Ubuntu 'Intrepid' (8.10) just came out.

Among other things, it sports a newer rt kernel: 2.6.27-3-rt (8.04 contained 2.6.24-19-rt).

This kernel seems to work very well for some people, but I'm experiencing noticeable latency/jitter when playing with MIDI.

The audio path:
- midi controller (tried 2 different ones)
- usb midi (either though a midisport 2x2 or directly)
- alsa midi
- zynaddsubfx
- jack
- alsa snd_intel8x0/snd_ac97_codec pcm

I went back to the older kernel (which conveniently seems to work fine) - I wonder what would be the best way of finding out where this regression comes from.

[thorgal]

I just compiled my own 2.6.27.4, customized to my h/w and CONFIG_PREEMPT=y
No RT patch (since there's none available at http://www.kernel.org/pub/linux/kernel/projects/rt/). Out of curiosity, I thought I would give it a try. And I am quite impressed, things just work! I was running things at 1ms latency no sweat.

I haven't noticed any MIDI problems but it's probably because I am not a heavy MIDI user, just rosegarden and a VSTi through dssi-vst, sync'ed with ardour.

To be sure, I played a drum track recorded in ardour from rosegarden a few days ago, together with the same track (MIDI) played by rosegarden and my drum VSTi via jack sync'ing. I could not hear any jitter. A tiny phase difference all along but no noticeable variations.

I will continue a few days with this kernel before I decide if it is worth but it's already quite promising. No bloody patch, that's really sweet!


PS: I forgot one thing. If you have an Intel gigabit ethernet adapter that was driven by the e1000 kernel module, you will have to change module to e1000e.
e1000 is still there but does not work anymore with my network chip. A big of googling pointed me to the module change.

[raboof]

On the rosegarden-user list, shane richards mentioned the jitter i'm seeing might be related to swapping - i'm going to look into that.

[thorgal]

... spoke too fast, I get some unstable results with random crashes of my VSTi. All righty, back to 2.6.24.

[raboof]

On the rosegarden-user list, shane richards mentioned the jitter i'm seeing might be related to swapping - i'm going to look into that.

OK, swapping isn't the problem, it's also there when swap is disabled entirely.

Unaffected:
- gtick (a DSP metronome)
Affected:
- keystation --alsamidi--> zynaddsubfx --jack--> alsa hw:0

I noticed 2.6.27-3-rt gives me XT-PIC-XT interrupts, while 2.6.24-19-rt gives me IO-APIC. I wonder if that makes a difference.

[raboof]

Is anyone aware of a Launchpad issue or something similar about this?

I've browsed around launchpad for a bit, but I can't even seem to get a list of bugs against the kernel package :S.

[raboof]

Looks like work on a new version of the -rt kernel can be previewed at https://launchpad.net/~abogani/+archive - haven't tried yet.

[raboof]

New observations:
- the problem is much more pronounced on my wind controller than on my midi keyboard. Both are attached though USB, but the wind controller emits much more information.
- the -generic kernel actually performs much better!
- the -generic kernel uses IO-APIC interrupts, the realtime one XT-PIC
- added a bug report at https://bugs.launchpad.net/ubuntu/+sour ... bug/300806

[emillo]

Same experience here.

MIDI keyboard is behringer UMX61

Sticking to 64studio for now

bye

[studio32]

[offtopic]he Emillo, welcome onboard it seems to be a nice website you have.. are you from Brazil?
Maybe you should introduce yourself...[/offtopic]

[emillo]

[offtopic]he Emillo, welcome onboard it seems to be a nice website you have.. are you from Brazil?
Maybe you should introduce yourself...[/offtopic]

Hi studio32.

No, I'm not from Brazil but from Italy.

Thank you for your compliments, I'm a long time linux advocate in the audio world and it has never been easy

The informations on my page are unfortunately rather old, but my daily duties leave me no time to update them.

I'm a hobbyst studio owner and multi-instrumentalist. I run GNU/Linux on my computers since years now, both for fun and for working too (not music/audio related though).

I have a small/mid home studio called cricketstudios: http://cricketstudios.it where you can find some samples of what I've done.

http://emillo.net/download/wip.mp3

That is one of the latest projects I'm involved with, featuring some nigerian friends

Cheers

[studio32]

@Emillo, ah Italiano, sorry I was misleaded by the colors of your website
I'll check out your links!

edit: Like it!
but I would skip the sentence 'throw your hands in the air, if you just don't care'... it's used by to many artists imho... but thatś not you right?

I really like the swing... nice sounds of the instruments, violin and especially the bass...

[matmc2]

Well, my 8.10RT works quite well... I'm using a VST host and Ardour2 through a Tascam US-122 and I have no problems at all, both with 8.10 and 8.04.1.

[freqrush]

Here the 2.6.27-3-rt kernel also uses just one of the 3 processors

Code: Select all

peter@ubustu:~$ dmesg | grep NR_CPU
[    0.000000] WARNING: NR_CPUS limit of 1 reached. Processor ignored.
peter@ubustu:~$ dmesg | grep Phenom
[    0.001216] CPU: AMD Phenom(tm) 8450 Triple-Core Processor stepping 03
[   26.323547] powernow-k8: Found 1 AMD Phenom(tm) 8450 Triple-Core Processor processors (1 cpu cores) (version 2.20.00)
peter@ubustu:~$ uname -a
Linux ubustu 2.6.27-3-rt #1 PREEMPT RT Mon Oct 27 03:02:33 UTC 2008 x86_64 GNU/Linux

Also changing midiconnections in qjackctl makes my mouse pointer disappear.

[raboof]

Here the 2.6.27-3-rt kernel also uses just one of the 3 processors

That's a known (and pretty serious!) limitation: https://bugs.launchpad.net/ubuntu/+sour ... bug/290498

Also changing midiconnections in qjackctl makes my mouse pointer disappear.

Amazing. No clue what could cause that.