[LAU] Ubuntu Hard real time kernel (2.6.24-21) poor performance.

After a number of years of making successful music recordings using a
combination of rosegarden and ardour on my Ubuntu Dapper Drake system,
and my home grown real-time kernels, I thought it was time to upgrade to
Hardy Heron and use the proper real time kernel that the experts had
built. What a disappointment!

I have an internal pci sound blaster live card (emu10k1 chip set) and a
M-audio audiophile USB sound module.

On my dapper installation, which happened to have 2.6.22.1 real time
patched kernel (the default ubuntu kernel was 2.6.15), I could get
latencies as low as 1.5ms (32 frames/period) on the sound blaster and
2.7ms ( 64 frames/period) on the USB Audiophile with very few xruns; so
few xruns that I used to wonder why people posted to mailing
lists/forums about the problem.

With the stock Ubuntu 2.6.24-21rt kernel the sound blaster gives xruns
every few seconds with no load (no recording and no playing back) at
23ms latency (512 frames/period) and the Audiophile struggles at 43ms
( 1024 frames/period); I am getting xruns every 10-20 seconds. Infact
the rt kernel gives little, if any, improvement over the generic kernel.

I have the following settings in my limits.conf
@audio - rtprio 95
@audio - memlock 512000
@audio - nice -19

And I have a script to set the real-time priorities for the interrupts
for the interrupts that the sound system use - usually IRQ17 and IRQ19.

My user is also a member of the audio group.

I have tried the nohz=no on the kernel start up command line as
discussed in numerous fora with no change. I did not expect one:

johnt@email-addr-hidden:/boot$ grep NO_HZ config-2.6.24-21-*
config-2.6.24-21-generic:CONFIG_NO_HZ=y
config-2.6.24-21-rt:# CONFIG_NO_HZ is not set

I have also tried the changes described in:

http://alsa.opensrc.org/index.php/Usb-audio

which also made no difference.

Any ideas what to try next? Have I got to go back and start build my own
kernels again (which always involved a battle between me and the nvidia
graphics card)?

John Tomlinson

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Received on Fri Oct 24 12:15:02 2008