Natty and RT Kernel (was "Maverick and RT")

Ralf Mardorf ralf.mardorf at alice-dsl.net
Thu Sep 30 08:41:35 BST 2010


On Thu, 2010-09-30 at 09:05 +0200, Alessio Igor Bogani wrote:
> Ralf,
> 
> 2010/9/30 Ralf Mardorf <ralf.mardorf at alice-dsl.net>:
> [...]
> > That's bad reasoning. Just because an app isn't ok, when using a
> > kernel-rt, low latency without rt isn't the better solution.
> > Independently, did you ensure that the kernel-rt runs with CPU frequency
> > scaling set up to performance? And did you test what will happen, if you
> > don't use rakarrack, but a heavy audio and MIDI set up? Did you compare
> > JACK1 and JACK2? Etc.?
> > Especially for external MIDI devices the so called Linux rt is far away
> > from hard rt.
> > Resume, even when using a kernel-rt, Linux is far away from hard rt, we
> > do need support of the kernel-rt for multimedia work and all the apps,
> > that do cause issues, when using a kernel-rt, need rework.
> 
> Sorry but my understanding of English is very limited.
> Could you explain your thoughts in a more simple manner?
> Thanks.
> 
> Ciao,
> Alessio

Hi Alessio :)

my English is terrible broken too :D.

I experienced the real-time kernels as the only valid kernels for audio
and MIDI recordings.

Anyway, when using a real-time kernel the set up needs some tuning and I
always have problems with this tuning.

First of all, "on-demand" for the CPU frequency scaling vs "performance"

cat config-[...] | grep CONFIG_CPU_FREQ_DEFAULT_GOV_PERFORMANCE

should be set up to 'y'.

I experienced that Rui's rtirq script doesn't have much impact, but it
anyway should be included.

On some machines JACK1 seem to work better and on other machines JACK2.

A big problem is MIDI, when controlling stand alone devices, then jitter
very often is an issue.

If we run uname -a and the kernel is just a

PREEMPT

but a

PREEMPT RT

'things' are more worse. IMO we only do need 'real' real-time kernels.

We can't get hard real-time for modern PCs. Hard real-time only is
possible when directly talking to the hardware, as it is done e.g. by
the C64 on Assembler ...

ask the UART if there's a byte ...

LDA the register
LSR
BCC to LDA

turn of the IRQ!!! ...

SEI

...

I'm unable to program for Linux, but for sure nor Linux, neither Windows
is able to do this kind of hard real-time, e.g. to turn of all IRQs.

The Linux folks who program the kernel-rt patches try to get as near as
possible to the oldish hard real-time programming, any other kernel, but
rt patched kernels aren't usable for music productions.

Some applications might not use JACK in the best way, so they could
cause xruns etc..

I'm really not the right person to teach this stuff, because I don't
have the needed knowledge myself, but for sure Paul Davis won't
recommend to run JACK and Ardour without a kernel-rt.

We don't need latest Desktop candy supported by generic kernels, but
well tuned disros + kernel-rt, to get a good audio workstation.

Cheers!

Ralf






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