Re: [Xenomai-help] [RTnet-users] Xenomai/rtnet vs. 2.6.21 realtime preempt patch

["M. Koehrer" <mathias_koehrer@domain.hid>]

Hi Jan,

> > I have tried the latest 2.6.21 kernel + Ingo Molnar's realtime preempt
> patch
> > (see: http://rt.wiki.kernel.org/index.php/RT_PREEMPT_HOWTO)
> > 
> > And I am really amazed about the results.
> > On my Pentium 4D (dual core) system, 3.2 GHz, I ran the cyclictest
> > application (which is something similar to the "latency" application of
> Xenomai/RTAI)
> > and I get values below 15 microseconds on my PC (for a single task running
> with 200 microseconds).
> > 
....
> > 
> > Currently, my application is working quite nice using Xenomai/rtnet
> however there are some
> > drawbacks like the issue with limited IRQs: Sharing of IRQs between
> Ethernet-Drivers of rtnet
> > and non-realtime drivers is not possible (at least I did not manage
> that...).
> > A smooth usage of real time features from a "standard" kernel could help
> here!
> 
> Nope, not really. As I think to have explained earlier, IRQ sharing
> between drivers that are designed for real-time and others that are not
> will never work deterministically. That has nothing to do with the
> design of your RTOS underneath. Actually, the same issue once came up
> for -rt over some ARM board that did poor IRQ line sharing as well.
> 
> Jan
I agree, the best is actually to have separate IRQs for real time and non real time drivers.
However, reality shows me, that this is very hard to get with standard PCs.
I can plug in one or two PCI boards to have an unique IRQ for them. However, if I want
to use more PCI boards IRQ sharing cannot be avoided.
As with the preempt patch, the duration of non-realtime IRQ routines seems to be fairly short.
>From this, I think it is at least an option to share IRQs between real time and non real time drivers
even if the IRQ routine of the non real time driver may lead to a delay of the real time IRQ routine.
It is a question of acceptable delays for the IRQs.

Regards

Mathias




-- 
Mathias Koehrer
mathias_koehrer@domain.hid


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