[Xenomai-help] Running real time task on one of the cores of a Dual Core CPU

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

Hi all,

I have some results and some questions on running Xenomai on a Dual Core CPU.
At my PC I have a Pentium D CPU (3.2 GHz), kernel 2.6.19.2, Xenomai 2.3 + some COW patches.
In the kernel I have enabled SMP.

The first interesting thing I noticed is that this kernel seems to run all standard processes
on core 0. The kernel does not automatically distribute processes on two cores.
I think I saw a different behaviour with an earlier kernel version (2.6.17 or even 2.4.33).
I do not have to use the isolcpus kernel parameter to force running all user space tasks on core 0.
This is exactly the thing I wanted to have. All "standard" linux tasks are running
on core 0.
Does anybody know if there was a change in the kernel?

Now I placed all real time tasks from Xenomai on core 1. This works fine.
My application calculates periodically a fairly complex simulation model. The interesting thing
is now, that the computation time needed for this model is much more constant in opposite to the
UP mode. On an UP system I had computation times between 111 and 170 microseconds.
With the SMP system I have now computation times between 111 and 115 microseconds.
Great! The worst case execution time is very close to the best case execution time!
I think this is caused by a more deterministic usage of the cache for the real time system.
In the UP system the cache cannot be reserved for the real time application, i.e. the real time
application has to re-read more often directly from memory.

Now my questions:
In a setup that uses core 0 for standard linux stuff and core 1 for Xenomai stuff the
Xenomai scheduler is running in SMP mode. However, I am not sure if this required in this setup.
As all real time code is bound to one core there does not have to be any multi processing
save synchronisation for the real time code.
My proposal is to support three different scheduling modes:
1 - UP as now
2 - SMP as now
3 - UP on a certain core: All real time code runs on one (runtime selectable) core even
if the linux kernel is fully SMP. This should increase the performance as inter CPU
protection is not required for the real time code.
Any comments on this idea are welcome!

I also use rtnet. Unfortunately I am not able to force rtnet (RTDM based) to run on CPU core 1
without patching the RTDM code.
A couple of months ago, I already asked for this issue.
Jan, can you tell me the latest news regarding this issue?

Thanks for all feedback on the topics

Regards

Mathias 

-- 
Mathias Koehrer
mathias_koehrer@domain.hid


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