Re: [Xenomai-help] measuring context switch primary<->secondary mode

[Markus Franke <Markus.Franke@domain.hid>]

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Ulrich Schwab wrote:
> I think that CONFIG_PREEMPT should reduce the jitter of Your measurement, most 
> importantly the worst case switch time, for wich You need a long test run 
> under heavy load.
> CONFIG_PREEMPT is there to provide more determinism, which usually has a price 
> in performance.
> Realtime is about determinism not performance (or speed).
> Its not maximum speed but guaranteed minimum speed.

You are absolutely right. I put my system under some load (dd) and made
two tests. One was with CONFIG_PREEMPT and one without. The worst case
switch time jitter was about 30% lower in the CONFIG_PREEMPT case, so
your prediction was right.

> Since the probability of a preemption during the mode switch without 
> CONFIG_PREEMPT is lower, it might take a long time until the worst case mode 
> switch time is observed.

Sorry but I don't understand what you are meaning. Let me explain my
point of view. When a realtime task in the Xenomai-Domain makes a
linux-systemcall the task is migrated fully automatically to the
Linux-Domain as soon as we reach the next preemption point in the
Linux-Kernel. Due to the fact that preemptability of the linux-kernel
increase when using CONFIG_PREEMPT, the time to the next preemption
point should decrease, right? I don't get the point of your last sentence.

Thanks for help,
Markus Franke


-- 
Nichts ist so praktisch wie eine gute Theorie!

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