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

[Ulrich Schwab <schwab@domain.hid>]

On Thursday 30 November 2006 14:48, Markus Franke wrote:
> 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?
Correct.
But I wrote:  "  without CONFIG_PREEMPT " 
meaning: the time to the next preemption point is bigger, which might get the 
requested system call done before the preemption is actually happening.

> I don't get the point of your last sentence.

Ulrich

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