Re: [Xenomai-help] How to chose between xenomai and preempt RT

[Gilles Chanteperdrix <gilles.chanteperdrix@xenomai.org>]

Jeff Angielski wrote:
> Gilles Chanteperdrix wrote:
>>> As time marches on, the CONFIG_PREEMPT is getting closer to hard 
>>> realtime, especially with the interrupt threading, but I don't think 
>>> that time is now.
>> threaded interrupts are no silver bullet, they are essentially replacing
>> interrupt latencies with kernel-space scheduling latencies; on x86, this
>> may not make that much of a difference, but on low-end platform it does.
>>
>> See also:
>> https://mail.gna.org/public/xenomai-help/2008-05/msg00043.html
>> https://mail.gna.org/public/xenomai-help/2009-06/msg00005.html
> 
> I meant to write PREEMPT_RT is getting closer to hard realtime.  Sorry 
> for the confusion.
> 
> As for the interrupt threads, the advantage is not in the latency, it is 
> in the ability to control the scheduling of the handlers.  In theory, 
> you can schedule your handler the have the highest priority handler.

The real advantage or the threaded interrupts is that the part of the
kernel-space code that need to protect from a particular interrupt
remains preemptible by other interrupts. The interrupt handler
themselves were already preemptible, if they did not use the
IRQF_DISABLED flag.

Note that the same effect could be obtained by disabling only the
particular interrupt at PIC level, but this would have mean a lot more
of code changes than what threaded interrupts need. But probably a lot
less run-time overhead.

-- 
					    Gilles.