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

[Jan Kiszka <jan.kiszka@domain.hid>]

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Jeff Angielski wrote:
> Adrien LECOINTRE wrote:
>> Hi,
>>
>> I've been testing for a while, Xenomai and Linux preempt RT latencies 
>> under different loads and I couldn't find any big differences. Latencies 
>> are almost the same.
>> So my question is, do you know any case where Xenomai is really better 
>> than a simple preempt RT?
>> Or which specification in a real time application should make me chose 
>> Xenomai instead of preempt RT?
> 
> I suspect your loads are not sufficiently exercising your system.

I may. But the question is also if the test is sufficiently close to the
final RT use case. One may think that running cyclictest alone gives the
full picture of -rt's determinism, but it only stresses one simple use
case of the kernel.

> 
> To answer your question, though, it all depends on your requirements, 
> whether you need hard or soft realtime.  In other words, is it ok to 
> miss your deadline every so often?  If so, CONFIG_PREEMPT would be fine 
> for you. If you can't miss any deadlines, use Xenomai.
> 
> Of course, just by choosing to use Xenomai, you don't get hard realtime. 
>   You still need to design your system and software correctly.
> 
> 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.
> 

Theoretically, -rt/CONFIG_PREEMPT_RT can provide this already - if you
do not hit implementation problems with the particular use case, or the
application uses services which do not provide real-time qualities on -rt.

Jan


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