Re: [Xenomai-help] Disadvantages of preempt_rt

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

On 04/07/2012 07:33 AM, Anisha Kaul wrote:
> Greetings,
> 
> From here: http://www.xenomai.org/index.php/Xenomai:Roadmap#Xenomai_3_FAQ
> 
> Quote:
>> the target hardware platform has limited horsepower, and/or you want the real-time job to put the smallest possible overhead on your system. This is where dual kernels are usually better than a native preemption system.
> 
> 
> Preempt_rt does preempt the whole Linux. In what way does preempting
> Linux put load on the system?

I think you are misunderstanding what preempt_rt is, preempt_rt does 
not preempt linux, preempt_rt is simply linux modified, when a linux 
kernel patched with preempt_rt runs, nothing else than linux runs, so 
nothing preempts the whole linux. This is achieved by means which have 
a global overhead on the whole system. The exact means are described on 
the rt wiki FAQ:
https://rt.wiki.kernel.org/articles/f/r/e/Frequently_Asked_Questions_7407.html

Note the answer to FAQ 3, which explains that improving latencies is a
trade off, and means degrading the average performances of the system.
That is the overhead we are talking about.

-- 
                                                                Gilles.