[Xenomai-help] Disadvantages of preempt_rt

[Xenomai-help] Disadvantages of preempt_rt

[Anisha Kaul]

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?

The FAQ there talks about the preempt_rt as compared to Xenomai.
Please enlighten.

I read this: http://lwn.net/Articles/146861/ but they don't seem to be
talking about the "disadvantage" of preempt_rt.

-Anisha

Re: [Xenomai-help] Disadvantages of preempt_rt

[Gilles Chanteperdrix]

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.

Re: [Xenomai-help] Disadvantages of preempt_rt

[Anisha Kaul]

> 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.


Thanks much for the hints.

What I have understood now is:
CONFIG_PREEMPT_RT patch is bound to cause a reduction in the overall
throughput of the system
***because there will be several context switches and also the lower
priority tasks won't be getting much a chance to get through***.

The reason I have understood is quoted within  ***.
Is this all correct or I am still missing a point?

-Anisha