Re: preempt rt in commercial use

[Armin Steinhoff <armin@steinhoff.de>]

  Nivedita Singhvi wrote:
> On 09/14/2010 07:49 AM, Jeff Angielski wrote:
>
>>> Isn't any case where preempt-rt does not behave as hard reatlime a 
>>> bug in
>>> preempt-rt, that should be reported to this list and eventually fixed?
>>
>> That is a philosophical question for the preempt-rt maintainers.
>>
>> I *believe* that the design goal for the preempt rt code is to minimize
>> kernel latency. It's not to make the kernel deterministic to support
>> hard realtime.
>
> Er, sort of, not quite.
>
> The design goal of the real-time kernel is most certainly to make the
> kernel more deterministic, to the extent we can in a general-purpose way.
>
> Determinism = capping max latencies.

   Capping max latencies doesn't help without a good real-time, event 
driven scheduler.

   But it helps to classify real-time operatings systems as so called 
hard or soft real-time operating systems.

   IMHO ... there is a common understanding that a RTOS can be 
considered as a hard-reatime OS if the
   max latency is < 15us  because it is able to server 80% (?) of all 
hard real-time applications in the field.
   All others are considered as soft real-time operating systems. From 
this point of view is PREEMPT_RT Linux
   a hard real-time OS ... if the hardware base is appropriate.

   BTW ... we use PREEMPT_RT Linux  as a base for our commercial 
solft-PLC called DACHSview: http://steinhoff-automation.com/Programming.htm

>
> It's better to have all hundred iterations of an operation take
> 45us each than to have 95 iterations take 30us and 5 iterations
> take 75us. You want to be able to say, "this operation will take
> at _most_ 50us" and have that be as true as possible.

   All these numbers have no meaning if you don't specify the hardware 
environment.

   --Armin

   http://www.steinhoff-automation.com



>
> We sacrifice overall throughput (ave latency) for determinism
> (low max latency).
>
> Not sure if that's what you intended to say, but hope that helps.
>
> thanks,
> Nivedita
>
>
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