Re: cyclictest better values with system load than without (OMAP3530 target)

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

On 11/29/2013 05:36 PM, Gilles Chanteperdrix wrote:
> On 11/29/2013 04:10 PM, Carsten Emde wrote:
>> BTW: Power saving and real-time do not necessarily exclude each other.
>> If a - still deterministic - but a little longer latency is acceptable,
>> some light sleep states and a somewhat lower clock frequency may be
>> allowed which still may result in considerable energy saving. If,
>> however, the fastest possible real-time response is required, C states
>> and P states must be disabled (or set to polling and maximum speed,
>> repsectively) and the power bill must be payed.
>
> Well, I do not fully agree. To be sure that you can clock down the
> processor for executing a task which has sufficient time to meet its
> deadline, your system must be "time triggered", all the timer events
> must be known in advance. Because on a fully dynamic system, you may
> make that decision, but a new timer may be scheduled which causes the
> system to miss its deadline whereas it would not have missed it if it
> had run at full speed.
>
And a second problem is that you must know the task WCET, which on a 
modern system:
- depends on the task frequency;
- depends on the IRQ load.
Again, only a time triggered system seems to make this possible.

-- 
					    Gilles.