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

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

On 11/29/2013 06:36 PM, Carsten Emde wrote:
> Hi Gilles,
> 
>> 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,
>>>> respectively) 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.
> Hmm, I'm not sure whether I correctly got your point.
> 
> Let me try an example: A 1-GHz CPU of a given systems runs at full speed 
> with frequency governor set to performance and provides the required 
> real-time capabilities. When a second system with the same capabilities 
> was needed, the 1-GHz CPU unfortunately was out of stock, and the 
> decision was made to buy the 2-GHz variant of the processor. To save 
> energy, however, the clock frequency of the second system was set to 1 
> GHz using the cpufreq interface. Are you arguing that the 2-GHz 
> processor that is throttled down to 1 GHz has a slower response time 
> than the 1-GHz processor that always runs at full speed?

I probably misread what you were saying and thought you were talking
about dynamically changing the processor frequency when knowing that the
WCET of a task allows running it with a smaller frequency and still meet
its deadline. The thing implemented here for instance:
https://code.google.com/p/xenomaiote/

So called OTE algorithm (but I do not find what this acronym means exactly).

-- 
                                                                Gilles.