Re: Ada.Execution_Time

["Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de>]

On Tue, 28 Dec 2010 16:46:20 +0200, Niklas Holsti wrote:

> And you seem to forget that Ada.Execution_Time may be implemented by 
> reading a real-time clock. As has been said before.

Only if you have control over the OS or else can hook on task switches. I
think it is doable under VxWorks. But I doubt that AdaCore would do this.

> Dmitry:
>>>> I don't
>>>> care how much processor time my control loop takes so long it manages to
>>>> write the outputs when the actuators expect them.
> Niklas:
>>> You should care, if the processor must also have time for some other 
>>> tasks of lower priority, which are preempted by the control-loop task.
> Dmitry:
>> Why? It is straightforward: the task of higher priority level owns the
>> processor.
> 
> Do you mean that your system has only one task with real-time deadlines, 
> and no CPU time has to be left for lower-priority tasks, and no CPU time 
> is taken by higher-priority tasks? Then scheduling is trivial for your 
> system and your system is a poor example for a discussion about scheduling.

Right, a real-time system is usually a bunch of tasks activated according
to their priority levels. This is why I doubted that Ada.Execution_Time
might be useful there.

> Niklas:
>>> For example, assume that the computation in a control algorithm consists 
>>> of two consecutive stages where the first stage processes the inputs 
>>> into a state model and the second stage computes the control outputs 
>>> from the state model. Using Ada.Execution_Time or 
>>> Ada.Execution_Time.Timers the program could detect an unexpectedly high 
>>> CPU usage in the first stage, and fall back to a simpler, faster 
>>> algorithm in the second stage, to ensure that some control outputs are 
>>> computed before the deadline.
> Dmitry:
>> No, in our systems we use a different schema.
> 
> So what? I said nothing (and know nothing) about your system, any 
> resemblance is coincidental. And there can be several valid schemas.

No, the point was rather that your schema is not typical for a real-time
system.

>> Consider a system with n-processors. The execution
>> time second will be 1/n of the real time second.
> 
> No. If you have n workers digging a ditch, you must pay each of them the 
> same amount of money each hour as if you had one worker. So the "digging 
> hour" is still one hour, although the total amount of work that can be 
> done in one hour is n digging-hours. You are confusing the total amount 
> of work with the amount of work per worker.

The ditch has 10 digging hours, this is a virtual time, which can be 1 real
hour if I had 10 workers or 26 hours if I have only one (26 = 24(8) + 2, 8
hours per day). With one worker it can even be 26 + 48 if he starts Friday,
or even more if she takes a leave for child rearing (:-)).

The sum of working hours is a measure of work. It is not a measure of time.

   Time = Work / Power

You can use it to estimate the real time required to complete the work ...
or just re-read the The Mythical Man-Month... (:-))

> With n processors the system can do n seconds worth of execution in one 
> real-time second. But each processor still executes for one second. And 
> as I understand the Ada task dispatching/scheduling model, one task 
> cannot execute at the same time on more than one processor, so one task 
> cannot accumulate more than one second of execution time in one second 
> of real time.

Yes.

-- 
Regards,
Dmitry A. Kazakov
http://www.dmitry-kazakov.de