Re: Using "delay until" in real-time

[Mike Silva <mjsilva@my-deja.com>]

It's Son of Patriot bug...

I'm as far from an expert as you can get, but what if you defined your
Iteration in terms of Tick (4*Tick in your case)?  Seems like that
should provide an exact representation (why do I feel like that comment
will get me nailed?).  Otherwise, keep your own Iteration count in a
scalar and add it (first converted to a Time_Span) to a Start_time for
each delay until.

It's a very interesting question, and if I'm wrong I look forward to
learning the right answer.

Mike

In article <915jl7$jt5$1@nnrp1.deja.com>,
  Ted Dennison <dennison@telepath.com> wrote:
> I came across an interesting problem the other day that I'd like to
> share, in the hopes that someone here has another solution that the
ones
> we thought up.
>
> We have a real-time scheduler that uses the "delay until" statement to
> perform its scheduling. So far, this shouldn't shock anyone, as
> real-time scheduling is supposedly what "delay until" was put in the
> language for.
>
> Our scheduling code looks something like this (very simplified, and
not
> compiled. My apologies ahead of time if Deja screws up the
formatting):
>
> ---
> Iteration_Hz : constant Float := 60.0;
>
> task Executive is
>    Iteration : constant Ada.Real_Time.Time_Span
>       := Ada.Real_Time.To_Time_Span (Duration(1.0/Iteration_Hz));
>
>    Next_Time : Ada.Real_Time.Time := Ada.Real_Time.Clock + Iteration;
> begin
>    loop
>       -- Perform scheduling tasks
>       ...
>
>       Next_Time := Next_Time + Iteration;
>       delay until Next_Time;
>    end loop;
> end Executive;
>
> ---
>
> That looks pretty straightforward, right? Well, unfortunately, its
*WRONG*.
>
> 1/60 works out to about 0.016(6-repeating). That can't be accurately
> represented in an IEEE floating-point register. So what we get is
> something like 0.016667 (forgive me if I'm rounding the wrong digit).
> That rouding error slowly accumulates over time, until after about 52
> seconds our poor Executive task skips a tick. On our system ticks come
> at 240Hz, so that means that once every 52 seconds the executive
> scheduler waits about 20ms instead of 16.6(6-repeating)ms. Of course
> this wreaks havoc in our hard-realtime system.
>
> So the question is, what's the best way to solve this problem? Here's
> what we came up with so far:
>
>   o  Use the RTOS' "taskDelay" function to delay the appropriate
amount
> of ticks (4).
>
> I don't like this one, as its a non-Ada solution, and thus renders our
> scheduler non-portable. Its also succeptable to drift if something
> causes an extra tick to pass before we get to the taskDelay.
>
>   o  Check the Ada.Real_Time.Clock every cycle when we awaken, and add
> the Iteration to that for the "delay until".
>
> I don't like this because the clock check is going to be fairly
> expensive, and this solution is also succeptable to drift.
>
>   o  Figure out what the denominator is (60 in this case), and redo
the
> calculation every time that number of runs have happened.
>
> For instance, in the above code, I'd add something like:
>       Runs := Runs + 1;
>       if Runs >= Natural(Iteration_Hz) then
>          -- Adjust for drift due to rounding errors.
>          Runs := 0;
>          Run_Time := Run_Time + Ada.Real_Time.To_Time_Span (1.0);
>          Next_Time := Run_Time;
>       else
>          Next_Time := Next_Time + Iteration;
>       end if;
>
> I kind of like this option, as it is a mathematical solution to what
is,
> in essence, a mathematical problem. This ought to work as long as the
> denominator can be counted on to be a whole (natural) number. In my
> case, I think it can.
>
> Since this problem should happen to anyone using "delay until" with an
> iteration rate divisible by 3, and 60, 30, and 15Hz are common rates
in
> the industry, I can't be the first person to stumble on this problem.
> How does everyone else handle it?
>
> --
> T.E.D.
>
> http://www.telepath.com/~dennison/Ted/TED.html
>
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