[GAP] Periodic task

[Tullio Vardanega]

There are two key issues that you have to mind here:
(1) that the absolute time on which your task wants to wait
starts from a "system-level" reference and, in case case of
a cyclic tasks, proceeds at the exact period from it
(2) that the activity you want the task to perform does fit
in the assigned period (we usually say C<T, where C is the
the task computation time and T is the task period).

If you have done (1), then the semantics of "delay until" on
a past time (when (2) was not true) is that the task continues
running bar preemption event (because invoking "delay until"
incurs a dispatching point).

Else you'll happily run at the desired frequency.
I attach some code templates that do the trick in case you cared
to look at them.

-- Tullio Vardanega

> Hello,
>
> I'm trying to write a periodic task. It has to send a UDP packet for
> every 10 ms time slice. But an obvious solution:
>
> 	declare
> 		Slice : Time_Span := Milliseconds (10);
> 		Slot : Time := Clock;
>
> 	...	
>
> 	loop
> 		-- send packet
>            	Send_Socket (Socket, Token, Last);
>
> 		-- wait until next slot time
> 		Slot := Slot + Slice;
> 	        delay until Slot;
>         end loop;
>
> doesn't work properly. Time period between sending packet is 7 - 20
> millisconds.
>
> How can I make a properly working periodic task?
>
> Regards,
> Daniel Lewandowski
>
>
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