Re: GNAT DOS Delay Problems

["Samuel T. Harris" <s_harris@hso.link.com>]

Gert Caspersen wrote:
> 
> When running the below program, no context switch takes place. Task T1 with
> the low priority is starved by task T2 with the high priority.
> 
> In my opinion, T1 should be scheduled when T2 executes delay 10.0. However,
> this appears never to happen. Rather, the entire program is suspended for
> ten seconds.
> 
> I am using GNAT version 3.07 for DOS.
> 
> Any ideas of the cause would be appreciated....
> 
> with Ada.Text_IO;
> procedure Dims is
> 
>    task T1 is
>       pragma Priority (2);
>    end T1;
> 
>    task T2 is
>       pragma Priority (3);
>    end T2;
> 
>    task body T1 is
>    begin  -- T1
>       loop
>          Ada.Text_IO.Put_Line("Task 1");
>          delay (1.0);
>       end loop;
>    exception
>       when others =>
>          Ada.Text_IO.Put_Line("Task 1 got exception");
>    end T1;
> 
>    task body T2 is
>    begin  -- T2
>       loop
>          Ada.Text_IO.Put_Line("Task 2");
>          delay (10.0);
>       end loop;
>    exception
>       when others =>
>          Ada.Text_IO.Put_Line("Task 2 got exception");
>    end T2;
> 
> begin  -- Dims
>    null;
> end Dims;

I remember using both Janus and Alsys Ada on PC before
they implementing time-slicing. Once a task took over,
it had control until it terminated or initiated a
rendevous. The run-time scheduler NEVER interrupted
the running task and only "scheduled" the next task
when a task terminated or a rendevous occured.

To get around that problem, I used a nil_task with
a single accept which basically did nothing. I then
"spinkled" the actual tasks and the main program with
entry calls to that do_nothing entry. This gave the
scheduler a chance to switch amoung the various tasks
in the system. Placement of these "scheduler" rendevous
was a black art and some playing around was needed to
get the desired behavior.

-- 
Samuel T. Harris, Senior Engineer
Hughes Training, Inc. - Houston Operations
2224 Bay Area Blvd. Houston, TX 77058-2099
"If you can make it, We can fake it!"