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Subject: non-preemptive tasking on GNAT 2020 Windows 10 multicore AMD
From: Dan Winslow <dandwinslow@gmail.com>
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I am trying to achieve true non-preemptive tasking using gnat 2020 CE on a =
windows 10 environment. I have placed this in the gnat.adc file:

pragma Task_Dispatching_Policy(Non_Preemptive_FIFO_Within_Priorities);

Without this gnat.adc setting, the tasks switched back and forth a great de=
al, as you would expect with preemptive tasking. Putting the pragma in the =
gnat.adc file seemed to affect the granularity of the task switching on my =
test program below, in that it lengthened the time that each task executed =
consecutive loops, but they still switched over to each other eventually. H=
ere is the test program:


with Ada.Text_IO; Use Ada.Text_Io;
=20
procedure Test is


   Task Type One is
   End; =20

   Task Type Two;

   Task body One is
     x : Integer :=3D 0;
   Begin
     put_line("one");
     Loop
       x:=3Dx+1;
       if x > 10000000 Then
         exit;
       end if; =20
     End Loop;
     Put_line("Task one done, x=3D" & x'img);
   End;

   Task body Two is
     x : Integer :=3D 0;
   Begin
     put_line("two");
     Loop
       x:=3Dx+1;
       if x > 1000 Then
         exit;
       end if; =20
     End Loop;
     Put_line("Task two done, x=3D" & x'img);
   End;

  a : One;
  B : two;
begin
  =20
  Null;
End;

Here is the compile line:

gnatmake -gnat2012 -gnatX -f -g test.adb -gnatwA -I. -I..  -D obj


And here is the output:

one
two
Task two done, x=3D 1001
Task one done, x=3D 10000001


I expected the opposite, that task one would execute first, which it did, b=
ut that it would also finish first because there's no reason for it to yiel=
d to two without preemption. It looks to me like I am not actually getting =
non-preemptive tasking, and I would like to know why.

After some looking I found the 2012 attribute 'with CPU', and produced test=
 code :

With System.Multiprocessors;     use System.Multiprocessors;

with Ada.Text_IO; Use Ada.Text_Io;
=20
procedure Test is

   Task Type One with Cpu=3D>1 is
   End; =20

   Task Type Two with Cpu=3D> 1 is
   end;
  =20
   x,y : integer :=3D 0;
   limit : integer :=3D 1000000;

   Task body One is
   Begin
     put_line("one");
     Loop
       if y > 0 then=20
         raise tasking_error;
       end if; =20
       x:=3Dx+1;
       if x > limit Then
         exit;
       end if; =20
     End Loop;
     Put_line("Task one done, x=3D" & x'img);
   Exception
     When others =3D>
     put_line("task one died, x=3D" & x'img); =20
   End;

   Task body Two is
   Begin
     put_line("two");
     Loop
       y:=3Dy+1;
       if y > limit Then
         exit;
       end if; =20
     End Loop;
     Put_line("Task two done, y=3D" & y'img);
   Exception
     When others =3D>
     put_line("task two died"); =20
   End;

  a : One;
  B : two;
begin
  put_line(Number_Of_CPUs'img & " cpu's");=20
  While (x < limit+1) or (y < limit+1) loop
    Delay 0.0;
  End Loop; =20
  put_line("main done, x " & x'img & " y " & y'img);
End;


which produces output


one
two
 24 cpu's
task one died, x=3D 310528
Task two done, y=3D 1000001
^C

(of course, I have to ctl-c out since main never finishes.)

This happens whether or not I have the scheduling pragma in gnat.adc. Does =
Windows just not respect processor assignment and/or the scheduling pragma?

(Sorry for no code blocks, not sure how to do that here)