Ada Tasking revisited (was: Re: Ada versus Java - Tasking)

[ohk@ultra.tfdt-o.nta.no (Ole-Hjalmar Kristensen FOU.TD/DELAB)]

In article <32E2E430.50EF@bix.com> Tom Moran <tmoran@bix.com> writes:


   > These task switching times are still pretty horrible though
   Yes, and Win95 takes (on my P60) over 700 microseconds just to call
   Ada.Calendar.Clock
     You can use an Ada compiler that doesn't map tasks to OS threads or,
   for really fast stuff, use DOS (where the OS doesn't get in the way).

Interesting thought. Develop on a workstation, then deploy on DOS for
maximum speed!

I have run a simple test of the effect of using protected types to
reduce task switching. The protected type is introduced as a buffer
between the two tasks, and even with a buffer size of 1 leads to a
significant reduction in time. My code is a somewhat simplified
version of an example which was posted earlier, in that it uses only 2
tasks:

with Interfaces;
with Ada.Calendar;
with Ada.Text_Io;

use Ada;
procedure Q_Prod_Cons is
   procedure Time_This is
-- Note different size of Data_Block
      type Data_Block is array (1 .. 4 ) of Interfaces.Unsigned_8;
      for Data_Block'Component_Size use Interfaces.Unsigned_8'Size;
      for Data_Block'Size use 4 * Interfaces.Unsigned_8'Size;

      protected type Buffer is
         entry Put(X: in Data_Block);
         entry Get(X: out Data_Block);
      private
         Data: Data_Block;
         Full: Boolean := False;
      end Buffer;

      protected body Buffer is
         entry Put(X: in Data_Block) when not Full is
         begin
            Data := X; Full := True;
         end Put;

         entry Get(X: out Data_Block) when Full is
         begin
            x:= Data; Full := False;
         end Get;
      end Buffer;
      type Buffer_Ptr is access all Buffer;

      task type Consumer_Agent (Q : access Buffer);

      task body Consumer_Agent is
         Local : Data_Block;
      begin -- Consumer_Agent
         for I in 1..100000 loop
            Q.Get(Local);
         end loop ;
      end Consumer_Agent;
      type Consumer_Ptr is access all Consumer_Agent;
      
      
      task type Producer_Agent (Q : access Buffer);

      task body Producer_Agent is
         Data : Data_Block := (others => 0);
      begin -- Producer_Agent
         for I in 1..100000 loop
            Q.Put(Data);
         end loop ;
      end Producer_Agent;
      type Producer_Ptr is access all Producer_Agent;
      
      Buf: Buffer_Ptr;
      Consumer : Consumer_Ptr;
      Producer : Producer_Ptr;
      
   begin -- Time_This
      Buf := new Buffer;
      Consumer := new Consumer_Agent(Q => Buf);
      Producer := new Producer_Agent (Q => Buf);
   end Time_This;
   
   Stop  : Calendar.Time;
   Start : Calendar.Time := Calendar.Clock;
   
   use type Calendar.Time;
begin -- Prod_Cons
   Time_This;
   Stop := Calendar.Clock;
   Text_Io.Put_Line (Duration'Image (Stop - Start) );
end Q_Prod_Cons;

The original version, modified to use only 2 tasks, and with 100000
iterations use ca. 23 seconds on my machine, while the above version
uses only 15 seconds on average. This is because a thread may execute
operations on behalf of another thread while it is executing inside
the protected type. 

With a longer queue, the effect would be even greater, of course.

Ole-Hj. Kristensen