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Dispatching#

The task dispatching model specifies task scheduling, based on conceptual priority-ordered ready queues. 

package Ada.Dispatching
   with Preelaborate, Nonblocking, Global => in out synchronized is

   procedure Yield with Nonblocking => False;

   Dispatching_Policy_Error: exception;
end Ada.Dispatching;

Earliest Deadline First#

The deadline of a task is an indication of the urgency of the task; it represents a point on an ideal physical time line. The deadline can affect how resources are allocated to the task.

This subclause presents Dispatching.EDF, a package for representing the deadline of a task and a dispatching policy that defines Earliest Deadline First (EDF) dispatching. The Relative_Deadline aspect is provided to assign an initial deadline to a task. A configuration pragma Generate_Deadlines is provided to specify that a task’s deadline is recomputed whenever it is made ready. 

with Ada.Real_Time;
with Ada.Task_Identification;

package Ada.Dispatching.EDF
   with Nonblocking, Global => in out synchronized is

   subtype Deadline          is Ada.Real_Time.Time;
   subtype Relative_Deadline is Ada.Real_Time.Time_Span;

   Default_Deadline         : constant Deadline          := Ada.Real_Time.Time_Last;
   Default_Relative_Deadline: constant Relative_Deadline := Ada.Real_Time.Time_Span_Last;

   procedure Set_Deadline(D: Deadline; T: Ada.Task_Identification.Task_Id :=
                                          Ada.Task_Identification.Current_Task);
   function Get_Deadline(T: Ada.Task_Identification.Task_Id :=
                            Ada.Task_Identification.Current_Task)
      return Deadline;
   procedure Set_Relative_Deadline(D: Relative_Deadline; T: Ada.Task_Identification.Task_Id :=
                                                            Ada.Task_Identification.Current_Task);
   function Get_Relative_Deadline(T: Ada.Task_Identification.Task_Id :=
                                     Ada.Task_Identification.Current_Task)
      return Relative_Deadline;
   procedure Delay_Until_And_Set_Deadline(Delay_Until_Time: Ada.Real_Time.Time; 
                                          Deadline_Offset : Ada.Real_Time.Time_Span)
      with Nonblocking => False;
   function Get_Last_Release_Time(T: Ada.Task_Identification.Task_Id :=
                                     Ada.Task_Identification.Current_Task)
      return Ada.Real_Time.Time;;
end Ada.Dispatching.EDF;

Legality Rules

The Relative_Deadline aspect shall not be specified on a task or protected interface type. If the Relative_Deadline aspect is specified for a subprogram, the aspect_definition shall be a static expression.

Erroneous Execution

If a value of Task_Id is passed as a parameter to any of the subprograms of this package and the corresponding task object no longer exists, the execution of the program is erroneous.