Earliest Deadline First Dispatching

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 might affect how resources are allocated to the task.

This subclause defines a package for representing the deadline of a task and a dispatching policy that defines Earliest Deadline First (EDF) dispatching. An aspect is defined to assign an initial deadline to a task.

Static Semantics

with Ada.Real_Time;
with Ada.Task_Identification;
package Ada.Dispatching.EDF is
  subtype Deadline is Ada.Real_Time.Time;
  Default_Deadline : constant Deadline :=
              Ada.Real_Time.Time_Last;
  procedure Set_Deadline (D : in Deadline;
              T : in Ada.Task_Identification.Task_Id :=
              Ada.Task_Identification.Current_Task);
  procedure Delay_Until_And_Set_Deadline (
              Delay_Until_Time : in Ada.Real_Time.Time;
              Deadline_Offset : in Ada.Real_Time.Time_Span);
  function Get_Deadline (T : Ada.Task_Identification.Task_Id :=
              Ada.Task_Identification.Current_Task) return Deadline;
end Ada.Dispatching.EDF;

For a task type (including the anonymous type of a single_task_declaration) or subprogram, the following language-defined representation aspect may be specified:

Legality Rules

Post-Compilation Rules

If the EDF_Across_Priorities policy is specified for a partition, then the Ceiling_Locking policy (see D.3) shall also be specified for the partition.

If the EDF_Across_Priorities policy appears in a Priority_Specific_Dispatching pragma (see D.2.2) in a partition, then the Ceiling_Locking policy (see D.3) shall also be specified for the partition.

Dynamic Semantics

The initial absolute deadline of a task for which aspect Relative_Deadline is specified is the value of Real_Time.Clock + the expression that is the value of the aspect, where this entire expression, including the call of Real_Time.Clock, is evaluated between task creation and the start of its activation. If the aspect Relative_Deadline is not specified, then the initial absolute deadline of a task is the value of Default_Deadline. The environment task is also given an initial deadline by this rule, using the value of the Relative_Deadline aspect of the main subprogram (if any).

The procedure Set_Deadline changes the absolute deadline of the task to D. The function Get_Deadline returns the absolute deadline of the task.

The procedure Delay_Until_And_Set_Deadline delays the calling task until time Delay_Until_Time. When the task becomes runnable again it will have deadline Delay_Until_Time + Deadline_Offset.

On a system with a single processor, the setting of the deadline of a task to the new value occurs immediately at the first point that is outside the execution of a protected action. If the task is currently on a ready queue it is removed and re-entered on to the ready queue determined by the rules defined below.

When EDF_Across_Priorities is specified for priority range Low..High all ready queues in this range are ordered by deadline. The task at the head of a queue is the one with the earliest deadline.

For a task T to which policy EDF_Across_Priorities applies, the base priority is not a source of priority inheritance; the active priority when first activated or while it is blocked is defined as the maximum of the following:

the highest priority P, if any, less than the base priority of T such that one or more tasks are executing within a protected object with ceiling priority P and task T has an earlier deadline than all such tasks; and furthermore T has an earlier deadline than all other tasks on ready queues with priorities in the given EDF_Across_Priorities range that are strictly less than P.

When a task T is first activated or becomes unblocked, it is added to the ready queue corresponding to this active priority. Until it becomes blocked again, the active priority of T remains no less than this value; it will exceed this value only while it is inheriting a higher priority.

When the setting of the base priority of a ready task takes effect and the new priority is in a range specified as EDF_Across_Priorities, the task is added to the ready queue corresponding to its new active priority, as determined above.

For all the operations defined in Dispatching.EDF, Tasking_Error is raised if the task identified by T has terminated. Program_Error is raised if the value of T is Null_Task_Id.

Bounded (Run-Time) Errors

If EDF_Across_Priorities is specified for priority range Low..High, it is a bounded error to declare a protected object with ceiling priority Low or to assign the value Low to attribute 'Priority. In either case either Program_Error is raised or the ceiling of the protected object is assigned the value Low+1.

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.

Documentation Requirements

On a multiprocessor, the implementation shall document any conditions that cause the completion of the setting of the deadline of a task to be delayed later than what is specified for a single processor.

18 If two adjacent priority ranges, A..B and B+1..C are specified to have policy EDF_Across_Priorities, then this is not equivalent to this policy being specified for the single range, A..C.

19 The above rules implement the preemption-level protocol (also called Stack Resource Policy protocol) for resource sharing under EDF dispatching. The preemption-level for a task is denoted by its base priority. The definition of a ceiling preemption-level for a protected object follows the existing rules for ceiling locking.