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D.2.5 Round Robin Dispatching

{AI95-00355-01} {AI05-0299-1} [This subclause defines the task dispatching policy Round_Robin_Within_Priorities and the package Round_Robin.] 

Static Semantics

{AI95-00355-01} The policy_identifier Round_Robin_Within_Priorities is a task dispatching policy.
{AI95-00355-01} The following language-defined library package exists: 
with System;
with Ada.Real_Time;
package Ada.Dispatching.Round_Robin is
  Default_Quantum : constant Ada.Real_Time.Time_Span :=
  procedure Set_Quantum (Pri     : in System.Priority;
                         Quantum : in Ada.Real_Time.Time_Span);
  procedure Set_Quantum (Low, High : in System.Priority;
                         Quantum   : in Ada.Real_Time.Time_Span);
  function Actual_Quantum (Pri : System.Priority)
             return Ada.Real_Time.Time_Span;
  function Is_Round_Robin (Pri : System.Priority) return Boolean;
end Ada.Dispatching.Round_Robin;
Implementation defined: The value of Default_Quantum in Dispatching.Round_Robin.
{AI95-00355-01} When task dispatching policy Round_Robin_Within_Priorities is the single policy in effect for a partition, each task with priority in the range of System.Interrupt_Priority is dispatched according to policy FIFO_Within_Priorities.

Dynamic Semantics

{AI95-00355-01} The procedures Set_Quantum set the required Quantum value for a single priority level Pri or a range of priority levels Low .. High. If no quantum is set for a Round Robin priority level, Default_Quantum is used.
{AI95-00355-01} The function Actual_Quantum returns the actual quantum used by the implementation for the priority level Pri.
{AI95-00355-01} {AI05-0264-1} The function Is_Round_Robin returns True if priority Pri is covered by task dispatching policy Round_Robin_Within_Priorities; otherwise, it returns False.
{AI95-00355-01} A call of Actual_Quantum or Set_Quantum raises exception Dispatching.Dispatching_Policy_Error if a predefined policy other than Round_Robin_Within_Priorities applies to the specified priority or any of the priorities in the specified range.
 {AI95-00355-01} For Round_Robin_Within_Priorities, the dispatching rules for FIFO_Within_Priorities apply with the following additional rules:
When a task is added or moved to the tail of the ready queue for its base priority, it has an execution time budget equal to the quantum for that priority level. This will also occur when a blocked task becomes executable again.
When a task is preempted (by a higher priority task) and is added to the head of the ready queue for its priority level, it retains its remaining budget.
While a task is executing, its budget is decreased by the amount of execution time it uses. The accuracy of this accounting is the same as that for execution time clocks (see D.14).
Ramification: Note that this happens even when the task is executing at a higher, inherited priority, and even if that higher priority is dispatched by a different policy than round robin. 
When a task has exhausted its budget and is without an inherited priority (and is not executing within a protected operation), it is moved to the tail of the ready queue for its priority level. This is a task dispatching point.
Ramification: In this case, it will be given a budget as described in the first bullet.
The rules for FIFO_Within_Priority (to which these bullets are added) say that a task that has its base priority set to a Round Robin priority is moved to the tail of the ready queue for its new priority level, and then will be given a budget as described in the first bullet. That happens whether or not the task's original base priority was a Round Robin priority. 

Implementation Requirements

 {AI95-00333-01} {AI95-00355-01} An implementation shall allow, for a single partition, both the task dispatching policy to be specified as Round_Robin_Within_Priorities and also the locking policy (see D.3) to be specified as Ceiling_Locking. 
Reason: This is the preferred combination of the Round_Robin_Within_Priorities policy with a locking policy, and we want that combination to be portable. 

Documentation Requirements

 {AI95-00355-01} An implementation shall document the quantum values supported. 
Documentation Requirement: The quantum values supported for round robin dispatching.
 {AI95-00355-01} An implementation shall document the accuracy with which it detects the exhaustion of the budget of a task. 
Documentation Requirement: The accuracy of the detection of the exhaustion of the budget of a task for round robin dispatching.
16  {AI95-00355-01} Due to implementation constraints, the quantum value returned by Actual_Quantum might not be identical to that set with Set_Quantum.
17  {AI95-00355-01} A task that executes continuously with an inherited priority will not be subject to round robin dispatching.

Extensions to Ada 95

{AI95-00355-01} Policy Round_Robin_Within_Priorities and package Dispatching.Round_Robin are new. 

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