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D.2.5 Round Robin Dispatching

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{00355AI95−00355−01} [This clause defines the task dispatching policy Round_Robin_Within_Priorities and the package Round_Robin.]

Static Semantics

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{00355AI95−00355−01} The policy_identifier Round_Robin_Within_Priorities is a task dispatching policy.

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{00355AI95−00355−01} The following language−defined library package exists:

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     with System;
     with Ada.Real_Time;
     package Ada.Dispatching.Round_Robin is  Default_Quantum constant Ada.Real_Time.Time_Span :=
                  implementation−defined;
       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;

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Implementation defined: The value of Default_Quantum in Dispatching.Round_Robin.

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{00355AI95−00355−01} 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

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{00355AI95−00355−01} 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.

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{00355AI95−00355−01} The function Actual_Quantum returns the actual quantum used by the implementation for the priority level Pri.

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{00355AI95−00355−01} The function Is_Round_Robin returns True if priority Pri is covered by task dispatching policy Round_Robin_Within_Priorities; otherwise it returns False.

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{00355AI95−00355−01} 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.

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{00355AI95−00355−01} For Round_Robin_Within_Priorities, the dispatching rules for FIFO_Within_Priorities apply with the following additional rules:

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13.a/2

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.

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14.a/2

Ramification: In this case, it will be given a budget as described in the first bullet.

14.b/2

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

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{00333AI95−00333−01} {00355AI95−00355−01} 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.

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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

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{00355AI95−00355−01} An implementation shall document the quantum values supported.

16.a.1/2

Documentation Requirement: The quantum values supported for round robin dispatching.

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{00355AI95−00355−01} An implementation shall document the accuracy with which it detects the exhaustion of the budget of a task.

17.a.1/2

Documentation Requirement: The accuracy of the detection of the exhaustion of the budget of a task for round robin dispatching.

     NOTES

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16  {00355AI95−00355−01} Due to implementation constraints, the quantum value returned by Actual_Quantum might not be identical to that set with Set_Quantum.

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17  {00355AI95−00355−01} A task that executes continuously with an inherited priority will not be subject to round robin dispatching.
Extensions to Ada 95

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{00355AI95−00355−01} {extensions to Ada 95} Policy Round_Robin_Within_Priorities and package Dispatching.Round_Robin are new.