Round Robin Dispatching
Author: JTC1/SC22/WG9/ARG, by Randall Brukardt, ARG Editor

Ada Reference Manual — Legal Information

# D.2.5 Round Robin Dispatching

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This clause defines the task dispatching policy Round_Robin_Within_Priorities and the package Round_Robin.

#### Static Semantics

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The policy_identifier Round_Robin_Within_Priorities is a task dispatching policy.
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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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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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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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The function Actual_Quantum returns the actual quantum used by the implementation for the priority level Pri.
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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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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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For Round_Robin_Within_Priorities, the dispatching rules for FIFO_Within_Priorities apply with the following additional rules:
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- 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.

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- 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.

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- 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).

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- 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.

#### Implementation Requirements

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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.

#### Documentation Requirements

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An implementation shall document the quantum values supported.
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An implementation shall document the accuracy with which it detects the exhaustion of the budget of a task.
NOTES
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16 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 A task that executes continuously with an inherited priority will not be subject to round robin dispatching.

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