Group Execution Time Budgets

{AI95-00354-01} This clause describes a language-defined package to assign execution time budgets to groups of tasks.

Static Semantics

{AI05-0169-1} with System;
with System.Multiprocessors;
package Ada.Execution_Time.Group_Budgets is

{AI05-0092-1} {AI05-0169-1}   type Group_Budget(CPU : System.Multiprocessors.CPU :=
                                  System.Multiprocessors.CPU'First)
    is tagged limited private;

type Group_Budget_Handler is access
protected procedure (GB : in out Group_Budget);

type Task_Array is array (Positive range <>) of
Ada.Task_Identification.Task_Id;

Min_Handler_Ceiling : constant System.Any_Priority :=
implementation-defined;

  procedure Add_Task (GB : in out Group_Budget;
                      T  : in Ada.Task_Identification.Task_Id);
  procedure Remove_Task (GB: in out Group_Budget;
                         T  : in Ada.Task_Identification.Task_Id);
  function Is_Member (GB : Group_Budget;
                      T : Ada.Task_Identification.Task_Id) return Boolean;
  function Is_A_Group_Member
     (T : Ada.Task_Identification.Task_Id) return Boolean;
  function Members (GB : Group_Budget) return Task_Array;

  procedure Replenish (GB : in out Group_Budget; To : in Time_Span);
  procedure Add (GB : in out Group_Budget; Interval : in Time_Span);
  function Budget_Has_Expired (GB : Group_Budget) return Boolean;
  function Budget_Remaining (GB : Group_Budget) return Time_Span;

  procedure Set_Handler (GB      : in out Group_Budget;
                         Handler : in Group_Budget_Handler);
  function Current_Handler (GB : Group_Budget)
     return Group_Budget_Handler;
  procedure Cancel_Handler (GB        : in out Group_Budget;
                            Cancelled : out Boolean);

Group_Budget_Error : exception;

private
-- not specified by the language
end Ada.Execution_Time.Group_Budgets;

{AI95-00354-01} The type Group_Budget represents an execution time budget to be used by a group of tasks. The type Group_Budget needs finalization (see 7.6). A task can belong to at most one group. Tasks of any priority can be added to a group.

{AI95-00354-01} An object of type Group_Budget has an associated nonnegative value of type Time_Span known as its budget, which is initially Time_Span_Zero. The type Group_Budget_Handler identifies a protected procedure to be executed by the implementation when the budget is exhausted, that is, reaches zero. Such a protected procedure is called a handler.

{AI95-00354-01} An object of type Group_Budget also includes a handler, which is a value of type Group_Budget_Handler. The handler of the object is said to be set if it is not null and cleared otherwise. The handler of all Group_Budget objects is initially cleared.

Discussion: Type Group_Budget is tagged. This makes it possible to share a handler between several events. In simple cases, 'Access can be used to compare the parameter with a specific group budget object (this works because a tagged type is a by-reference type). In more complex cases, a type extension of type Group_Budget can be declared; a double type conversion can be used to access the extension data. An example of how this can be done can be found for the similar type Timing_Event, see D.15.

Dynamic Semantics

{AI95-00354-01} The procedure Add_Task adds the task identified by T to the group GB; if that task is already a member of some other group, Group_Budget_Error is raised.

{AI95-00354-01} The procedure Remove_Task removes the task identified by T from the group GB; if that task is not a member of the group GB, Group_Budget_Error is raised. After successful execution of this procedure, the task is no longer a member of any group.

{AI95-00354-01} {AI05-0264-1} The function Is_Member returns True if the task identified by T is a member of the group GB; otherwise, it returns return False.

{AI95-00354-01} {AI05-0264-1} The function Is_A_Group_Member returns True if the task identified by T is a member of some group; otherwise, it returns False.

{AI95-00354-01} The function Members returns an array of values of type Task_Identification.Task_Id identifying the members of the group GB. The order of the components of the array is unspecified.

{AI95-00354-01} {AI05-0092-1} {AI05-0169-1} The procedure Replenish loads the group budget GB with To as the Time_Span value. The exception Group_Budget_Error is raised if the Time_Span value To is non-positive. Any execution on CPU of any member of the group of tasks results in the budget counting down, unless exhausted. When the budget becomes exhausted (reaches Time_Span_Zero), the associated handler is executed if the handler of group budget GB is set. Nevertheless, the tasks continue to execute.

{AI95-00354-01} The procedure Add modifies the budget of the group GB. A positive value for Interval increases the budget. A negative value for Interval reduces the budget, but never below Time_Span_Zero. A zero value for Interval has no effect. A call of procedure Add that results in the value of the budget going to Time_Span_Zero causes the associated handler to be executed if the handler of the group budget GB is set.

{AI95-00354-01} {AI05-0264-1} The function Budget_Has_Expired returns True if the budget of group GB is exhausted (equal to Time_Span_Zero); otherwise, it returns False.

{AI95-00354-01} The function Budget_Remaining returns the remaining budget for the group GB. If the budget is exhausted it returns Time_Span_Zero. This is the minimum value for a budget.

{AI95-00354-01} {AI05-0264-1} The procedure Set_Handler associates the handler Handler with the Group_Budget GB:; if Handler is null, the handler of Group_Budget is cleared;, otherwise, it is set.

{AI95-00354-01} A call of Set_Handler for a Group_Budget that already has a handler set replaces the handler; if Handler is not null, the handler for Group_Budget remains set.

{AI95-00354-01} {AI05-0264-1} The function Current_Handler returns the handler associated with the group budget GB if the handler for that group budget is set; otherwise, it returns null.

{AI95-00354-01} {AI05-0264-1} The procedure Cancel_Handler clears the handler for the group budget if it is set. Cancelled is assigned True if the handler for the group budget was set prior to it being cleared; otherwise, it is assigned False.

{AI95-00354-01} The constant Min_Handler_Ceiling is the minimum ceiling priority required for a protected object with a handler to ensure that no ceiling violation will occur when that handler is invoked.

{AI95-00354-01} The precision of the accounting of task execution time to a Group_Budget is the same as that defined for execution-time clocks from the parent package.

{AI95-00354-01} As part of the finalization of an object of type Group_Budget all member tasks are removed from the group identified by that object.

{AI95-00354-01} {AI05-0264-1} If a task is a member of a Group_Budget when it terminates, then as part of the finalization of the task it is removed from the group.

{AI95-00354-01} For all the operations defined in this package, Tasking_Error is raised if the task identified by T has terminated, and Program_Error is raised if the value of T is Task_Identification.Null_Task_Id.

{AI95-00354-01} An exception propagated from a handler invoked when the budget of a group of tasks becomes exhausted has no effect.

Erroneous Execution

{AI95-00354-01} For a call of any of the subprograms defined in this package, if the task identified by T no longer exists, the execution of the program is erroneous.

Implementation Requirements

{AI95-00354-01} For a given Group_Budget object, the implementation shall perform the operations declared in this package atomically with respect to any of these operations on the same Group_Budget object. The replacement of a handler, by a call of Set_Handler, shall be performed atomically with respect to the execution of the handler.

Reason: This prevents various race conditions. In particular it ensures that if the budget is exhausted when Set_Handler is changing the handler then either the new or old handler is executed and the exhausting event is not lost.

48 {AI95-00354-01} Clearing or setting of the handler of a group budget does not change the current value of the budget. Exhaustion or loading of a budget does not change whether the handler of the group budget is set or cleared.

Extensions to Ada 95

Inconsistencies With Ada 2005

{AI05-0169-1} A Group_Budget is now defined to work on a single processor. If an implementation managed to make this package work on for programs running on a multiprocessor system, and a program depends on that fact, it could fail when ported to Ada 2012. We believe it is unlikely that such an implementation exists because of the difficulty of signalling other processors when the time reaches zero; in any case, depending on such an implementation is not portable.