D.14.2 Group Execution Time Budgets
This subclause describes a language-defined package
to assign execution time budgets to groups of tasks.
The following language-defined
library package exists:
Nonblocking, Global => in out synchronized is
Group_Budget(CPU : System.Multiprocessors.CPU :=
is tagged limited private
Group_Budget_Handler is access
(GB : in out
Nonblocking => False;
Task_Array is array
Min_Handler_Ceiling : constant
Add_Task (GB : in out
T : in
Remove_Task (GB: in out
T : in
Is_Member (GB : Group_Budget;
T : Ada.Task_Identification.Task_Id) return
(T : Ada.Task_Identification.Task_Id) return
Members (GB : Group_Budget) return
Replenish (GB : in out
Group_Budget; To : in
Add (GB : in out
Group_Budget; Interval : in
Budget_Has_Expired (GB : Group_Budget) return
Budget_Remaining (GB : Group_Budget) return
Set_Handler (GB : in out
Handler : in
Current_Handler (GB : Group_Budget)
Cancel_Handler (GB : in out
Cancelled : out
Group_Budget_Error : exception
-- not specified by the language
The type Group_Budget represents an execution time
budget to be used by a group of tasks. The type Group_Budget needs finalization
). A task can belong to at most one
group. Tasks of any priority can be added to a group.
An object of type Group_Budget has an associated
nonnegative value of type Time_Span known as its budget
is initially Time_Span_Zero. The type Group_Budget_Handler identifies
a protected procedure to be executed by the implementation when the budget
, that is, reaches zero. Such a protected procedure
is called a handler
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
The handler of all Group_Budget objects is initially cleared.
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.
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.
The function Is_Member returns True if the task identified
by T is a member of the group GB; otherwise, it returns False.
The function Is_A_Group_Member returns True if the
task identified by T is a member of some group; otherwise, it returns
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.
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 nonpositive. 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.
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.
The function Budget_Has_Expired returns True if the
budget of group GB is exhausted (equal to Time_Span_Zero); otherwise,
it returns False.
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.
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.
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.
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.
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.
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.
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.
As part of the finalization of an object of type
Group_Budget all member tasks are removed from the group identified by
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.
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.
An exception propagated from a handler invoked when
the budget of a group of tasks becomes exhausted has no effect.
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.
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.
NOTE 1 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.
NOTE 2 A Group_Budget_Handler can
be associated with several Group_Budget objects.
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