D.14.2 Group Execution Time Budgets
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This subclause describes a language-defined package to assign execution
time budgets to groups of tasks.
Static Semantics
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The following language-defined library package exists:
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with System;
with System.Multiprocessors;
package Ada.Execution_Time.Group_Budgets
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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;
Implementation defined: The value of
Min_Handler_Ceiling in Execution_Time.Group_Budgets.
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;
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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.
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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.
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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
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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.
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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.
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The function Is_Member returns True if the task identified by T is a
member of the group GB; otherwise, it returns False.
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The function Is_A_Group_Member returns True if the task identified by
T is a member of some group; otherwise, it returns False.
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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.
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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.
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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.
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The function Budget_Has_Expired returns True if the budget of group GB
is exhausted (equal to Time_Span_Zero); otherwise, it returns False.
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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As part of the finalization of an object of type Group_Budget all member
tasks are removed from the group identified by that object.
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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.
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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.
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An exception propagated from a handler invoked when the budget of a group
of tasks becomes exhausted has no effect.
Erroneous Execution
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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
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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.
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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.
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A Group_Budget_Handler can be associated with several Group_Budget objects.
Extensions to Ada 95
Inconsistencies With Ada 2005
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A Group_Budget is now defined to work on a single
processor. If an implementation managed to make this package work 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.
Ada 2005 and 2012 Editions sponsored in part by Ada-Europe
