D.14.2 Group Execution Time Budgets
1/2
{
AI95-00354-01}
This clause describes a language-defined package
to assign execution time budgets to groups of tasks. Static Semantics
2/2
{
AI95-00354-01}
The following language-defined library package
exists: 3/2
with System;
package Ada.Execution_Time.Group_Budgets is
4/2
type Group_Budget is tagged limited private;
5/2
type Group_Budget_Handler is access
protected procedure (GB : in out Group_Budget);
6/2
type Task_Array is array (Positive range <>) of
Ada.Task_Identification.Task_Id;
7/2
Min_Handler_Ceiling : constant System.Any_Priority :=
implementation-defined;
8/2
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;
9/2
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;
10/2
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);
11/2
Group_Budget_Error : exception;
12/2
private
-- not specified by the language
end Ada.Execution_Time.Group_Budgets;
13/2
{
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.14/2
{
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.{budget}
{exhaust (a budget)}
{handler (group budget)
[partial]} 15/2
{
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.
{set (group budget object) [partial]}
{clear (group budget
object) [partial]} 15.a/2
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
16/2
{
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.17/2
{
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.18/2
{
AI95-00354-01}
The function Is_Member returns True if the task
identified by T is a member of the group GB; otherwise it return False.19/2
{
AI95-00354-01}
The function Is_A_Group_Member returns True if
the task identified by T is a member of some group; otherwise it returns
False.20/2
{
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.21/2
{
AI95-00354-01}
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 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.22/2
{
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.23/2
{
AI95-00354-01}
The function Budget_Has_Expired returns True if
the budget of group GB is exhausted (equal to Time_Span_Zero); otherwise
it returns False.24/2
{
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.25/2
{
AI95-00354-01}
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.26/2
{
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.27/2
{
AI95-00354-01}
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.28/2
{
AI95-00354-01}
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.29/2
{
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.30/2
{
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.31/2
{
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.32/2
{
AI95-00354-01}
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.33/2
{
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.34/2
{
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
35/2
{
AI95-00354-01}
{erroneous execution
(cause) [partial]} 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
36/2
{
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. 36.a/2
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.
37/2
47 {
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.38/2
48 {
AI95-00354-01}
A Group_Budget_Handler can be associated with several
Group_Budget objects. Extensions to Ada 95
38.a/2
{
AI95-00354-01}
{extensions to Ada 95} The
package Execution_Time.Group_Budgets is new.
Sponsored by Ada-Europe