D.14 Execution Time
1/2
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This clause describes a language-defined package
to measure execution time. Static Semantics
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The following language-defined library package
exists: 3/2
with Ada.Task_Identification;
with Ada.Real_Time; use Ada.Real_Time;
package Ada.Execution_Time is
4/2
type CPU_Time is private;
CPU_Time_First : constant CPU_Time;
CPU_Time_Last : constant CPU_Time;
CPU_Time_Unit : constant := implementation-defined-real-number;
CPU_Tick : constant Time_Span;
5/2
function Clock
(T : Ada.Task_Identification.Task_Id
:= Ada.Task_Identification.Current_Task)
return CPU_Time;
6/2
function "+" (Left : CPU_Time; Right : Time_Span) return CPU_Time;
function "+" (Left : Time_Span; Right : CPU_Time) return CPU_Time;
function "-" (Left : CPU_Time; Right : Time_Span) return CPU_Time;
function "-" (Left : CPU_Time; Right : CPU_Time) return Time_Span;
7/2
function "<" (Left, Right : CPU_Time) return Boolean;
function "<=" (Left, Right : CPU_Time) return Boolean;
function ">" (Left, Right : CPU_Time) return Boolean;
function ">=" (Left, Right : CPU_Time) return Boolean;
8/2
procedure Split
(T : in CPU_Time; SC : out Seconds_Count; TS : out Time_Span);
9/2
function Time_Of (SC : Seconds_Count;
TS : Time_Span := Time_Span_Zero) return CPU_Time;
9.1/3
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Interrupt_Clocks_Supported : constant Boolean := implementation-defined;9.2/3
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Separate_Interrupt_Clocks_Supported : constant Boolean :=
implementation-defined;9.3/3
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function Clock_For_Interrupts return CPU_Time;10/2
private
... -- not specified by the language
end Ada.Execution_Time;
11/3
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The execution
time or CPU time of a given task is defined as the time spent by
the system executing that task, including the time spent executing run-time
or system services on its behalf. The mechanism used to measure execution
time is implementation defined. The Boolean
constant Interrupt_Clocks_Supported is set to True if the implementation
separately accounts for the execution time of interrupt handlers. If
it is set to False it It is implementation defined which task, if any, is charged the execution
time that is consumed by interrupt handlers.
The Boolean constant Separate_Interrupt_Clocks_Supported is set to True
if the implementation separately accounts for the execution time of individual
interrupt handlers and run-time services on behalf of the system.
11.a/2
Discussion: The
implementation-defined properties above and of the values declared in
the package are repeated in Documentation Requirements, so we don't mark
them as implementation-defined.
12/2
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The type CPU_Time represents the execution time
of a task. The set of values of this type corresponds one-to-one with
an implementation-defined range of mathematical integers.13/2
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The CPU_Time value I represents the half-open execution-time
interval that starts with I*CPU_Time_Unit and is limited by (I+1)*CPU_Time_Unit,
where CPU_Time_Unit is an implementation-defined real number. For each
task, the execution time value is set to zero at the creation of the
task.13.a/2
Ramification: Since
it is implementation-defined which task is charged execution time for
system services, the execution time value may become non-zero even before
the start of the activation of the task.
14/2
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CPU_Time_First and CPU_Time_Last are the smallest
and largest values of the CPU_Time type, respectively. 14.1/3
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The execution time value for the function Clock_For_Interrupts
is initialized to zero.Dynamic Semantics
15/2
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CPU_Time_Unit is the smallest
amount of execution time representable by the CPU_Time type; it is expressed
in seconds. A CPU clock tick is an execution time interval during
which the clock value (as observed by calling the Clock function) remains
constant. CPU_Tick is the average length of such intervals.16/2
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The effects of the operators on CPU_Time and Time_Span
are as for the operators defined for integer types.17/2
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The function Clock returns the current execution
time of the task identified by T; Tasking_Error is raised if that task
has terminated; Program_Error is raised if the value of T is Task_Identification.Null_Task_Id.18/2
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The effects of the Split and Time_Of operations
are defined as follows, treating values of type CPU_Time, Time_Span,
and Seconds_Count as mathematical integers. The effect of Split (T, SC,
TS) is to set SC and TS to values such that T*CPU_Time_Unit = SC*1.0
+ TS*CPU_Time_Unit, and 0.0 <= TS*CPU_Time_Unit < 1.0. The value
returned by Time_Of(SC,TS) is the execution-time value T such that T*CPU_Time_Unit=SC*1.0
+ TS*CPU_Time_Unit.18.1/3
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The function Clock_For_Interrupts returns the total
cumulative time spent executing within all interrupt handlers. This time
is not allocated to any task execution time clock. If Interrupt_Clocks_Supported
is set to False the function raises Program_Error.Erroneous Execution
19/2
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For a call of Clock, if the
task identified by T no longer exists, the execution of the program is
erroneous. Implementation Requirements
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The range of CPU_Time values shall be sufficient
to uniquely represent the range of execution times from the task start-up
to 50 years of execution time later. CPU_Tick shall be no greater than
1 millisecond. Documentation Requirements
21/2
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The implementation shall document the values of
CPU_Time_First, CPU_Time_Last, CPU_Time_Unit, and CPU_Tick. 21.a/2
Documentation Requirement:
The values of CPU_Time_First, CPU_Time_Last,
CPU_Time_Unit, and CPU_Tick of package Execution_Time.
22/2
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The implementation shall document the properties
of the underlying mechanism used to measure execution times, such as
the range of values supported and any relevant aspects of the underlying
hardware or operating system facilities used. 22.a/3
Documentation Requirement: The
properties of the mechanism used to implement package Execution_Time,
including the values of the constants defined in the package.
Metrics
23/2
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The implementation shall document the following
metrics:24/2
An upper bound on the execution-time
duration of a clock tick. This is a value D such that if t1 and t2 are
any execution times of a given task such that t1 < t2 and Clockt1
= Clockt2
then t2 – t1 <= D.
25/2
An upper bound on the size
of a clock jump. A clock jump is the difference between two successive
distinct values of an execution-time clock (as observed by calling the
Clock function with the same Task_Id).
26/2
An upper bound on the execution
time of a call to the Clock function, in processor clock cycles.
27/2
Upper bounds on the execution
times of the operators of the type CPU_Time, in processor clock cycles.
27.a/2
Documentation Requirement:
The metrics for execution time.
Implementation Permissions
28/2
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Implementations targeted to machines with word
size smaller than 32 bits need not support the full range and granularity
of the CPU_Time type.Implementation Advice
29/2
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When appropriate, implementations should provide
configuration mechanisms to change the value of CPU_Tick. 29.a/2
Implementation Advice:
When appropriate, implementations should
provide configuration mechanisms to change the value of Execution_Time.CPU_Tick.
Extensions to Ada 95
29.b/2
Incompatibilities With Ada 2005
29.c/3
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Function Clock_For_Interrupts,
and constants Interrupt_Clocks_Supported and Separate_Interrupt_Clocks_Supported
are newly added to Execution_Time. If Execution_Time is referenced in
a use_clause,
and an entity E with a defining_identifier
of one of the newly added entities is defined in a package that is also
referenced in a use_clause,
the entity E may no longer be use-visible, resulting in errors.
This should be rare and is easily fixed if it does occur. Wording Changes from Ada 2005
29.d/3
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If Interrupt_Clocks_Supported is True, it is now
possible to determine the execution time of interrupt handlers. This
is not an inconsistency, as not charging any task for such time was a
legitimate implementation for Ada 2005.
Ada 2005 and 2012 Editions sponsored in part by Ada-Europe
