D.14 Execution Time
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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
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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;
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function Clock
(T : Ada.Task_Identification.Task_Id
:= Ada.Task_Identification.Current_Task)
return CPU_Time;
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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;
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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;
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procedure Split
(T : in CPU_Time; SC : out Seconds_Count; TS : out Time_Span);
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function Time_Of (SC : Seconds_Count;
TS : Time_Span := Time_Span_Zero) return CPU_Time;
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private
... -- not specified by the language
end Ada.Execution_Time;
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{execution time
(of a task)} {CPU
time (of a task)} 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. It is implementation defined which task, if
any, is charged the execution time that is consumed by 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.
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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.
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CPU_Time_First and CPU_Time_Last are the smallest
and largest values of the CPU_Time type, respectively. Dynamic Semantics
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{CPU clock tick}
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.Erroneous Execution
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{erroneous execution
(cause) [partial]} 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
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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.
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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/2
Documentation Requirement:
The properties of the mechanism used
to implement package Execution_Time.
Metrics
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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).
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- An upper bound
on the execution time of a call to the Clock function, in processor clock
cycles.
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- Upper bounds
on the execution times of the operators of the type CPU_Time, in processor
clock cycles.
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Documentation Requirement:
The metrics for execution time.
Implementation Permissions
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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
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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
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{extensions to Ada 95} The
package Execution_Time is new.
Sponsored by Ada-Europe