D.14 Execution Time
This clause describes a language-defined package
to measure execution time.
The following language-defined
library package exists:
CPU_Time is private
CPU_Time_First : constant
CPU_Time_Last : constant
CPU_Time_Unit : constant
CPU_Tick : constant
(T : Ada.Task_Identification.Task_Id
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;
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;
(T : in
CPU_Time; SC : out
Seconds_Count; TS : out
Time_Of (SC : Seconds_Count;
TS : Time_Span := Time_Span_Zero) return
... -- not specified by the language
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.
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.
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
CPU_Time_First and CPU_Time_Last are the smallest
and largest values of the CPU_Time type, respectively.
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.
The effects of the operators on CPU_Time and Time_Span
are as for the operators defined for integer types.
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.
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
For a call of Clock, if the
task identified by T no longer exists, the execution of the program is
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
The implementation shall document the values of
CPU_Time_First, CPU_Time_Last, CPU_Time_Unit, and CPU_Tick.
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.
shall document the following metrics:
- 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
then t2 – t1 <= D.
- 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).
- An upper bound on the execution time
of a call to the Clock function, in processor clock cycles.
- Upper bounds on the execution times
of the operators of the type CPU_Time, in processor clock cycles.
Implementations targeted to machines with word size
smaller than 32 bits need not support the full range and granularity
of the CPU_Time type.
When appropriate, implementations should provide
configuration mechanisms to change the value of CPU_Tick.