D.14 Execution Time
This subclause describes a language-defined package
to measure execution time.
Static Semantics
The following language-defined
library package exists:
with Ada.Task_Identification;
with Ada.Real_Time;
use Ada.Real_Time;
package Ada.Execution_Time
is
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;
function Clock
(T : Ada.Task_Identification.Task_Id
:= Ada.Task_Identification.Current_Task)
return CPU_Time;
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;
procedure Split
(T :
in CPU_Time; SC :
out Seconds_Count; TS :
out Time_Span);
function Time_Of (SC : Seconds_Count;
TS : Time_Span := Time_Span_Zero)
return CPU_Time;
Interrupt_Clocks_Supported :
constant Boolean :=
implementation-defined;
Separate_Interrupt_Clocks_Supported :
constant Boolean :=
implementation-defined;
function Clock_For_Interrupts
return CPU_Time;
private
... -- not specified by the language
end Ada.Execution_Time;
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 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 (see
D.14.3).
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
task.
CPU_Time_First and CPU_Time_Last are the smallest
and largest values of the CPU_Time type, respectively.
The execution time value for the function Clock_For_Interrupts
is initialized to zero.
Dynamic Semantics
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
+ TS*CPU_Time_Unit.
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
For a call of Clock, if the
task identified by T no longer exists, the execution of the program is
erroneous.
Implementation Requirements
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
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.
Metrics
The implementation
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
= Clockt2
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.
Implementation Permissions
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
When appropriate, implementations should provide
configuration mechanisms to change the value of CPU_Tick.
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