Manual Reference Pages - cpu_time (3fortran)

NAME

CPU_TIME(3) - [SYSTEM:TIME] Return CPU processor time used in seconds

SYNOPSIS

call cpu_time(time)

          subroutine cpu_time(time)

           real,intent(out) :: time

CHARACTERISTICS

o TIME is a real of any kind

DESCRIPTION

CPU_TIME(3) returns a real value representing the elapsed CPU time in seconds. This is useful for testing segments of code to determine execution time.
If no time source is available, TIME is set to a negative value.
The exact definition of time is left imprecise because of the variability in what different processors are able to provide.
Note that TIME may contain a system dependent, arbitrary offset and may not start with 0.0. For CPU_TIME(3) the absolute value is meaningless. Only differences between subsequent calls, as shown in the example below, should be used.

PARALLEL PROCESSING

Whether the value assigned is an approximation to the amount of time used by the invoking image, or the amount of time used by the whole program, is processor dependent.
A processor for which a single result is inadequate (for example, a parallel processor) might choose to provide an additional version for which TIME is an array.

RESULT

o TIME : is assigned a processor-dependent approximation to the processor time in seconds. If the processor cannot return a meaningful time, a processor-dependent negative value is returned.
: The start time is left imprecise because the purpose is to time sections of code, as in the example. This might or might not include system overhead time.

EXAMPLES

Sample program:

program demo_cpu_time use, intrinsic :: iso_fortran_env, only : real32,real64,real128 implicit none real :: start, finish real(kind=real64) :: startd, finishd ! call cpu_time(start) call cpu_time(startd) ! put code to time here call cpu_time(finish) call cpu_time(finishd) ! ! writes processor time taken by the piece of code.

! the accuracy of the clock and whether it includes system time ! as well as user time is processor dependent. Accuracy up to ! milliseconds is common but not guaranteed, and may be much ! higher or lower print ’("Processor Time = ",f6.3," seconds.")’,finish-start

! see your specific compiler documentation for how to measure ! parallel jobs and for the precision of the time returned print ’("Processor Time = ",g0," seconds.")’,finish-start print ’("Processor Time = ",g0," seconds.")’,finishd-startd end program demo_cpu_time

Results:

The precision of the result, some aspects of what is returned, and what if any options there are for parallel applications may very from system to system. See compiler-specific for details.

       > Processor Time =  0.000 seconds.
       > Processor Time = .4000030E-05 seconds.
       > Processor Time = .2000000000000265E-05 seconds.

STANDARD

Fortran 95