Timing tools

⚠ INFORMATION
This page was translated by an AI (LLM) with a cursory human check and is awaiting full review.

Unix command to measure time

The Unix command time displays the execution time of an executable after it has finished running. Here is an example:

time ./mon_executable    real    0m1.43s    user    0m0.08s    sys     0m0.16s

• The wall-clock time or Elapsed time corresponds to line real;
• The system time corresponds to line sys, which gives an indication of the load related to input/output (read/write operations on the hard disk).

Wall-clock time in Fortran: SYSTEM_CLOCK

Fortran 90 intrinsic subroutine that provides an Elapsed time by returning the number of clock periods consumed. Here is an example of its use:

INTEGER(kind=8) :: &
  nb_periodes_initial, & ! valeur initiale du compteur de périodes d'horloge
  nb_periodes_final,   & ! valeur finale   du compteur de périodes d'horloge
  nb_periodes_max,     & ! valeur maximale du compteur d'horloge
  nb_periodes_sec,     & ! nombre de périodes d'horloge par seconde
  nb_periodes            ! nombre de périodes d'horloge du code
REAL(kind=8) :: temps_elapsed  ! temps réel en secondes

! Initialisations
CALL SYSTEM_CLOCK(COUNT_RATE=nb_periodes_sec, COUNT_MAX=nb_periodes_max)
...
CALL SYSTEM_CLOCK(COUNT=nb_periodes_initial)
...
!      <<<<<<<<<<<<< partie du code à évaluer >>>>>>>>>>>>>
...
CALL SYSTEM_CLOCK(COUNT=nb_periodes_final)
nb_periodes = nb_periodes_final - nb_periodes_initial
IF (nb_periodes_final < nb_periodes_initial) &
        nb_periodes = nb_periodes + nb_periodes_max
temps_elapsed   = REAL(nb_periodes) / nb_periodes_sec
...

CPU time in Fortran: CPU_TIME

Fortran 90 intrinsic subroutine that measures the CPU time of a portion of code enclosed between two calls to CPU_TIME. Example of implementation:

implicit none
integer               :: j, n
real,dimension(10000) :: x
real                  :: t1,t2
...
call CPU_TIME( t1 )
!do j=1,10000
!  x(j)=log(23.)*j
!end do
x(:) = log(23.)*[ (j,j=1,10000) ]
call CPU_TIME( t2 )
...
read *, n ; print *, x(n)
print *,t2 - t1
...

note

• The variables t1 and t2 are of real type; the result of CPU_TIME is measured in seconds.
• If the returned argument is negative, the functionality is not available.