Table des matières

Jean Zay: Time measurement tools

Unix time measurement command

The Unix time command prints the running time of an executable file, after its own outputs.
Here is an example (without any outputs from the executable): 

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

The elapsed time corresponds here to the real line.
The system time corresponds to the sys line; it gives an indication of the load linked to inputs/outputs (disk operations).

Clock time in Fortran: ''SYSTEM_CLOCK''

SYSTEM_CLOCK is an intrinsic subroutine of Fortran 90 which gives the elapsed time by returning the number of clock periods consumed.
Here is a usage example:

example.f90
  . . .
  INTEGER(kind=8) :: &
     nb_periodes_initial, & ! initial value of the counter of clock periods 
     nb_periodes_final,   & ! final value of the compter of clock periods 
     nb_periodes_max,     & ! maximum value of the clock counter
     nb_periodes_sec,     & ! number of clock periods per second
     nb_periodes            ! number of clock periods of the code
  REAL(kind=8) :: elapsed_time  ! real time in seconds
 
  ! Initialisations
 
  CALL SYSTEM_CLOCK(COUNT_RATE=nb_periodes_sec, COUNT_MAX=nb_periodes_max)
  . . .
  CALL SYSTEM_CLOCK(COUNT=nb_periodes_initial)
  . . .
  !      <<<<<<<<<<<<< part of the code to evaluate >>>>>>>>>>>>>
  . . .
  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''

CPU_TIME is a Fortran 90 intrinsic subroutine which measures the CPU time of a code portion framed by two calls to CPU_TIME.
Implementation:

example.f90
  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
  . . . .

Notes:

  • The t1 and t2 types are real in Fortran; the result of CPU_TIME is measured in seconds.
  • If the returned argument is a negative value, it means that the functionality is not available.