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User Commands  - howlong (1)

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help_text=[ CHARACTER(LEN=128) :: &

NAME

howlong(1) - display wallclock time taken to execute a system command

CONTENTS

Synopis
Description
Sample Subject Program
Footnotes
Options
Example
See Also

SYNOPIS

howlong SYSTEM_COMMAND

   or

howlong --help|--version

DESCRIPTION

Perhaps the simplest performance metric is to just measure the wallclock time taken by a program. This could literally be measured using a conventional clock or stopwatch. This is difficult to automate! So typically a command like the GNU/Linux or Unix command time(1) is used.

Commands such as time(1) often provide more than wallclock times too. But lets construct a wallclock timing tool of our own using standard Fortran (not even the ISO_C_Binding interface will be called upon) that will measure the run time of a command.

Once passed a command to time on the command line, it will then run the command and report the wallclock time use by the program, and echo the command.

Next we provide a simple program that calls the routine(s) of interest enough times to get useful timing information and time it.

So lets say we compiled up the test program using two different sets of compiler options:

    f90 little_test.f90 -O0 -o little_test0
    f90 little_test.f90 -O3 -o little_test3

Now to run the programs via our timing utility only takes a few commands:

    howlong ./little_test0
    Wallclock: 0-00:00:25.461 :command: ./little_test0
    howlong ./little_test3
    Wallclock: 0-00:00:10.274 :command: ./little_test3

SAMPLE SUBJECT PROGRAM

An uninstrumented test program for timing :

   program little_test
   use,intrinsic :: iso_fortran_env, only : int8
   implicit none
   character(len=*),parameter             :: original = "abcdxyz ZXYDCBA _!@"
   integer,parameter                      :: how_many_times = 100000000
   character(len=:),volatile,allocatable  :: t
   integer                                :: i
      do i=1,how_many_times
        t=upper(original)
        t=lower(original)
      enddo
   contains
   function upper(str) result(translated)
   integer(kind=int8), parameter :: &
   & ascii_diff = abs(iachar(’A’,kind=int8) - iachar(’a’,kind=int8))
   character(*), intent(in) :: str
   integer :: i
   character(len=len(str)) :: translated

      translated=str
      do i = 1, len(str)
        select case(str(i:i))
          case("a":"z")
            translated(i:i) = achar(iachar(str(i:i))-ascii_diff)
        end select
      enddo
   end function upper

   function lower(str) result(translated)
   integer(kind=int8), parameter :: &
   & ascii_diff = abs(iachar(’A’,kind=int8) - iachar(’a’,kind=int8))
   character(*), intent(in) :: str
   integer :: i
   character(len=len(str)) :: translated

      translated=str
      do i = 1, len(str)
        select case(str(i:i))
          case("A":"Z")
            translated(i:i) = achar(iachar(str(i:i))+ascii_diff)
        end select
      enddo
   end function lower

   end program little_test

FOOTNOTES

Note that in many HPC environments programs are often run via a job scheduler like Slurm, LSF, PBS, Torque, ... . In these environments there are usually account records of each job that provide resource usage statistics.

OPTIONS

--help	display this help and exit
--version
	output version information and exit

EXAMPLE

howlong ./myprogram

SEE ALSO

gprof(1), gcov(1)

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Nemo Release 3.1

howlong (1)

February 23, 2025

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