jburkardt-f77/timer/timer.html at master · Code-Sample-Collection/jburkardt-f77 · GitHub

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<html>

  <head>
    <title>
      TIMER - Compute Elapsed Time
    </title>
  </head>

  <body bgcolor="#EEEEEE" link="#CC0000" alink="#FF3300" vlink="#000055">

    <h1 align = "center">
      TIMER <br> Compute Elapsed Time
    </h1>

    <hr>

    <p>
      <b>TIMER</b>
      is a directory of FORTRAN77 programs which
      compute the elapsed CPU time or wallclock time
      of a part of a calculation.
    </p>

    <p>
      The idea is that you want to
      determine the amount of CPU time taken by a piece of your code, so
      you write lines like this:
      <pre><tt>
        call timer ( t_start)
        do i = 1, n
          ...some big calculation...
        end do
        call timer ( t_stop )
        write ( *, * ) 'Elapsed CPU time = ', t_stop - t_start
      </pre></tt>
    </p>

    <p>
      Up until FORTRAN90, the FORTRAN language did not specify a standard
      way to access a system timer.  Users of FORTRAN on UNIX systems
      could access a system library routine called ETIME for this purpose,
      but ETIME was not part of the language, and was not to be found on
      other operating systems.
    </p>

    <p>
      On various
      computers, there are often vendor-supplied routines to do this chore.
      You will often find that the timer is inaccurate; some operations will
      be reported as taking zero time; other operations will be reported
      with times that differ by 5 or 10 percent.  Moreover, two timers that
      profess to measure the same thing may produce results that are
      consistently different by a factor of 40 percent.
    </p>

    <p>
      Another problem is "wrap around".  Many timers reach a maximum value,
      and then reset themselves to zero.  If this happens to you, (it's
      happened to me many times!) you may find that a certain procedure
      seems to take negative time!
    </p>

    <p>
      Some timers return CPU time, that is, the amount of elapsed computer
      time that was used by your program; other routines return "real" time
      or "wall clock" time, which will not account for situations in which
      your program started, and then was paused for some reason (swapped out,
      waiting for I/O, or other system functions), and then finished.
    </p>

    <p>
      For parallel programming, the important thing to measure is the elapsed
      wallclock time.  This can be found by subtracting an initial reading of
      the wallclock time from a final one.
    </p>

    <p>
      The OpenMP system provides a function used as follows:
      <pre>
        seconds = omp_get_wtime ( )
        operations to time;
        seconds = omp_get_wtime ( ) - seconds;
      </pre>
      while the MPI system provides a similar function used as:
      <pre>
        seconds = MPI_Wtime ( );
        operations;
        seconds = MPI_Wtime ( ) - seconds;
      </pre>
      and in MATLAB, wallclock time can be taken with "tic" and "toc":
      <pre>
        tic;
        operation;
        seconds = toc;
      </pre>
    </p>

    <h3 align = "center">
      Licensing:
    </h3>

    <p>
      The computer code and data files described and made available on this web page
      are distributed under
      <a href = "../../txt/gnu_lgpl.txt">the GNU LGPL license.</a>
    </p>

    <h3 align = "center">
      Languages:
    </h3>

    <p>
      <b>TIMER</b> is available in
      <a href = "../../c_src/timer/timer.html">a C version</a> and
      <a href = "../../cpp_src/timer/timer.html">a C++ version</a> and
      <a href = "../../f77_src/timer/timer.html">a FORTRAN77 version</a> and
      <a href = "../../f_src/timer/timer.html">a FORTRAN90 version</a> and
      <a href = "../../m_src/timer/timer.html">a MATLAB version</a>.
    </p>

    <h3 align = "center">
      Related Data and Programs:
    </h3>

    <p>
      <a href = "../../f77_src/linpack_bench/linpack_bench.html">
      LINPACK_BENCH</a>,
      a FORTRAN77 program which
      measures the time taken by <b>LINPACK</b> to solve a particular linear system.
    </p>

    <p>
      <a href = "../../f77_src/matmul/matmul.html">
      MATMUL</a>,
      a FORTRAN77 program which
      is an interactive matrix multiplication benchmark program.
    </p>

    <p>
      <a href = "../../f77_src/mdbnch/mdbnch.html">
      MDBNCH</a>,
      a FORTRAN77 program which
      is a benchmark code for a molecular dynamics calculation.
    </p>

    <p>
      <a href = "../../f_src/memory_test/memory_test.html">
      MEMORY_TEST</a>,
      a FORTRAN90 program which
      declares and uses a sequence of larger
      and larger vectors, to see how big a vector can be used on a given
      machine and compiler.
    </p>

    <p>
      <a href = "../../f77_src/mpi/mpi.html">
      MPI</a>,
      FORTRAN77 programs which
      illustrate the use of the MPI application program interface
      for carrying out parallel computations in a distributed memory environment.
    </p>

    <p>
      <a href = "../../f77_src/openmp/openmp.html">
      OPENMP</a>,
      FORTRAN77 programs which
      illustrate the use of the OpenMP application program interface
      for carrying out parallel computations in a shared memory environment.
    </p>

    <p>
      <a href = "../../f77_src/sum_million/sum_million.html">
      SUM_MILLION</a>,
      a FORTRAN77 program which
      sums the integers from 1 to 1,000,000, as a demonstration of how
      to rate a computer's speed;
    </p>

    <p>
      <a href = "../../f77_src/timestamp/timestamp.html">
      TIMESTAMP</a>,
      a FORTRAN77 library which
      returns the YMDHMS date as a timestamp.
    </p>

    <p>
      <a href = "../../f77_src/wtime/wtime.html">
      WTIME</a>,
      a FORTRAN77 library which
      returns a reading of the wall clock time in seconds.
    </p>

    <h3 align = "center">
      Examples and Tests:
    </h3>

    <p>
      <b>TIMER_ETIME</b> uses the ETIME routine, which is only available
      on UNIX systems:
      <ul>
        <li>
          <a href = "timer_etime.f">timer_etime.f</a>, the test;
        </li>
        <li>
          <a href = "timer_etime.sh">timer_etime.sh</a>,
          commands to compile and run the test;
        </li>
        <li>
          <a href = "timer_etime_output.txt">timer_etime_output.txt</a>,
          the output file;
        </li>
      </ul>
    </p>

    <p>
      <b>TIMER_OMP_GET_WTIME</b> uses the OpenMP wall clock function <b>omp_get_wtime()</b>:
      <ul>
        <li>
          <a href = "timer_omp_get_wtime.f">timer_omp_get_wtime.f</a>, the test;
        </li>
        <li>
          <a href = "timer_omp_get_wtime.sh">timer_omp_get_wtime.sh</a>,
          commands to compile and run the test;
        </li>
        <li>
          <a href = "timer_omp_get_wtime_output.txt">timer_omp_get_wtime_output.txt</a>,
          the output file.
        </li>
      </ul>
    </p>

    <p>
      You can go up one level to <a href = "../f77_src.html">
      the FORTRAN77 source codes</a>.
    </p>

    <hr>

    <i>
      Last revised on 06 March 2008.
    </i>

    <!-- John Burkardt -->

  </body>

</html>