dtime, etime: Elapsed Execution Time

Both functions have return values of elapsed time (or -1.0 as error indicator). The time is in seconds. The resolution is to a nanosecond.

dtime: Elapsed Time Since the Last dtime Call

For dtime, the elapsed time is:

• First call: elapsed time since start of execution

• Subsequent calls: elapsed time since the last call to dtime

• Single processor: time used by the CPU

• Multiple Processor: the sum of times for all the CPUs, which is not useful data; use etime instead.

  ---

  Note -

  Do not call dtime from within a parallelized loop.

  ---

The function is called by:

Example: dtime(), single processor:

    real e, dtime, t(2)
    print *, 'elapsed:', e, ', user:', t(1), ', sys:', t(2)
    do i = 1, 10000
        k=k+1
    end do
    e = dtime( t )
    print *, 'elapsed:', e, ', user:', t(1), ', sys:', t(2)
    end
demo% f77 -silent tdtime.f
demo% a.out
elapsed:  0., user:  0., sys:  0.
elapsed:   0.180000, user:    6.00000E-02, sys:   0.120000
demo%

etime: Elapsed Time Since Start of Execution

For etime, the elapsed time is:

• Single Processor-CPU time for the calling process

• Multiple Processors--wallclock time while processing your program

Here is how FORTRAN decides single processor or multiple processor:

For a parallelized FORTRAN program linked with libF77_mt, if the environment variable PARALLEL is:

• Undefined, the current run is single processor.

• Defined and in the range 1, 2, 3, ..., the current run is multiple processor.

• Defined, but some value other than 1, 2, 3, ..., the results are unpredictable.

The function is called by:

Take note that the initial call to etime will be inaccurate. It merely enables the system clock. Do not use the value returned by the initial call to etime.

Example: etime(), single processor:

    real e, etime, t(2)
    e = etime(t)         !  Startup etime - do not use result
    do i = 1, 10000
        k=k+1
    end do
    e = etime( t )
    print *, 'elapsed:', e, ', user:', t(1), `, sys:', t(2)
    end
demo% f77 -silent tetime.f
demo% a.out
elapsed:   0.190000, user:    6.00000E-02, sys:   0.130000
demo% 

See also times(2), f77(1), and the Fortran Programming Guide.