1.4.8 dtime, etime: Elapsed Execution Time (Sun Studio 12: Fortran Library Reference)

Sun Studio 12: Fortran Library Reference

1.4.8 `dtime`, `etime`: Elapsed Execution Time

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

Versions of dtime and etime used by Fortran 95 use the system’s low resolution clock by default. The resolution is one hundreth of a second. However, if the program is run under the Sun OS^TM operating system utility ptime(1), (/usr/proc/bin/ptime), the high resolution clock is used.

1.4.8.1 `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 –
Calling dtime from within a parallelized loop gives non-deterministic results, since the elapsed time counter is global to all threads participating in the loop

The function is called by:

e = dtime( tarray )

tarray

real(2)

Output

e= -1.0: Error: tarray values are undefined

e≠ -1.0: User time in tarray(1) if no error. System time in tarray(2) if no error

Return value

real

Output

e= -1.0: Error

e≠ -1.0: The sum of tarray(1) and tarray(2)

Example: dtime(), single processor:

demo% cat tdtime.f
       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% f95 tdtime.f
demo% a.out
elapsed: 0.0E+0 , user: 0.0E+0 , sys: 0.0E+0
 elapsed: 0.03 , user: 0.01 , sys: 0.02
demo%

1.4.8.2 `etime`: Elapsed Time Since Start of Execution

For etime, the elapsed time is:

Single Processor Execution—CPU time for the calling process
Multiple Processor Execution—wallclock time while processing your program

The runtime library determines that a program is executing in a multiprocessor mode if either the PARALLEL or OMP_NUM_THREADS environment variables are defined to some integer value greater than 1.

The function is called by:

e = etime( tarray )

tarray

real(2)

Output

e= -1.0: Error: tarray values are undefined.

e≠ -1.0: Single Processor: User time in tarray(1). System time in tarray(2)

Multiple Processor: Wall clock time in tarray(1), 0.0 in tarray(2)

Return value

real

Output

e= -1.0: Error

e≠ -1.0: The sum of tarray(1) and tarray(2)

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:

demo% cat tetime.f
       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% f95 tetime.f
demo% a.out
elapsed: 0.02 , user: 0.01 , sys: 0.01
demo%

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