Returns an elapsed time on the calling processor
Time in seconds since an arbitrary time in the past.
This is intended to be a high-resolution, elapsed (or wall) clock.
See MPI_WTICK to determine the resolution of MPI_WTIME.
If the attribute MPI_WTIME_IS_GLOBAL is defined and true, then
thevalue is synchronized across all processes in MPI_COMM_WORLD.
Notes for Fortran
This is a function, declared as DOUBLE PRECISION MPI_WTIME() in Fortran.
also: MPI_Wtick, MPI_Attr_get