NAME

       MPI_Wtime - Returns an elapsed time on the calling processor.

SYNTAX

C Syntax

       #include <mpi.h>
       double MPI_Wtime()

Fortran Syntax

       USE MPI
       ! or the older form: INCLUDE 'mpif.h'
       DOUBLE PRECISION MPI_WTIME()

Fortran 2008 Syntax

       USE mpi_f08
       DOUBLE PRECISION MPI_WTIME()

C++ Syntax

       #include <mpi.h>
       double MPI::Wtime()

RETURN VALUE

       Time in seconds since an arbitrary time in the past.

DESCRIPTION

       MPI_Wtime returns a floating-point number of seconds, representing elapsed wall-clock time
       since some time in the past.

       The "time in the past" is guaranteed not to change during the life  of  the  process.  The
       user  is  responsible  for  converting large numbers of seconds to other units if they are
       preferred.

       This function is portable (it returns seconds, not "ticks"), it  allows  high  resolution,
       and carries no unnecessary baggage. One would use it like this:

           {
              double starttime, endtime;
              starttime = MPI_Wtime();
               ....  stuff to be timed  ...
              endtime   = MPI_Wtime();
              printf("That took %f seconds\n",endtime-starttime);
           }

       The  times  returned  are local to the node that called them. There is no requirement that
       different nodes return the "same" time.

NOTES

       The boolean variable  MPI_WTIME_IS_GLOBAL,  a  predefined  attribute  key  that  indicates
       whether  clocks  are  synchronized, does not have a valid value in Open MPI, as the clocks
       are not guaranteed to be synchronized.

       This function is intended to be a high-resolution, elapsed (or wall) clock. See  MPI_Wtick
       to determine the resolution of MPI_Wtime.

       On  POSIX  platforms, this function may utilize a timer that is cheaper to invoke than the
       gettimeofday() system call, but  will  fall  back  to  gettimeofday()  if  a  cheap  high-
       resolution  timer is not available.  The ompi_info command can be consulted to see if Open
       MPI supports a native high-resolution timer on your platform; see the value for "MPI_WTIME
       support"  (or  "options:mpi-wtime"  when  viewing  the parsable output).  If this value is
       "native", a method that is likely to be cheaper than gettimeofday() will be used to obtain
       the time when MPI_Wtime is invoked.

       For  example,  on  platforms that support it, the clock_gettime() function will be used to
       obtain a monotonic clock value with whatever  precision  is  supported  on  that  platform
       (e.g., nanoseconds).

       Note,  too,  that  the  MCA  parameter  opal_timer_require_monotonic  can  influcence this
       behavior.  It defaults to true, but if set to false, Open  MPI  may  use  a  finer-grained
       timing  mechanism  (e.g.,  the  RDTSC/RDTSCP  clock ticks on x86_64 platforms), but is not
       guaranteed to be monotonic in some cases (e.g., if the MPI  process  is  not  bound  to  a
       single processor core).

       This  function  does  not  return  an  error value. Consequently, the result of calling it
       before MPI_Init or after MPI_Finalize is undefined.

SEE ALSO

       MPI_Wtick