Q. Timing in MPI program?

MPI_Wtime ( ) returns lapsed wall clock time in seconds because some random point in past. Elapsed time for program section is given by difference between MPI_Wtime values at starting and end of process. Process clocks aren't necessarily synchronised, so clock values aren't necessarily comparable across the processes and care should be taken in determining overall running time for parallel program. Even if clocks are openly synchronised variation across clocks still can't be expected to be considerably less than round-trip time for zero-length message between the processes.