What is the point of designing and implementing parallel algorithms? Of course, to save a little running time from the serial algorithm. so MPI_Wtime() function's man page will give:
Time in seconds since an arbitrary time in the past.and the routine will return a double precision number. So to clock a block of code in MPI, just record the MPI_Wtime() value at the start, and subtract it from the time returned when the run finished.