MPI_Wtime

突然發現, 原來MPI有提供一個函式MPI_Wtime(), 可以用來計算運行過的時間, 單位是秒.
它的回傳值是double, 並且需要確定你的系統環境是MPI_WTIME_IS_GLOBAL是有被定義的.

/* C Example */
#include <mpi.h>
#include <stdio.h>

int main (int argc, char* argv[]){
        int rank, size;
        double t1;
        void *v;
        int flag;

        MPI_Init (&argc, &argv);      /* starts MPI */
        MPI_Comm_rank (MPI_COMM_WORLD, &rank);        /* get current process id */
        MPI_Comm_size (MPI_COMM_WORLD, &size);        /* get number of processes */
        t1 = MPI_Wtime();
        MPI_Attr_get( MPI_COMM_WORLD, MPI_WTIME_IS_GLOBAL, &v, &flag );
        printf( "Hello world from process %d of %d, time = %f, global time = %d\n", rank, size, MPI_Wtime() - t1, flag);
        MPI_Finalize();
        return 0;
}