4.6.1. Examples using MPI_SCATTER, MPI_SCATTERV


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Example

The reverse of Example Examples using MPI_GATHER, MPI_GATHERV . Scatter sets of 100 ints from the root to each process in the group. See figure 8 .


    MPI_Comm comm; 
    int gsize,*sendbuf; 
    int root, rbuf[100]; 
    ... 
    MPI_Comm_size( comm, &gsize); 
    sendbuf = (int *)malloc(gsize*100*sizeof(int)); 
    ... 
    MPI_Scatter( sendbuf, 100, MPI_INT, rbuf, 100, MPI_INT, root, comm); 


Figure 8: The root process scatters sets of 100  ints to each process in the group.


Example

The reverse of Example Examples using MPI_GATHER, MPI_GATHERV . The root process scatters sets of 100 ints to the other processes, but the sets of 100 are stride ints apart in the sending buffer. Requires use of MPI_SCATTERV. Assume . See figure 9 .


    MPI_Comm comm; 
    int gsize,*sendbuf; 
    int root, rbuf[100], i, *displs, *scounts; 
 
    ... 
 
    MPI_Comm_size( comm, &gsize); 
    sendbuf = (int *)malloc(gsize*stride*sizeof(int)); 
    ... 
    displs = (int *)malloc(gsize*sizeof(int)); 
    scounts = (int *)malloc(gsize*sizeof(int)); 
    for (i=0; i<gsize; ++i) { 
        displs[i] = i*stride; 
        scounts[i] = 100; 
    } 
    MPI_Scatterv( sendbuf, scounts, displs, MPI_INT, rbuf, 100, MPI_INT, 
                                                              root, comm); 


Figure 9: The root process scatters sets of 100  ints, moving by  stride ints from send to send in the scatter.


Example

The reverse of Example Examples using MPI_GATHER, MPI_GATHERV . We have a varying stride between blocks at sending (root) side, at the receiving side we receive into the ith column of a 100×150 C array. See figure 10 .


    MPI_Comm comm; 
    int gsize,recvarray[100][150],*rptr; 
    int root, *sendbuf, myrank, bufsize, *stride; 
    MPI_Datatype rtype; 
    int i, *displs, *scounts, offset; 
    ... 
    MPI_Comm_size( comm, &gsize); 
    MPI_Comm_rank( comm, &myrank ); 
 
    stride = (int *)malloc(gsize*sizeof(int)); 
    ... 
    /* stride[i] for i = 0 to gsize-1 is set somehow 
     * sendbuf comes from elsewhere 
     */ 
    ... 
    displs = (int *)malloc(gsize*sizeof(int)); 
    scounts = (int *)malloc(gsize*sizeof(int)); 
    offset = 0; 
    for (i=0; i<gsize; ++i) { 
        displs[i] = offset; 
        offset += stride[i]; 
        scounts[i] = 100 - i; 
    } 
    /* Create datatype for the column we are receiving 
     */ 
    MPI_Type_vector( 100-myrank, 1, 150, MPI_INT, &rtype); 
    MPI_Type_commit( &rtype ); 
    rptr = &recvarray[0][myrank]; 
    MPI_Scatterv( sendbuf, scounts, displs, MPI_INT, rptr, 1, rtype, 
                                                            root, comm); 
 


Figure 10: The root scatters blocks of 100-i ints into column  i of a 100$ x $150 C array. At the sending side, the blocks are  stride[i] ints apart.



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