Examples using MPI_SCATTER, MPI_SCATTERV

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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=mycoll-fig7.ps,width=3.50in
Figure 8: The root process scatters sets of 100 ints to each process in the group.

  
 
 
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=mycoll-fig8.ps,width=3.50in
Figure 9: The root process scatters sets of 100 ints, moving by stride ints from send to send in the scatter.

  
 
 
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=mycoll-fig9.ps,width=4.00in
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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