4.5.1. Examples using MPI_GATHER, MPI_GATHERV

Example

Gather 100 ints from every process in group to root. See figure 3 .

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

Example

Previous example modified -- only the root allocates memory for the receive buffer.

MPI_Comm comm; 
    int gsize,sendarray[100]; 
    int root, myrank, *rbuf; 
    ... 
    MPI_Comm_rank( comm, myrank); 
    if ( myrank == root) { 
       MPI_Comm_size( comm, &gsize); 
       rbuf = (int *)malloc(gsize*100*sizeof(int)); 
       } 
    MPI_Gather( sendarray, 100, MPI_INT, rbuf, 100, MPI_INT, root, comm); 

Figure 3: The root process gathers 100
 int
s from each process in the group.

Example

Do the same as the previous example, but use a derived datatype. Note that the type cannot be the entire set of gsize*100 ints since type matching is defined pairwise between the root and each process in the gather.

MPI_Comm comm; 
    int gsize,sendarray[100]; 
    int root, *rbuf; 
    MPI_Datatype rtype; 
    ... 
    MPI_Comm_size( comm, &gsize); 
    MPI_Type_contiguous( 100, MPI_INT, &rtype ); 
    MPI_Type_commit( &rtype ); 
    rbuf = (int *)malloc(gsize*100*sizeof(int)); 
    MPI_Gather( sendarray, 100, MPI_INT, rbuf, 1, rtype, root, comm); 

Example

Now have each process send 100 ints to root, but place each set (of 100) stride ints apart at receiving end. Use MPI_GATHERV and the displs argument to achieve this effect. Assume . See figure 4 .

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

Figure 4: The root process gathers 100
 int
s from each process in the group, each set is placed
 stride
 ints apart.

Example

Same as Example Examples using MPI_GATHER, MPI_GATHERV on the receiving side, but send the 100 ints from the 0th column of a 100 150 int array, in C. See figure 5 .

MPI_Comm comm; 
    int gsize,sendarray[100][150]; 
    int root, *rbuf, stride; 
    MPI_Datatype stype; 
    int *displs,i,*rcounts; 
 
... 
 
MPI_Comm_size( comm, &gsize); 
    rbuf = (int *)malloc(gsize*stride*sizeof(int)); 
    displs = (int *)malloc(gsize*sizeof(int)); 
    rcounts = (int *)malloc(gsize*sizeof(int)); 
    for (i=0; i<gsize; ++i) { 
        displs[i] = i*stride; 
        rcounts[i] = 100; 
    } 
    /* Create datatype for 1 column of array 
     */ 
    MPI_Type_vector( 100, 1, 150, MPI_INT, &stype); 
    MPI_Type_commit( &stype ); 
    MPI_Gatherv( sendarray, 1, stype, rbuf, rcounts, displs, MPI_INT, 
                                                             root, comm); 

Figure 5: The root process gathers column
 0
 of a 100$ x $150 C array, and each set is placed
 stride
 ints apart.

Example

Process i sends (100-i) ints from the ith column of a 100 150 int array, in C. It is received into a buffer with stride, as in the previous two examples. See figure 6 .

MPI_Comm comm; 
    int gsize,sendarray[100][150],*sptr; 
    int root, *rbuf, stride, myrank; 
    MPI_Datatype stype; 
    int *displs,i,*rcounts; 
 
... 
 
MPI_Comm_size( comm, &gsize); 
    MPI_Comm_rank( comm, &myrank ); 
    rbuf = (int *)malloc(gsize*stride*sizeof(int)); 
    displs = (int *)malloc(gsize*sizeof(int)); 
    rcounts = (int *)malloc(gsize*sizeof(int)); 
    for (i=0; i<gsize; ++i) { 
        displs[i] = i*stride; 
        rcounts[i] = 100-i;     /* note change from previous example */ 
    } 
    /* Create datatype for the column we are sending 
     */ 
    MPI_Type_vector( 100-myrank, 1, 150, MPI_INT, &stype); 
    MPI_Type_commit( &stype ); 
    /* sptr is the address of start of "myrank" column 
     */ 
    sptr = &sendarray[0][myrank]; 
    MPI_Gatherv( sptr, 1, stype, rbuf, rcounts, displs, MPI_INT, 
       root, comm); 

Figure 6: The root process gathers 100-i ints from column
 i
 of a 100$ x $150 C array, and each set is placed
 stride
 ints apart.

Example

Same as Example Examples using MPI_GATHER, MPI_GATHERV , but done in a different way at the sending end. We create a datatype that causes the correct striding at the sending end so that that we read a column of a C array. A similar thing was done in Example Examples , Section Examples .

MPI_Comm comm; 
    int gsize,sendarray[100][150],*sptr; 
    int root, *rbuf, stride, myrank, disp[2], blocklen[2]; 
    MPI_Datatype stype,type[2]; 
    int *displs,i,*rcounts; 
 
... 
 
MPI_Comm_size( comm, &gsize); 
    MPI_Comm_rank( comm, &myrank ); 
    rbuf = (int *)malloc(gsize*stride*sizeof(int)); 
    displs = (int *)malloc(gsize*sizeof(int)); 
    rcounts = (int *)malloc(gsize*sizeof(int)); 
    for (i=0; i<gsize; ++i) { 
        displs[i] = i*stride; 
        rcounts[i] = 100-i; 
    } 
    /* Create datatype for one int, with extent of entire row 
     */ 
    disp[0] = 0;       disp[1] = 150*sizeof(int); 
    type[0] = MPI_INT; type[1] = MPI_UB; 
    blocklen[0] = 1;   blocklen[1] = 1; 
    MPI_Type_struct( 2, blocklen, disp, type, &stype ); 
    MPI_Type_commit( &stype ); 
    sptr = &sendarray[0][myrank]; 
    MPI_Gatherv( sptr, 100-myrank, stype, rbuf, rcounts, displs, MPI_INT, 
                                                               root, comm); 

Example

Same as Example Examples using MPI_GATHER, MPI_GATHERV at sending side, but at receiving side we make the stride between received blocks vary from block to block. See figure 7 .

MPI_Comm comm; 
    int gsize,sendarray[100][150],*sptr; 
    int root, *rbuf, *stride, myrank, bufsize; 
    MPI_Datatype stype; 
    int *displs,i,*rcounts,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 
     */ 
 
/* set up displs and rcounts vectors first 
     */ 
    displs = (int *)malloc(gsize*sizeof(int)); 
    rcounts = (int *)malloc(gsize*sizeof(int)); 
    offset = 0; 
    for (i=0; i<gsize; ++i) { 
        displs[i] = offset; 
        offset += stride[i]; 
        rcounts[i] = 100-i; 
    } 
    /* the required buffer size for rbuf is now easily obtained 
     */ 
    bufsize = displs[gsize-1]+rcounts[gsize-1]; 
    rbuf = (int *)malloc(bufsize*sizeof(int)); 
    /* Create datatype for the column we are sending 
     */ 
    MPI_Type_vector( 100-myrank, 1, 150, MPI_INT, &stype); 
    MPI_Type_commit( &stype ); 
    sptr = &sendarray[0][myrank]; 
    MPI_Gatherv( sptr, 1, stype, rbuf, rcounts, displs, MPI_INT, 
                                                        root, comm); 

Figure 7: The root process gathers 100-i ints from column
 i
 of a 100$ x $150 C array, and each set is placed
 stride[i]
 ints apart (a varying stride).

Example

Process i sends num ints from the ith column of a 100 150 int array, in C. The complicating factor is that the various values of num are not known to root, so a separate gather must first be run to find these out. The data is placed contiguously at the receiving end.

MPI_Comm comm; 
    int gsize,sendarray[100][150],*sptr; 
    int root, *rbuf, stride, myrank, disp[2], blocklen[2]; 
    MPI_Datatype stype,types[2]; 
    int *displs,i,*rcounts,num; 
 
... 
 
MPI_Comm_size( comm, &gsize); 
    MPI_Comm_rank( comm, &myrank ); 
 
/* First, gather nums to root 
     */ 
    rcounts = (int *)malloc(gsize*sizeof(int)); 
    MPI_Gather( &num, 1, MPI_INT, rcounts, 1, MPI_INT, root, comm); 
    /* root now has correct rcounts, using these we set displs[] so 
     * that data is placed contiguously (or concatenated) at receive end 
     */ 
    displs = (int *)malloc(gsize*sizeof(int)); 
    displs[0] = 0; 
    for (i=1; i<gsize; ++i) { 
        displs[i] = displs[i-1]+rcounts[i-1]; 
    } 
    /* And, create receive buffer 
     */ 
    rbuf = (int *)malloc(gsize*(displs[gsize-1]+rcounts[gsize-1]) 
            *sizeof(int)); 
    /* Create datatype for one int, with extent of entire row 
     */ 
    disp[0] = 0;       disp[1] = 150*sizeof(int); 
    type[0] = MPI_INT; type[1] = MPI_UB; 
    blocklen[0] = 1;   blocklen[1] = 1; 
    MPI_Type_struct( 2, blocklen, disp, type, &stype ); 
    MPI_Type_commit( &stype ); 
    sptr = &sendarray[0][myrank]; 
    MPI_Gatherv( sptr, num, stype, rbuf, rcounts, displs, MPI_INT, 
                                                               root, comm);