95. Examples using MPI_GATHER, MPI_GATHERV


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The examples in this section use intracommunicators.
Example Gather 100 ints from every process in group to root. See figure 4 .


    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 4: The root process gathers 100  ints 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 5 .


    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); 
Note that the program is erroneous if stride < 100.


Figure 5: The root process gathers 100  ints 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 6 .


    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 6: 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 i-th 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 7 .


    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); 
Note that a different amount of data is received from each process.


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 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 we read a column of a C array. A similar thing was done in Example Examples , Section Examples .


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 8 .


    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 8: 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 i-th 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.



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