MPI_Gatherv(3)                     Open MPI                     MPI_Gatherv(3)



NAME
       MPI_Gatherv - Gathers varying amounts of data from all processes to the
       root process


SYNTAX
C Syntax
       #include <mpi.h>
       int MPI_Gatherv(void *sendbuf, int sendcount, MPI_Datatype sendtype,
            void *recvbuf, int *recvcounts, int *displs, MPI_Datatype recvtype,
            int root, MPI_Comm comm)


Fortran Syntax
       INCLUDE 'mpif.h'
       MPI_GATHERV(SENDBUF, SENDCOUNT, SENDTYPE, RECVBUF, RECVCOUNTS,
                 DISPLS, RECVTYPE, ROOT, COMM, IERROR)
            <type>    SENDBUF(*), RECVBUF(*)
            INTEGER   SENDCOUNT, SENDTYPE, RECVCOUNTS(*), DISPLS(*)
            INTEGER   RECVTYPE, ROOT, COMM, IERROR


C++ Syntax
       #include <mpi.h>
       void MPI::Comm::Gatherv(const void* sendbuf, int sendcount,
            const MPI::Datatype& sendtype, void* recvbuf,
            const int recvcounts[], const int displs[],
            const MPI::Datatype& recvtype, int root) const = 0


INPUT PARAMETERS
       sendbuf   Starting address of send buffer (choice).

       sendcount Number of elements in send buffer (integer).

       sendtype  Datatype of send buffer elements (handle).

       recvcounts
                 Integer array (of length group size) containing the number of
                 elements that are received from each process (significant
                 only at root).

       displs    Integer array (of length group size). Entry i specifies the
                 displacement relative to recvbuf at which to place the
                 incoming data from process i (significant only at root).

       recvtype  Datatype of recv buffer elements (significant only at root)
                 (handle).

       root      Rank of receiving process (integer).

       comm      Communicator (handle).


OUTPUT PARAMETERS
       recvbuf   Address of receive buffer (choice, significant only at root).

       IERROR    Fortran only: Error status (integer).


DESCRIPTION
       MPI_Gatherv extends the functionality of MPI_Gather by allowing a
       varying count of data from each process, since recvcounts is now an
       array. It also allows more flexibility as to where the data is placed
       on the root, by providing the new argument, displs.

       The outcome is as if each process, including the root process, sends a
       message to the root,

           MPI_Send(sendbuf, sendcount, sendtype, root, ...)

       and the root executes n receives,

           MPI_Recv(recvbuf + disp[i] * extent(recvtype), \
                    recvcounts[i], recvtype, i, ...)

       Messages are placed in the receive buffer of the root process in rank
       order, that is, the data sent from process j is placed in the jth
       portion of the receive buffer recvbuf on process root. The jth portion
       of recvbuf begins at offset displs[j] elements (in terms of recvtype)
       into recvbuf.

       The receive buffer is ignored for all nonroot processes.

       The type signature implied by sendcount, sendtype on process i must be
       equal to the type signature implied by recvcounts[i], recvtype at the
       root. This implies that the amount of data sent must be equal to the
       amount of data received, pairwise between each process and the root.
       Distinct type maps between sender and receiver are still allowed, as
       illustrated in Example 2, below.

       All arguments to the function are significant on process root, while on
       other processes, only arguments sendbuf, sendcount, sendtype, root,
       comm are significant. The arguments root and comm must have identical
       values on all processes.

       The specification of counts, types, and displacements should not cause
       any location on the root to be written more than once. Such a call is
       erroneous.

       Example 1:  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 stride >= 100.

             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.

       Example 2: Same as Example 1 on the receiving side, but send the 100
       ints from the 0th column of a 100 * 150 int array, in C.

             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);

       Example 3: Process i sends (100-i) ints from the ith column of a 100 x
       150 int array, in C. It is received into a buffer with stride, as in
       the previous two examples.

             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.

       Example 4: Same as Example 3, 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.

             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 *)alloc(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 5: Same as Example 3 at sending side, but at receiving side we
       make the  stride between received blocks vary from block to block.

             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);

       Example 6: Process i sends num ints from the ith column of a 100 x 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);

USE OF IN-PLACE OPTION
       The in-place option operates in the same way as it does for MPI_Gather.
       When the communicator is an intracommunicator, you can perform a gather
       operation in-place (the output buffer is used as the input buffer).
       Use the variable MPI_IN_PLACE as the value of the root process sendbuf.
       In this case, sendcount and sendtype are ignored, and the contribution
       of the root process to the gathered vector is assumed to already be in
       the correct place in the receive buffer.

       Note that MPI_IN_PLACE is a special kind of value; it has the same
       restrictions on its use as MPI_BOTTOM.

       Because the in-place option converts the receive buffer into a send-
       and-receive buffer, a Fortran binding that includes INTENT must mark
       these as INOUT, not OUT.


WHEN COMMUNICATOR IS AN INTER-COMMUNICATOR
       When the communicator is an inter-communicator, the root process in the
       first group gathers data from all the processes in the second group.
       The first group defines the root process.  That process uses MPI_ROOT
       as the value of its root argument.  The remaining processes use
       MPI_PROC_NULL as the value of their root argument.  All processes in
       the second group use the rank of that root process in the first group
       as the value of their root argument.   The send buffer argument of the
       processes in the first group must be consistent with the receive buffer
       argument of the root process in the second group.



ERRORS
       Almost all MPI routines return an error value; C routines as the value
       of the function and Fortran routines in the last argument. C++
       functions do not return errors. If the default error handler is set to
       MPI::ERRORS_THROW_EXCEPTIONS, then on error the C++ exception mechanism
       will be used to throw an MPI:Exception object.

       Before the error value is returned, the current MPI error handler is
       called. By default, this error handler aborts the MPI job, except for
       I/O function errors. The error handler may be changed with
       MPI_Comm_set_errhandler; the predefined error handler MPI_ERRORS_RETURN
       may be used to cause error values to be returned. Note that MPI does
       not guarantee that an MPI program can continue past an error.


SEE ALSO
       MPI_Gather
       MPI_Scatter
       MPI_Scatterv




1.4.1                            Jan 14, 2010                   MPI_Gatherv(3)
