MPI_Alltoallv(3)                   Open MPI                   MPI_Alltoallv(3)



NAME
       MPI_Alltoallv - All processes send different amount of data to, and
       receive different amount of data from, all processes

SYNTAX
C Syntax
       #include <mpi.h>
       int MPI_Alltoallv(void *sendbuf, int *sendcounts,
            int *sdispls, MPI_Datatype sendtype,
            void *recvbuf, int *recvcounts,
            int *rdispls, MPI_Datatype recvtype, MPI_Comm comm)


Fortran Syntax
       INCLUDE 'mpif.h'

       MPI_ALLTOALLV(SENDBUF, SENDCOUNTS, SDISPLS, SENDTYPE,
            RECVBUF, RECVCOUNTS, RDISPLS, RECVTYPE, COMM, IERROR)

            <type>    SENDBUF(*), RECVBUF(*)
            INTEGER   SENDCOUNTS(*), SDISPLS(*), SENDTYPE
            INTEGER   RECVCOUNTS(*), RDISPLS(*), RECVTYPE
            INTEGER   COMM, IERROR


C++ Syntax
       #include <mpi.h>
       void MPI::Comm::Alltoallv(const void* sendbuf,
            const int sendcounts[], const int displs[],
            const MPI::Datatype& sendtype, void* recvbuf,
            const int recvcounts[], const int rdispls[],
            const MPI::Datatype& recvtype)


INPUT PARAMETERS
       sendbuf     Starting address of send buffer.

       sendcounts  Integer array, where entry i specifies the number of
                   elements to send to rank i.

       sdispls     Integer array, where entry i specifies the displacement
                   (offset from sendbuf, in units of sendtype) from which to
                   send data to rank i.

       sendtype    Datatype of send buffer elements.

       recvcounts  Integer array, where entry j specifies the number of
                   elements to receive from rank j.

       rdispls     Integer array, where entry j specifies the displacement
                   (offset from recvbuf, in units of recvtype) to which data
                   from rank j should be written.

       recvtype    Datatype of receive buffer elements.

       comm        Communicator over which data is to be exchanged.


OUTPUT PARAMETERS
       recvbuf     Address of receive buffer.

       IERROR      Fortran only: Error status.


DESCRIPTION
       MPI_Alltoallv is a generalized collective operation in which all
       processes send data to and receive data from all other processes. It
       adds flexibility to MPI_Alltoall by allowing the user to specify data
       to send and receive vector-style (via a displacement and element
       count). The operation of this routine can be thought of as follows,
       where each process performs 2n (n being the number of processes in
       communicator comm) independent point-to-point communications (including
       communication with itself).

            MPI_Comm_size(comm, &n);
            for (i = 0, i < n; i++)
                MPI_Send(sendbuf + sdispls[i] * extent(sendtype),
                    sendcounts[i], sendtype, i, ..., comm);
            for (i = 0, i < n; i++)
                MPI_Recv(recvbuf + rdispls[i] * extent(recvtype),
                    recvcounts[i], recvtype, i, ..., comm);

       Process j sends the k-th block of its local sendbuf to process k, which
       places the data in the j-th block of its local recvbuf.

       When a pair of processes exchanges data, each may pass different
       element count and datatype arguments so long as the sender specifies
       the same amount of data to send (in bytes) as the receiver expects to
       receive.

       Note that process i may send a different amount of data to process j
       than it receives from process j. Also, a process may send entirely
       different amounts of data to different processes in the communicator.


       WHEN COMMUNICATOR IS AN INTER-COMMUNICATOR

       When the communicator is an inter-communicator, the gather operation
       occurs in two phases.  The data is gathered from all the members of the
       first group and received by all the members of the second group.  Then
       the data is gathered from all the members of the second group and
       received by all the members of the first.  The operation exhibits a
       symmetric, full-duplex behavior.

       The first group defines the root process.  The root process uses
       MPI_ROOT as the value of root.  All other processes in the first group
       use MPI_PROC_NULL as the value of root.  All processes in the second
       group use the rank of the root process in the first group as the value
       of root.

       When the communicator is an intra-communicator, these groups are the
       same, and the operation occurs in a single phase.


NOTES
       The MPI_IN_PLACE option is not available for any form of all-to-all
       communication.

       The specification of counts and displacements should not cause any
       location to be written more than once.

       All arguments on all processes are significant. The comm argument, in
       particular, must describe the same communicator on all processes.

       The offsets of sdispls and rdispls are measured in units of sendtype
       and recvtype, respectively. Compare this to MPI_Alltoallw, where these
       offsets are measured in bytes.


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_Alltoall
       MPI_Alltoallw




1.4.1                            Jan 14, 2010                 MPI_Alltoallv(3)
