MPI_Alltoall(3)                    Open MPI                    MPI_Alltoall(3)



NAME
       MPI_Alltoall - All processes send data to all processes


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


Fortran Syntax
       INCLUDE 'mpif.h'
       MPI_ALLTOALL(SENDBUF, SENDCOUNT, SENDTYPE, RECVBUF, RECVCOUNT,
            RECVTYPE, COMM, IERROR)

            <type>    SENDBUF(*), RECVBUF(*)
            INTEGER   SENDCOUNT, SENDTYPE, RECVCOUNT, RECVTYPE
            INTEGER   COMM, IERROR


C++ Syntax
       #include <mpi.h>
       void MPI::Comm::Alltoall(const void* sendbuf, int sendcount,
            const MPI::Datatype& sendtype, void* recvbuf,
            int recvcount, const MPI::Datatype& recvtype)


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

       sendcount   Number of elements to send to each process (integer).

       sendtype    Datatype of send buffer elements (handle).

       recvcount   Number of elements to receive from each process (integer).

       recvtype    Datatype of receive buffer elements (handle).

       comm        Communicator over which data is to be exchanged (handle).


OUTPUT PARAMETERS
       recvbuf     Starting address of receive buffer (choice).

       IERROR      Fortran only: Error status (integer).


DESCRIPTION
       MPI_Alltoall is a collective operation in which all processes send the
       same amount of data to each other, and receive the same amount of data
       from each other. The operation of this routine can be represented 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 + i * sendcount * extent(sendtype),
                    sendcount, sendtype, i, ..., comm);
            for (i = 0, i < n; i++)
                MPI_Recv(recvbuf + i * recvcount * extent(recvtype),
                    recvcount, recvtype, i, ..., comm);

       Each process breaks up its local sendbuf into n blocks - each
       containing sendcount elements of type sendtype - and divides its
       recvbuf similarly according to recvcount and recvtype. 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. The amount of data sent must be
       equal to the amount of data received, pairwise, between every pair of
       processes.

       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 this function.

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

       There are two MPI library functions that are more general than
       MPI_Alltoall. MPI_Alltoallv allows all-to-all communication to and from
       buffers that need not be contiguous; different processes may send and
       receive different amounts of data. MPI_Alltoallw expands
       MPI_Alltoallv's functionality to allow the exchange of data with
       different datatypes.


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_Alltoallv
       MPI_Alltoallw




1.4.1                            Jan 14, 2010                  MPI_Alltoall(3)
