Ubuntu Manpage: MPI_Alltoall - All processes send data to all processes

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.