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

NAME

       MPI_Alltoall, MPI_Ialltoall - All processes send data to all processes

SYNTAX

C Syntax

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

       int MPI_Ialltoall(const void *sendbuf, int sendcount,
            MPI_Datatype sendtype, void *recvbuf, int recvcount,
            MPI_Datatype recvtype, MPI_Comm comm, MPI_Request *request)

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

       MPI_IALLTOALL(SENDBUF, SENDCOUNT, SENDTYPE, RECVBUF, RECVCOUNT,
            RECVTYPE, COMM, REQUEST, IERROR)

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

       request     Request (handle, non-blocking only).

       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.

USE OF IN-PLACE OPTION

       When the communicator is an intracommunicator, you can perform an all-to-all operation in-
       place (the output buffer is used as the input buffer).  Use the variable  MPI_IN_PLACE  as
       the  value  of sendbuf.  In this case, sendcount and sendtype are ignored.  The input data
       of each process is assumed to be in the area where that  process  would  receive  its  own
       contribution to the receive buffer.

NOTES

       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.