Ubuntu Manpage: MPI_Neighbor_alltoall, MPI_Ineighbor_alltoall - All processes send data to neighboring processes in a

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NAME

       MPI_Neighbor_alltoall,  MPI_Ineighbor_alltoall  -  All  processes send data to neighboring processes in a
       virtual topology communicator

SYNTAX

C Syntax

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

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

Fortran Syntax

       USE MPI
       ! or the older form: INCLUDE 'mpif.h'
       MPI_NEIGHBOR_ALLTOALL(SENDBUF, SENDCOUNT, SENDTYPE, RECVBUF, RECVCOUNT,
            RECVTYPE, COMM, IERROR)

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

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

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

Fortran 2008 Syntax

       USE mpi_f08
       MPI_Neighbor_alltoall(sendbuf, sendcount, sendtype, recvbuf, recvcount,
                 recvtype, comm, ierror)

            TYPE(*), DIMENSION(..), INTENT(IN) :: sendbuf
            TYPE(*), DIMENSION(..) :: recvbuf
            INTEGER, INTENT(IN) :: sendcount, recvcount
            TYPE(MPI_Datatype), INTENT(IN) :: sendtype, recvtype
            TYPE(MPI_Comm), INTENT(IN) :: comm
            INTEGER, OPTIONAL, INTENT(OUT) :: ierror

       MPI_Ineighbor_alltoall(sendbuf, sendcount, sendtype, recvbuf, recvcount,
                 recvtype, comm, request, ierror)

            TYPE(*), DIMENSION(..), INTENT(IN), ASYNCHRONOUS :: sendbuf
            TYPE(*), DIMENSION(..), ASYNCHRONOUS :: recvbuf
            INTEGER, INTENT(IN) :: sendcount, recvcount
            TYPE(MPI_Datatype), INTENT(IN) :: sendtype, recvtype
            TYPE(MPI_Comm), INTENT(IN) :: comm
            TYPE(MPI_Request), INTENT(OUT) :: request
            INTEGER, OPTIONAL, INTENT(OUT) :: ierror

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_Neighbor_alltoall is a collective operation in which all processes send and receive the  same  amount
       of data to each neighbor. The operation of this routine can be represented as follows, where each process
       performs  2n  (n  being  the  number  of  neighbors  in  communicator  comm)  independent  point-to-point
       communications. The neighbors and buffer layout are determined by the topology of comm.

       Example of MPI_Neighbor_alltoall semantics for cartesian topologies:

               MPI_Cart_get(comm, maxdims, dims, periods, coords);
               for (dim = 0, i = 0 ; dim < dims ; ++dim) {
                   MPI_Cart_shift(comm, dim, 1, &r0, &r1);
                   MPI_Isend(sendbuf + i * sendcount * extent(sendtype),
                             sendcount, sendtype, r0, ..., comm, ...);
                   MPI_Irecv(recvbuf + i * recvcount * extent(recvtype),
                             recvcount, recvtype, r0, ..., comm, ...);
                   ++i;
                   MPI_Isend(sendbuf + i * sendcount * extent(sendtype),
                             sendcount, sendtype, r1, ..., comm, &req[i]);
                   MPI_Irecv(recvbuf + i * recvcount * extent(recvtype),
                             recvcount, recvtype, r1, ..., comm, ...);
                   ++i;
               }

               MPI_Waitall (...);

       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 neighbor 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.

NEIGHBOR ORDERING

For a distributed graph topology, created with MPI_Dist_graph_create, the sequence of neighbors in the
send and receive buffers at each process is defined as the sequence returned by MPI_Dist_graph_neighbors
for destinations and sources, respectively. For a general graph topology, created with MPI_Graph_create,
the order of neighbors in the send and receive buffers is defined as the sequence of neighbors as
returned by MPI_Graph_neighbors. Note that general graph topologies should generally be replaced by the
distributed graph topologies.

For a Cartesian topology, created with MPI_Cart_create, the sequence of neighbors in the send and receive
buffers at each process is defined by order of the dimensions, first the neighbor in the negative
direction and then in the positive direction with displacement 1. The numbers of sources and destinations
in the communication routines are 2*ndims with ndims defined in MPI_Cart_create. If a neighbor does not
exist, i.e., at the border of a Cartesian topology in the case of a non-periodic virtual grid dimension
(i.e., periods[...]==false), then this neighbor is defined to be MPI_PROC_NULL.

If a neighbor in any of the functions is MPI_PROC_NULL, then the neighborhood collective communication
behaves like a point-to-point communication with MPI_PROC_NULL in this direction. That is, the buffer is
still part of the sequence of neighbors but it is neither communicated nor updated.

NOTES

       The MPI_IN_PLACE option for sendbuf is not meaningful for this function.

       All arguments on all processes are significant. The comm argument, in particular, must describe the  same
       communicator on all processes. comm must be either a cartesian, graph, or dist graph communicator.

       There   are   two   MPI   library   functions   that   are   more   general  than  MPI_Neighbor_alltoall.
       MPI_Neighbor_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_Neighbor_alltoallw expands
       MPI_Neighbor_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.

       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.