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

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

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.