MPI_Neighbor_alltoallw(3)

NAME
MPI_Neighbor_alltoallw, MPI_Ineighbor_alltoallw - All processes send data of different types to, and receive data of different types from, all processes

SYNTAX

C Syntax
#include <mpi.h>
int MPI_Neighbor_alltoallw(const void *sendbuf, const int sendcounts[],
	const int sdispls[], const MPI_Datatype sendtypes[],
	void *recvbuf, const int recvcounts[], const int rdispls[],
        const MPI_Datatype recvtypes[], MPI_Comm comm)
int MPI_Ineighbor_alltoallw(const void *sendbuf, const int sendcounts[],
	const int sdispls[], const MPI_Datatype sendtypes[],
	void *recvbuf, const int recvcounts[], const int rdispls[],
        const MPI_Datatype recvtypes[], MPI_Comm comm, MPI_Request *request)

Fortran Syntax
INCLUDE 'mpif.h'
MPI_NEIGHBOR_ALLTOALLW(SENDBUF, SENDCOUNTS, SDISPLS, SENDTYPES,
	RECVBUF, RECVCOUNTS, RDISPLS, RECVTYPES, COMM, IERROR)
	<type>	SENDBUF(*), RECVBUF(*)
	INTEGER	SENDCOUNTS(*), SDISPLS(*), SENDTYPES(*)
	INTEGER	RECVCOUNTS(*), RDISPLS(*), RECVTYPES(*)
	INTEGER	COMM, IERROR
MPI_INEIGHBOR_ALLTOALLW(SENDBUF, SENDCOUNTS, SDISPLS, SENDTYPES,
	RECVBUF, RECVCOUNTS, RDISPLS, RECVTYPES, COMM, REQUEST, IERROR)
	<type>	SENDBUF(*), RECVBUF(*)
	INTEGER	SENDCOUNTS(*), SDISPLS(*), SENDTYPES(*)
	INTEGER	RECVCOUNTS(*), RDISPLS(*), RECVTYPES(*)
	INTEGER	COMM, REQUEST, IERROR

INPUT PARAMETERS

sendbuf
    Starting address of send buffer.

sendcounts
    Integer array, where entry i specifies the number of elements to send to neighbor i.

sdispls
    Integer array, where entry i specifies the displacement (in bytes, offset from sendbuf) from which to send data to neighbor i.

sendtypes
    Datatype array, where entry i specifies the datatype to use when sending data to neighbor i.

recvcounts
    Integer array, where entry j specifies the number of elements to receive from neighbor j.

rdispls
    Integer array, where entry j specifies the displacement (in bytes, offset from recvbuf) to which data from neighbor j should be written.

recvtypes
    Datatype array, where entry j specifies the datatype to use when receiving data from neighbor j.

comm
    Communicator over which data is to be exchanged.

OUTPUT PARAMETERS

recvbuf
    Address of receive buffer.

request
    Request (handle, non-blocking only).

IERROR
    Fortran only: Error status.

DESCRIPTION
MPI_Neighbor_alltoallw is a generalized collective operation in which all processes send data to and receive data from all neighbors. It adds flexibility to MPI_Neighbor_alltoallv by allowing the user to specify the datatype of individual data blocks (in addition to displacement and element count). Its operation can be thought of in the following way, where each process performs 2n (n being the number of neighbors in the topology of communicator comm) independent point-to-point communications. The neighbors and buffer layout are determined by the topology of comm.

        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 + sdispls[i]  * extent(sendtype),
                      sendcount, sendtypes[i], r0, ..., comm, ...);
            MPI_Irecv(recvbuf + rdispls[i] * extent(recvtype),
                      recvcount, recvtypes[i], r0, ..., comm, ...);
            ++i;
            MPI_Isend(sendbuf + sdispls[i] * extent(sendtype),
                      sendcount, sendtypes[i], r1, ..., comm, &req[i]);
            MPI_Irecv(recvbuf + rdispls[i] * extent(recvtype),
                      recvcount, recvtypes[i], r1, ..., comm, ...);
            ++i;
        }
        MPI_Wait_all (...);
	MPI_Comm_size(comm, &n);
	for (i = 0, i < n; i++)
	    MPI_Send(sendbuf + sdispls[i], sendcounts[i],
	        sendtypes[i], i, ..., comm);
	for (i = 0, i < n; i++)
	    MPI_Recv(recvbuf + rdispls[i], recvcounts[i],
	        recvtypes[i], i, ..., comm);

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.

When a pair of processes exchanges data, each may pass different element count and datatype arguments so long as the sender specifies the same amount of data to send (in bytes) as the receiver expects to receive.

Note that process i may send a different amount of data to process j than it receives from process j. Also, a process may send entirely different amounts and types of data to different processes in the communicator.

NOTES
The MPI_IN_PLACE option for sendbuf is not meaningful for this operation

The specification of counts, types, and displacements should not cause any location to be written more than once.

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

The offsets of sdispls and rdispls are measured in bytes. Compare this to MPI_Neighbor_alltoallv, where these offsets are measured in units of sendtype and recvtype, respectively.

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.

SEE ALSO
MPI_Neighbor_alltoall
MPI_Neighbor_alltoallv
MPI_Cart_create
MPI_Graph_create
MPI_Dist_graph_create