MPI_Neighbor_alltoallv(3)

NAME
MPI_Neighbor_alltoallv, MPI_Ineighbor_alltoallv - All processes send different amounts of data to, and receive different amounts of data from, all neighbors

SYNTAX

C Syntax
#include <mpi.h>
int MPI_Neighbor_alltoallv(const void *sendbuf, const int sendcounts[],
	const int sdispls[], MPI_Datatype sendtype,
	void *recvbuf, const int recvcounts[],
	const int rdispls[], MPI_Datatype recvtype, MPI_Comm comm)
int MPI_Ineighbor_alltoallv(const void *sendbuf, const int sendcounts[],
	const int sdispls[], MPI_Datatype sendtype,
	void *recvbuf, const int recvcounts[],
	const int rdispls[], MPI_Datatype recvtype, MPI_Comm comm,
	MPI_Request *request)

Fortran Syntax
INCLUDE 'mpif.h'
MPI_NEIGHBOR_ALLTOALLV(SENDBUF, SENDCOUNTS, SDISPLS, SENDTYPE,
	RECVBUF, RECVCOUNTS, RDISPLS, RECVTYPE, COMM, IERROR)
	<type>	SENDBUF(*), RECVBUF(*)
	INTEGER	SENDCOUNTS(*), SDISPLS(*), SENDTYPE
	INTEGER	RECVCOUNTS(*), RDISPLS(*), RECVTYPE
	INTEGER	COMM, IERROR
MPI_INEIGHBOR_ALLTOALLV(SENDBUF, SENDCOUNTS, SDISPLS, SENDTYPE,
	RECVBUF, RECVCOUNTS, RDISPLS, RECVTYPE, COMM, REQUEST, IERROR)
	<type>	SENDBUF(*), RECVBUF(*)
	INTEGER	SENDCOUNTS(*), SDISPLS(*), SENDTYPE
	INTEGER	RECVCOUNTS(*), RDISPLS(*), RECVTYPE
	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 (offset from sendbuf, in units of sendtype) from which to send data to neighbor i.

sendtype
    Datatype of send buffer elements.

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 (offset from recvbuf, in units of recvtype) to which data from neighbor j should be written.

recvtype
    Datatype of receive buffer elements.

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_alltoallv is a generalized collective operation in which all processes send data to and receive data from all neighbors. It adds flexibility to MPI_Neighbor_alltoall by allowing the user to specify data to send and receive vector-style (via a displacement and element count). The operation of this routine can be thought of as follows, where each process performs 2n (n being the number of neighbors in to 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, sendtype, r0, ..., comm, ...);
            MPI_Irecv(recvbuf + rdispls[i] * extent(recvtype),
                      recvcount, recvtype, r0, ..., comm, ...);
            ++i;
            MPI_Isend(sendbuf + sdispls[i] * extent(sendtype),
                      sendcount, sendtype, r1, ..., comm, &req[i]);
            MPI_Irecv(recvbuf + rdispls[i] * extent(recvtype),
                      recvcount, recvtype, r1, ..., comm, ...);
            ++i;
        }

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 of data to different processes in the communicator.

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 operation.

The specification of counts 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 units of sendtype and recvtype, respectively. Compare this to MPI_Neighbor_alltoallw, where these offsets are measured in bytes.

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_alltoallw
MPI_Cart_create
MPI_Graph_create
MPI_Dist_graph_create