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MPI_Neighbor_alltoall, MPI_Ineighbor_alltoall  All processes
send data to neighboring processes in a virtual topology communicator
#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)
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
 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).
 recvbuf
 Starting address of receive buffer (choice).
 request
 Request (handle, nonblocking only).
 IERROR
 Fortran only: Error status (integer).
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
pointtopoint 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 kth block of its
local sendbuf to neighbor k, which places the data in the jth 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.
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 nonperiodic 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 pointtopoint 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.
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 alltoall 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.
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.
MPI_Neighbor_alltoallv
MPI_Neighbor_alltoallw
MPI_Cart_create
MPI_Graph_create
MPI_Dist_graph_create
MPI_Dist_graph_create_adjacent
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