MPI_Neighbor_alltoall(3) Open MPI MPI_Neighbor_alltoall(3)NAME
MPI_Neighbor_alltoall, MPI_Ineighbor_alltoall - All processes send data
to neighboring processes in a virtual topology communicator
SYNTAXC 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).
DESCRIPTIONMPI_Neighbor_alltoall is a collective operation in which all processes
send and receive the same amount of data to each neighbor. The opera‐
tion 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 contain‐
ing sendcount elements of type sendtype - and divides its recvbuf simi‐
larly 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 pro‐
cesses.
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 topol‐
ogy, 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 gen‐
erally 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 neigh‐
borhood collective communication behaves like a point-to-point communi‐
cation 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 func‐
tion.
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 commu‐
nication to and from buffers that need not be contiguous; different
processes may send and receive different amounts of data. MPI_Neigh‐
bor_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.
SEE ALSO
MPI_Neighbor_alltoallv
MPI_Neighbor_alltoallw
MPI_Cart_create
MPI_Graph_create
MPI_Dist_graph_create
MPI_Dist_graph_create_adjacent
1.7.4 Feb 04, 2014 MPI_Neighbor_alltoall(3)
