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NAME
MPI_Alltoall, MPI_Ialltoall - All processes send data to all processes
SYNTAX
C Syntax
#include <mpi.h>
int MPI_Alltoall(const void *sendbuf, int sendcount,
MPI_Datatype sendtype, void *recvbuf, int recvcount,
MPI_Datatype recvtype, MPI_Comm comm)
int MPI_Ialltoall(const void *sendbuf, int sendcount,
MPI_Datatype sendtype, void *recvbuf, int recvcount,
MPI_Datatype recvtype, MPI_Comm comm, MPI_Request *request)
Fortran Syntax
USE MPI
! or the older form: INCLUDE 'mpif.h'
MPI_ALLTOALL(SENDBUF, SENDCOUNT, SENDTYPE, RECVBUF, RECVCOUNT,
RECVTYPE, COMM, IERROR)
<type> SENDBUF(*), RECVBUF(*)
INTEGER SENDCOUNT, SENDTYPE, RECVCOUNT, RECVTYPE
INTEGER COMM, IERROR
MPI_IALLTOALL(SENDBUF, SENDCOUNT, SENDTYPE, RECVBUF, RECVCOUNT,
RECVTYPE, COMM, REQUEST, IERROR)
<type> SENDBUF(*), RECVBUF(*)
INTEGER SENDCOUNT, SENDTYPE, RECVCOUNT, RECVTYPE
INTEGER COMM, REQUEST, IERROR
Fortran 2008 Syntax
USE mpi_f08
MPI_Alltoall(sendbuf, sendcount, sendtype, recvbuf, recvcount, recvtype,
comm, ierror)
TYPE(*), DIMENSION(..), INTENT(IN) :: sendbuf
TYPE(*), DIMENSION(..) :: recvbuf
INTEGER, INTENT(IN) :: sendcount, recvcount
TYPE(MPI_Datatype), INTENT(IN) :: sendtype, recvtype
TYPE(MPI_Comm), INTENT(IN) :: comm
INTEGER, OPTIONAL, INTENT(OUT) :: ierror
MPI_Ialltoall(sendbuf, sendcount, sendtype, recvbuf, recvcount, recvtype,
comm, request, ierror)
TYPE(*), DIMENSION(..), INTENT(IN), ASYNCHRONOUS :: sendbuf
TYPE(*), DIMENSION(..), ASYNCHRONOUS :: recvbuf
INTEGER, INTENT(IN) :: sendcount, recvcount
TYPE(MPI_Datatype), INTENT(IN) :: sendtype, recvtype
TYPE(MPI_Comm), INTENT(IN) :: comm
TYPE(MPI_Request), INTENT(OUT) :: request
INTEGER, OPTIONAL, INTENT(OUT) :: ierror
C++ Syntax
#include <mpi.h>
void MPI::Comm::Alltoall(const void* sendbuf, int sendcount,
const MPI::Datatype& sendtype, void* recvbuf,
int recvcount, const MPI::Datatype& recvtype)
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_Alltoall is a collective operation in which all processes send the same amount of data
to each other, and receive the same amount of data from each other. The operation of this
routine can be represented as follows, where each process performs 2n (n being the number
of processes in communicator comm) independent point-to-point communications (including
communication with itself).
MPI_Comm_size(comm, &n);
for (i = 0, i < n; i++)
MPI_Send(sendbuf + i * sendcount * extent(sendtype),
sendcount, sendtype, i, ..., comm);
for (i = 0, i < n; i++)
MPI_Recv(recvbuf + i * recvcount * extent(recvtype),
recvcount, recvtype, i, ..., comm);
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 process 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.
WHEN COMMUNICATOR IS AN INTER-COMMUNICATOR
When the communicator is an inter-communicator, the gather operation occurs in two phases.
The data is gathered from all the members of the first group and received by all the
members of the second group. Then the data is gathered from all the members of the second
group and received by all the members of the first. The operation exhibits a symmetric,
full-duplex behavior.
The first group defines the root process. The root process uses MPI_ROOT as the value of
root. All other processes in the first group use MPI_PROC_NULL as the value of root. All
processes in the second group use the rank of the root process in the first group as the
value of root.
When the communicator is an intra-communicator, these groups are the same, and the
operation occurs in a single phase.
USE OF IN-PLACE OPTION
When the communicator is an intracommunicator, you can perform an all-to-all operation in-
place (the output buffer is used as the input buffer). Use the variable MPI_IN_PLACE as
the value of sendbuf. In this case, sendcount and sendtype are ignored. The input data
of each process is assumed to be in the area where that process would receive its own
contribution to the receive buffer.
NOTES
All arguments on all processes are significant. The comm argument, in particular, must
describe the same communicator on all processes.
There are two MPI library functions that are more general than MPI_Alltoall. MPI_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_Alltoallw expands
MPI_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. C++ functions do not return errors. If the default
error handler is set to MPI::ERRORS_THROW_EXCEPTIONS, then on error the C++ exception
mechanism will be used to throw an MPI::Exception object.
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_Alltoallv
MPI_Alltoallw