bionic (3) MPI_Alltoallw.openmpi.3.gz
NAME
MPI_Alltoallw, MPI_Ialltoallw - 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_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_Ialltoallw(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_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_IALLTOALLW(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
C++ Syntax
#include <mpi.h>
void MPI::Comm::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[])
INPUT PARAMETERS
sendbuf Starting address of send buffer.
sendcounts Integer array, where entry i specifies the number of elements to send to rank i.
sdispls Integer array, where entry i specifies the displacement (in bytes, offset from sendbuf) from
which to send data to rank i.
sendtypes Datatype array, where entry i specifies the datatype to use when sending data to rank i.
recvcounts Integer array, where entry j specifies the number of elements to receive from rank j.
rdispls Integer array, where entry j specifies the displacement (in bytes, offset from recvbuf) to
which data from rank j should be written.
recvtypes Datatype array, where entry j specifies the datatype to use when receiving data from rank 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_Alltoallw is a generalized collective operation in which all processes send data to and receive data
from all other processes. It adds flexibility to MPI_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 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 + 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 process 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.
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, sendcounts, sdispls, and sendtypes 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
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_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. 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_Alltoall
MPI_Alltoallv