:ref:`MPI_Alltoallw`, :ref:`MPI_Ialltoallw`, :ref:`MPI_Alltoallw_init` - All processes send data of different types to, and receive data of different types from, all processes
#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)
int MPI_Alltoallw_init(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_Info info,
MPI_Request *request)USE MPI
! or the older form: 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
MPI_ALLTOALLW_INIT(SENDBUF, SENDCOUNTS, SDISPLS, SENDTYPES,
RECVBUF, RECVCOUNTS, RDISPLS, RECVTYPES, COMM, INFO, REQUEST, IERROR)
<type> SENDBUF(*), RECVBUF(*)
INTEGER SENDCOUNTS(*), SDISPLS(*), SENDTYPES(*)
INTEGER RECVCOUNTS(*), RDISPLS(*), RECVTYPES(*)
INTEGER COMM, INFO, REQUEST, IERRORUSE mpi_f08
MPI_Alltoallw(sendbuf, sendcounts, sdispls, sendtypes, recvbuf, recvcounts,
rdispls, recvtypes, comm, ierror)
TYPE(*), DIMENSION(..), INTENT(IN) :: sendbuf
TYPE(*), DIMENSION(..) :: recvbuf
INTEGER, INTENT(IN) :: sendcounts(*), sdispls(*), recvcounts(*),
rdispls(*)
TYPE(MPI_Datatype), INTENT(IN) :: sendtypes(*)
TYPE(MPI_Datatype), INTENT(IN) :: recvtypes(*)
TYPE(MPI_Comm), INTENT(IN) :: comm
INTEGER, OPTIONAL, INTENT(OUT) :: ierror
MPI_Ialltoallw(sendbuf, sendcounts, sdispls, sendtypes, recvbuf,
recvcounts, rdispls, recvtypes, comm, request, ierror)
TYPE(*), DIMENSION(..), INTENT(IN), ASYNCHRONOUS :: sendbuf
TYPE(*), DIMENSION(..), ASYNCHRONOUS :: recvbuf
INTEGER, INTENT(IN), ASYNCHRONOUS :: sendcounts(*), sdispls(*),
recvcounts(*), rdispls(*)
TYPE(MPI_Datatype), INTENT(IN), ASYNCHRONOUS :: sendtypes(*),
recvtypes(*)
TYPE(MPI_Comm), INTENT(IN) :: comm
TYPE(MPI_Request), INTENT(OUT) :: request
INTEGER, OPTIONAL, INTENT(OUT) :: ierror
MPI_Alltoallw_init(sendbuf, sendcounts, sdispls, sendtypes, recvbuf,
recvcounts, rdispls, recvtypes, comm, fIinfo, request, ierror)
TYPE(*), DIMENSION(..), INTENT(IN), ASYNCHRONOUS :: sendbuf
TYPE(*), DIMENSION(..), ASYNCHRONOUS :: recvbuf
INTEGER, INTENT(IN), ASYNCHRONOUS :: sendcounts(*), sdispls(*),
recvcounts(*), rdispls(*)
TYPE(MPI_Datatype), INTENT(IN), ASYNCHRONOUS :: sendtypes(*),
recvtypes(*)
TYPE(MPI_Comm), INTENT(IN) :: comm
TYPE(MPI_Info), INTENT(IN) :: info
TYPE(MPI_Request), INTENT(OUT) :: request
INTEGER, OPTIONAL, INTENT(OUT) :: ierrorsendbuf: 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.info: Info (handle, persistent only)
recvbuf: Address of receive buffer.request: Request (handle, non-blocking only).ierror: Fortran only: Error status.
:ref:`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 :ref:`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.
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.
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 :ref:`MPI_Alltoallv`, where these offsets are measured in units of sendtype and recvtype, respectively.
.. seealso::
* :ref:`MPI_Alltoall`
* :ref:`MPI_Alltoallv`