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MPI_Alltoallv.3.rst

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MPI_Alltoallv

:ref:`MPI_Alltoallv`, :ref:`MPI_Ialltoallv`, :ref:`MPI_Alltoallv_init` - All processes send different amount of data to, and receive different amount of data from, all processes

SYNTAX

C Syntax

#include <mpi.h>

int MPI_Alltoallv(const void *sendbuf, const int sendcounts[],
     const int sdispls[], MPI_Datatype sendtype,
     void *recvbuf, const int recvcounts[],
     const int rdispls[], MPI_Datatype recvtype, MPI_Comm comm)

int MPI_Ialltoallv(const void *sendbuf, const int sendcounts[],
     const int sdispls[], MPI_Datatype sendtype,
     void *recvbuf, const int recvcounts[],
     const int rdispls[], MPI_Datatype recvtype, MPI_Comm comm,
     MPI_Request *request)

int MPI_Alltoallv_init(const void *sendbuf, const int sendcounts[],
     const int sdispls[], MPI_Datatype sendtype,
     void *recvbuf, const int recvcounts[],
     const int rdispls[], MPI_Datatype recvtype, MPI_Comm comm,
     MPI_Info info, MPI_Request *request)

Fortran Syntax

USE MPI
! or the older form: INCLUDE 'mpif.h'
MPI_ALLTOALLV(SENDBUF, SENDCOUNTS, SDISPLS, SENDTYPE,
     RECVBUF, RECVCOUNTS, RDISPLS, RECVTYPE, COMM, IERROR)

     <type>  SENDBUF(*), RECVBUF(*)
     INTEGER SENDCOUNTS(*), SDISPLS(*), SENDTYPE
     INTEGER RECVCOUNTS(*), RDISPLS(*), RECVTYPE
     INTEGER COMM, IERROR

MPI_IALLTOALLV(SENDBUF, SENDCOUNTS, SDISPLS, SENDTYPE,
     RECVBUF, RECVCOUNTS, RDISPLS, RECVTYPE, COMM, REQUEST, IERROR)

     <type>  SENDBUF(*), RECVBUF(*)
     INTEGER SENDCOUNTS(*), SDISPLS(*), SENDTYPE
     INTEGER RECVCOUNTS(*), RDISPLS(*), RECVTYPE
     INTEGER COMM, REQUEST, IERROR

MPI_ALLTOALLV_INIT(SENDBUF, SENDCOUNTS, SDISPLS, SENDTYPE,
     RECVBUF, RECVCOUNTS, RDISPLS, RECVTYPE, COMM, INFO, REQUEST, IERROR)

     <type>  SENDBUF(*), RECVBUF(*)
     INTEGER SENDCOUNTS(*), SDISPLS(*), SENDTYPE
     INTEGER RECVCOUNTS(*), RDISPLS(*), RECVTYPE
     INTEGER COMM, INFO, REQUEST, IERROR

Fortran 2008 Syntax

USE mpi_f08
MPI_Alltoallv(sendbuf, sendcounts, sdispls, sendtype, recvbuf, recvcounts,
             rdispls, recvtype, comm, ierror)

     TYPE(*), DIMENSION(..), INTENT(IN) :: sendbuf
     TYPE(*), DIMENSION(..) :: recvbuf
     INTEGER, INTENT(IN) :: sendcounts(*), sdispls(*), recvcounts(*),
     rdispls(*)
     TYPE(MPI_Datatype), INTENT(IN) :: sendtype, recvtype
     TYPE(MPI_Comm), INTENT(IN) :: comm
     INTEGER, OPTIONAL, INTENT(OUT) :: ierror

MPI_Ialltoallv(sendbuf, sendcounts, sdispls, sendtype, recvbuf, recvcounts,
             rdispls, recvtype, 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) :: sendtype, recvtype
     TYPE(MPI_Comm), INTENT(IN) :: comm
     TYPE(MPI_Request), INTENT(OUT) :: request
     INTEGER, OPTIONAL, INTENT(OUT) :: ierror

MPI_Alltoallv_init(sendbuf, sendcounts, sdispls, sendtype, recvbuf, recvcounts,
                     rdispls, recvtype, comm, info, 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) :: sendtype, recvtype
     TYPE(MPI_Comm), INTENT(IN) :: comm
     TYPE(MPI_Info), INTENT(IN) :: info
     TYPE(MPI_Request), INTENT(OUT) :: request
     INTEGER, OPTIONAL, INTENT(OUT) :: ierror

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 (offset from sendbuf, in units of sendtype) from which to send data to rank i.
  • sendtype: Datatype of send buffer elements.
  • 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 (offset from recvbuf, in units of recvtype) to which data from rank j should be written.
  • recvtype: Datatype of receive buffer elements.
  • comm: Communicator over which data is to be exchanged.
  • info: Info (handle, persistent only)

OUTPUT PARAMETERS

  • recvbuf: Address of receive buffer.
  • request: Request (handle, non-blocking only).
  • ierror: Fortran only: Error status.

DESCRIPTION

:ref:`MPI_Alltoallv` 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_Alltoall` by allowing the user to specify data to send and receive vector-style (via a displacement and element count). The operation of this routine can be thought of 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 + sdispls[i] * extent(sendtype),
        sendcounts[i], sendtype, i, ..., comm);
for (i = 0, i < n; i++)
    MPI_Recv(recvbuf + rdispls[i] * extent(recvtype),
        recvcounts[i], recvtype, 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 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 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

The specification of counts 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 units of sendtype and recvtype, respectively. Compare this to :ref:`MPI_Alltoallw`, where these offsets are measured in bytes.

ERRORS

.. seealso::
   * :ref:`MPI_Alltoall`
   * :ref:`MPI_Alltoallw`