MPI_Alltoallw function

Gathers data from and scatters data to all members of a group. The MPI_Alltoallw function is the most general form of complete data exchange in this API. MPI_Alltoallw enables separate specification of the count, displacement, and data type.

Syntax

int MPIAPI MPI_Alltoallw(
  _In_  void         *sendbuf,
  _In_  int          *sendcounts[],
  _In_  int          *sdispls[],
  _In_  MPI_Datatype sendtypes[],
  _Out_ void         *recvbuf,
  _In_  int          *recvcounts[],
  _In_  int          *rdispls[],
  _In_  MPI_Datatype recvtypes[],
        MPI_Comm     comm
);

Parameters

• sendbuf [in]
  The pointer to the data to be sent to all processes in the group. The number and data type of the elements in the buffer are specified in the sendcount and sendtype parameters. Each element in the buffer corresponds to a process in the group.

  If the comm parameter references an intracommunicator, you can specify an in-place option by specifying MPI_IN_PLACE in all processes. The sendcount, sdispls and sendtype parameters are ignored. Each process enters data in the corresponding receive buffer element.

  Data that is sent and received must have the same type map, as specified by the recvcounts array and the recvtype parameter, and is read from the locations of the receive buffer as specified by the rdispls parameter.

• sendcounts [in]
  The number of data elements that this process sends in the buffer as specified in the sendbuf parameter. If an element in sendcount is zero, the data part of the message from that process is empty.

• sdispls [in]
  The location, in bytes, relative to the sendbuf parameter, of the data for each communicator process.

  Entry j specifies the displacement relative to the sendbuf parameter from which to take the outgoing data that is destined for process j.

• sendtypes [in]
  The data type for each of the elements in the send buffer. Entry j specifies the type of data that is sent to process j in the group.

• recvbuf [out]
  The pointer to a buffer that contains the data that is received from each process. The number and data type of the elements in the buffer are specified in the recvcount and recvtype parameters.

• recvcounts [in]
  The number of data elements from each communicator process in the receive buffer.

• rdispls [in]
  The location, in bytes, relative to the recvbuf parameter, of the data from each communicator process. Entry i specifies the displacement relative to the recvbuf parameter at which to place the incoming data from process i.

  In the recvbuf, recvcounts, and rdispls parameter arrays, the nth element of each array refers to the data that is received from the nth communicator process.

• recvtypes [in]
  The data type of each element in buffer. Entry i specifies the type of data that is received from process i.

• comm
  The MPI_Comm communicator handle.

Return value

Returns MPI_SUCCESS on success. Otherwise, the return value is an error code.

In Fortran, the return value is stored in the IERROR parameter.

Fortran

    MPI_ALLTOALLW(SENDBUF, SENDCOUNT, SDISPLS, SENDTYPE, RECVBUF, RECVCOUNTS, RDISPLS, RECVTYPE,COMM, IERROR)
        <type> SENDBUF(*), R.ECVBUF(*)
        INTEGER SENDCOUNT, SENDTYPES(*), SDISPLS(*), RECVCOUNTS(*), RDISPLS(*), RECVTYPE, COMM, IERROR

Remarks

All parameters are significant on all processes. The comm parameter must be identical on all processes.

If the comm parameter references an intracommunicator, then the jth block that is sent from process i is received by process j and is placed in the ith block of the receive buffer. These blocks do not all have to have the same size.

The type signature that is specified by the sendcounts[j] and sendtypes[j] parameters for process i must be equal to the type signature that is associated with the recvcounts[i] and recvtypes[i] at process j. Therefore, the amount of data sent must be equal to the amount of data that is received between any pair of processes. Distinct type maps between sender and receiver are still allowed.

The result of a call to the MPI_Alltoallw function is as if each process sent a message to every other process with MPI_Send(sendbuf+sdispls[i],sendcounts[i],sendtypes[i] ,i,...), and received a message from every other process with a call to MPI_Recv(recvbuf+rdispls[i],recvcounts[i],recvtypes[i] ,i,...).

Specifying the in-place option indicates that the same amount and type of data is sent and received between any two processes in the group of the communicator. Different pairs of processes can exchange different amounts of data. Users must ensure that recvcounts[j] and recvtype on process i match recvcounts[i] and recvtype on process j.

If the comm parameter references an intercommunicator, then the outcome is as if each process in group A sends a message to each process in group B, and vice versa. The jth send buffer of process i in group A should be consistent with the ith receive buffer of process j in group B, and vice versa.

Requirements

See also

MPI Collective Functions

MPI_Datatype

MPI_Alltoallv

MPI_Gather

MPI_Scatter

MPI_Send

MPI_Recv