MPI_Alltoall function

Article
09/14/2018

Gathers data from and scatters data to all members of a group. The MPI_Alltoall is an extension of the MPI_Allgather function. Each process sends distinct data to each of the receivers. 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.

Syntax

int MPIAPI MPI_Alltoall(
  _In_  void         *sendbuf,
        int          sendcount,
        MPI_Datatype sendtype,
  _Out_ void         *recvbuf,
        int          recvcount,
        MPI_Datatype recvtype,
        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.

If the comm parameter references an intracommunicator, you can specify an in-place option by specifying MPI_IN_PLACE in all processes. The sendcount and sendtype parameters are ignored. Each process enters data in the corresponding receive buffer element. The nth process sends data to the nth element of the receive buffer.
sendcount
The number of elements in the buffer that is specified in the sendbuf parameter. If sendcount is zero, the data part of the message is empty.
sendtype
The MPI data type of the elements in the send buffer.
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.
recvcount
The number of elements in the receive buffer. If the count is zero, the data part of the message is empty.
recvtype
The MPI data type of the elements in the receive buffer.
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_ALLTOALL(SENDBUF, SENDCOUNT, SENDTYPE, RECVBUF, RECVCOUNT, RECVTYPE,
                COMM, IERROR)
        <type> SENDBUF(*), R.ECVBUF(*)
        INTEGER SENDCOUNT, SENDTYPE, RECVCOUNT, RECVTYPE, COMM, IERROR

Remarks

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

The type signature that is specified by the sendcount, and sendtype parameters for a process must be equal to the type signature that is specified by the recvcount, and recvtype parameters. 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.

If the comm parameter references an intracommunicator, the outcome of a call to MPI_ALLGATHER(...) is as if each process executed a send to each process including itself by using MPI_Send(sendbuf + i*sendcount*extent(sendtype), sendcount, sendtype, I, …), and receiving from every other process by using MPI_Recv(recvbuf + i*recvcount*extent(recvtype), recvcount, recvtype, I, …).

If the comm parameter references an intercommunicator, then the result 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.

The number of items that is sent by processes in group A, do not have to be equal the number of items that is sent by processes in group B. In particular, you can move data in only one direction by specifying sendcount == 0 for the communication in the reverse direction.

Requirements

Product	HPC Pack 2012 MS-MPI Redistributable Package, HPC Pack 2008 R2 MS-MPI Redistributable Package, HPC Pack 2008 MS-MPI Redistributable Package or HPC Pack 2008 Client Utilities
Header	Mpi.h; Mpif.h
Library	Msmpi.lib
DLL	Msmpi.dll