SYNTAX¶

#include <mpi.h>
int MPI_Alltoall(const void *sendbuf, int sendcount,


     MPI_Datatype sendtype, void *recvbuf, int recvcount,


     MPI_Datatype recvtype, MPI_Comm comm)
int MPI_Ialltoall(const void *sendbuf, int sendcount,


     MPI_Datatype sendtype, void *recvbuf, int recvcount,


     MPI_Datatype recvtype, MPI_Comm comm, MPI_Request *request)
int MPI_Alltoall_init(const void *sendbuf, int sendcount,


     MPI_Datatype sendtype, void *recvbuf, int recvcount,


     MPI_Datatype recvtype, MPI_Comm comm, MPI_Info info, MPI_Request *request)

USE MPI
! or the older form: INCLUDE 'mpif.h'
MPI_ALLTOALL(SENDBUF, SENDCOUNT, SENDTYPE, RECVBUF, RECVCOUNT,


     RECVTYPE, COMM, IERROR)


     <type>  SENDBUF(*), RECVBUF(*)


     INTEGER SENDCOUNT, SENDTYPE, RECVCOUNT, RECVTYPE


     INTEGER COMM, IERROR
MPI_IALLTOALL(SENDBUF, SENDCOUNT, SENDTYPE, RECVBUF, RECVCOUNT,


     RECVTYPE, COMM, REQUEST, IERROR)


     <type>  SENDBUF(*), RECVBUF(*)


     INTEGER SENDCOUNT, SENDTYPE, RECVCOUNT, RECVTYPE


     INTEGER COMM, REQUEST, IERROR
MPI_ALLTOALL_INIT(SENDBUF, SENDCOUNT, SENDTYPE, RECVBUF, RECVCOUNT,


     RECVTYPE, COMM, INFO, REQUEST, IERROR)


     <type>  SENDBUF(*), RECVBUF(*)


     INTEGER SENDCOUNT, SENDTYPE, RECVCOUNT, RECVTYPE


     INTEGER COMM, INFO, REQUEST, IERROR

USE mpi_f08
MPI_Alltoall(sendbuf, sendcount, sendtype, recvbuf, recvcount, recvtype,


             comm, ierror)


     TYPE(*), DIMENSION(..), INTENT(IN) :: sendbuf


     TYPE(*), DIMENSION(..) :: recvbuf


     INTEGER, INTENT(IN) :: sendcount, recvcount


     TYPE(MPI_Datatype), INTENT(IN) :: sendtype, recvtype


     TYPE(MPI_Comm), INTENT(IN) :: comm


     INTEGER, OPTIONAL, INTENT(OUT) :: ierror
MPI_Ialltoall(sendbuf, sendcount, sendtype, recvbuf, recvcount, recvtype,


             comm, request, ierror)


     TYPE(*), DIMENSION(..), INTENT(IN), ASYNCHRONOUS :: sendbuf


     TYPE(*), DIMENSION(..), ASYNCHRONOUS :: recvbuf


     INTEGER, INTENT(IN) :: sendcount, recvcount


     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_Alltoall_init(sendbuf, sendcount, sendtype, recvbuf, recvcount, recvtype,


                     comm, info, request, ierror)


     TYPE(*), DIMENSION(..), INTENT(IN), ASYNCHRONOUS :: sendbuf


     TYPE(*), DIMENSION(..), ASYNCHRONOUS :: recvbuf


     INTEGER, INTENT(IN) :: sendcount, recvcount


     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¶

OUTPUT PARAMETERS¶

DESCRIPTION¶

MPI_Alltoall is a collective operation in which all processes send the same amount of data to each other, and receive the same amount of data from each other. The operation of this routine can be represented 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 + i * sendcount * extent(sendtype),


        sendcount, sendtype, i, ..., comm);
for (i = 0, i < n; i++)


    MPI_Recv(recvbuf + i * recvcount * extent(recvtype),


        recvcount, recvtype, i, ..., comm);

Each process breaks up its local sendbuf into n blocks - each containing sendcount elements of type sendtype - and divides its recvbuf similarly according to recvcount and recvtype. 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. The amount of data sent must be equal to the amount of data received, pairwise, between every pair of processes.

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, sendcount 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¶

All arguments on all processes are significant. The comm argument, in particular, must describe the same communicator on all processes.

There are two MPI library functions that are more general than MPI_Alltoall. MPI_Alltoallv allows all-to-all communication to and from buffers that need not be contiguous; different processes may send and receive different amounts of data. MPI_Alltoallw expands MPI_Alltoallv’s functionality to allow the exchange of data with different datatypes.

ERRORS¶

Almost all MPI routines return an error value; C routines as the return result of the function and Fortran routines in the last argument.

Before the error value is returned, the current MPI error handler associated with the communication object (e.g., communicator, window, file) is called. If no communication object is associated with the MPI call, then the call is considered attached to MPI_COMM_SELF and will call the associated MPI error handler. When MPI_COMM_SELF is not initialized (i.e., before MPI_Init/MPI_Init_thread, after MPI_Finalize, or when using the Sessions Model exclusively) the error raises the initial error handler. The initial error handler can be changed by calling MPI_Comm_set_errhandler on MPI_COMM_SELF when using the World model, or the mpi_initial_errhandler CLI argument to mpiexec or info key to MPI_Comm_spawn/MPI_Comm_spawn_multiple. If no other appropriate error handler has been set, then the MPI_ERRORS_RETURN error handler is called for MPI I/O functions and the MPI_ERRORS_ABORT error handler is called for all other MPI functions.

Open MPI includes three predefined error handlers that can be used:

MPI applications can also implement their own error handlers by calling:

Note that MPI does not guarantee that an MPI program can continue past an error.

See the MPI man page for a full list of MPI error codes.

See the Error Handling section of the MPI-3.1 standard for more information.

SEE ALSO:

COPYRIGHT¶

2003-2024, The Open MPI Community