plucky (3) MPI_Ialltoall.openmpi.3.gz

Provided by: openmpi-doc_5.0.7-1_all bug

SYNTAX

   C 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)

   Fortran Syntax
          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

   Fortran 2008 Syntax
          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

sendbuf: Starting address of send buffer (choice).

       • sendcount: Number of elements to send to each process (integer).

       • sendtype: Datatype of send buffer elements (handle).

       • recvcount: Number of elements to receive from each process (integer).

       • recvtype: Datatype of receive buffer elements (handle).

       • comm: Communicator over which data is to be exchanged (handle).

       • info: Info (handle, persistent only)

OUTPUT PARAMETERS

recvbuf: Starting address of receive buffer (choice).

       • request: Request (handle, non-blocking only).

       • ierror: Fortran only: Error status (integer).

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_ERRORS_ARE_FATAL Causes the program to abort all connected MPI processes.

       • MPI_ERRORS_ABORT An error handler that can be invoked on a communicator, window, file, or session. When
         called on a communicator, it acts as if MPI_Abort was called on  that  communicator.  If  called  on  a
         window  or file, acts as if MPI_Abort was called on a communicator containing the group of processes in
         the corresponding window or file. If called on a session, aborts only the local process.

       • MPI_ERRORS_RETURN Returns an error code to the application.

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

       • MPI_Comm_create_errhandler then MPI_Comm_set_errhandlerMPI_File_create_errhandler then MPI_File_set_errhandlerMPI_Session_create_errhandler then MPI_Session_set_errhandler or at MPI_Session_initMPI_Win_create_errhandler then MPI_Win_set_errhandler

       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:MPI_AlltoallvMPI_Alltoallw

       2003-2025, The Open MPI Community

                                                  Feb 17, 2025                                  MPI_IALLTOALL(3)