trusty (3) MPI_Alltoallv.openmpi.3.gz

NAME

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

Fortran Syntax

       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

C++ Syntax

       #include <mpi.h>
       void MPI::Comm::Alltoallv(const void* sendbuf,
            const int sendcounts[], const int displs[],
            const MPI::Datatype& sendtype, void* recvbuf,
            const int recvcounts[], const int rdispls[],
            const MPI::Datatype& recvtype)

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.

OUTPUT PARAMETERS

       recvbuf     Address of receive buffer.

       IERROR      Fortran only: Error status.

DESCRIPTION

       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 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.

NOTES

       The MPI_IN_PLACE option is not available for any form of all-to-all communication.

       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 MPI_Alltoallw, where these offsets are measured in bytes.

ERRORS

       Almost all MPI routines return an error value; C routines as  the  value  of  the  function  and  Fortran
       routines in the last argument. C++ functions do not return errors. If the default error handler is set to
       MPI::ERRORS_THROW_EXCEPTIONS, then on error the  C++  exception  mechanism  will  be  used  to  throw  an
       MPI:Exception object.

       Before  the  error  value  is  returned,  the current MPI error handler is called. By default, this error
       handler aborts the MPI job, except for I/O function  errors.  The  error  handler  may  be  changed  with
       MPI_Comm_set_errhandler; the predefined error handler MPI_ERRORS_RETURN may be used to cause error values
       to be returned. Note that MPI does not guarantee that an MPI program can continue past an error.

SEE ALSO

       MPI_Alltoall
       MPI_Alltoallw