Ubuntu Manpage: MPI_Neighbor_alltoallv, MPI_Ineighbor_alltoallv - All processes send different amounts of data to, and

NAME

       MPI_Neighbor_alltoallv,  MPI_Ineighbor_alltoallv  -  All processes send different amounts of data to, and
       receive different amounts of data from, all neighbors

SYNTAX

C Syntax

       #include <mpi.h>
       int MPI_Neighbor_alltoallv(const void *sendbuf, const int sendcounts[],
            const int sdispls[], MPI_Datatype sendtype,
            void *recvbuf, const int recvcounts[],
            const int rdispls[], MPI_Datatype recvtype, MPI_Comm comm)

       int MPI_Ineighbor_alltoallv(const void *sendbuf, const int sendcounts[],
            const int sdispls[], MPI_Datatype sendtype,
            void *recvbuf, const int recvcounts[],
            const int rdispls[], MPI_Datatype recvtype, MPI_Comm comm,
            MPI_Request *request)

Fortran Syntax

       INCLUDE 'mpif.h'

       MPI_NEIGHBOR_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

       MPI_INEIGHBOR_ALLTOALLV(SENDBUF, SENDCOUNTS, SDISPLS, SENDTYPE,
            RECVBUF, RECVCOUNTS, RDISPLS, RECVTYPE, COMM, REQUEST, IERROR)

            <type>    SENDBUF(*), RECVBUF(*)
            INTEGER   SENDCOUNTS(*), SDISPLS(*), SENDTYPE
            INTEGER   RECVCOUNTS(*), RDISPLS(*), RECVTYPE
            INTEGER   COMM, REQUEST, IERROR

INPUT PARAMETERS

       sendbuf     Starting address of send buffer.

       sendcounts  Integer array, where entry i specifies the number of elements to send to neighbor i.

       sdispls     Integer array, where entry i specifies the displacement (offset from  sendbuf,  in  units  of
                   sendtype) from which to send data to neighbor i.

       sendtype    Datatype of send buffer elements.

       recvcounts  Integer array, where entry j specifies the number of elements to receive from neighbor j.

       rdispls     Integer  array,  where  entry  j specifies the displacement (offset from recvbuf, in units of
                   recvtype) to which data from neighbor 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.

       request   Request (handle, non-blocking only).

       IERROR      Fortran only: Error status.

DESCRIPTION

       MPI_Neighbor_alltoallv is a generalized collective operation in which all  processes  send  data  to  and
       receive  data  from  all  neighbors. It adds flexibility to MPI_Neighbor_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
       neighbors in to topology of communicator comm) independent point-to-point communications.  The  neighbors
       and buffer layout are determined by the topology of comm.

               MPI_Cart_get(comm, maxdims, dims, periods, coords);
               for (dim = 0, i = 0 ; dim < dims ; ++dim) {
                   MPI_Cart_shift(comm, dim, 1, &r0, &r1);
                   MPI_Isend(sendbuf + sdispls[i]  * extent(sendtype),
                             sendcount, sendtype, r0, ..., comm, ...);
                   MPI_Irecv(recvbuf + rdispls[i] * extent(recvtype),
                             recvcount, recvtype, r0, ..., comm, ...);
                   ++i;
                   MPI_Isend(sendbuf + sdispls[i] * extent(sendtype),
                             sendcount, sendtype, r1, ..., comm, &req[i]);
                   MPI_Irecv(recvbuf + rdispls[i] * extent(recvtype),
                             recvcount, recvtype, r1, ..., comm, ...);
                   ++i;
               }

       Process  j  sends  the  k-th  block of its local sendbuf to neighbor 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.

NEIGHBOR ORDERING

For a distributed graph topology, created with MPI_Dist_graph_create, the sequence of neighbors in the
send and receive buffers at each process is defined as the sequence returned by MPI_Dist_graph_neighbors
for destinations and sources, respectively. For a general graph topology, created with MPI_Graph_create,
the order of neighbors in the send and receive buffers is defined as the sequence of neighbors as
returned by MPI_Graph_neighbors. Note that general graph topologies should generally be replaced by the
distributed graph topologies.

For a Cartesian topology, created with MPI_Cart_create, the sequence of neighbors in the send and receive
buffers at each process is defined by order of the dimensions, first the neighbor in the negative
direction and then in the positive direction with displacement 1. The numbers of sources and destinations
in the communication routines are 2*ndims with ndims defined in MPI_Cart_create. If a neighbor does not
exist, i.e., at the border of a Cartesian topology in the case of a non-periodic virtual grid dimension
(i.e., periods[...]==false), then this neighbor is defined to be MPI_PROC_NULL.

If a neighbor in any of the functions is MPI_PROC_NULL, then the neighborhood collective communication
behaves like a point-to-point communication with MPI_PROC_NULL in this direction. That is, the buffer is
still part of the sequence of neighbors but it is neither communicated nor updated.

NOTES

       The MPI_IN_PLACE option for sendbuf is not meaningful for this operation.

       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_Neighbor_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.

       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.