Gather, Vector Variant

  gather, vector variant

  MPI_GATHER(sendbuf, sendcount, sendtype, recvbuf, recvcounts, displs, recvtype, root, comm)

    IN       sendbuf          starting address of send buffer
    IN       sendcount        number of elements in send buffer
    IN       sendtype         datatype of send buffer elements 
    OUT      recvbuf          address of receive buffer
    IN       recvcounts       integer array
    IN       displs           integer array of displacements
    IN       recvtype         data type of recv buffer elements
    IN       root             rank of receiving process
    IN       comm             communicator

MPI_GATHERV(SENDBUF, SENDCOUNT, SENDTYPE, RECVBUF, RECVCOUNTS, DISPLS, RECVTYPE, ROOT, COMM, IERROR) <type> SENDBUF(*), RECVBUF(*)
INTEGER SENDCOUNT, SENDTYPE, RECVCOUNTS(*), DISPLS(*), RECVTYPE, ROOT, COMM, IERROR

MPI_GATHERV extends the functionality of MPI_GATHER by allowing a varying count of data from each process, since recvcounts is now an array. It also allows more flexibility as to where the data is placed on the root, by providing the new argument, displs.

The outcome is as if each process, including the root process, sends a message to the root, MPI_Send(sendbuf, sendcount, sendtype, root, ...) and the root executes n receives, MPI_Recv(recvbuf+displs[i]-extent(recvtype), recvcounts[i], recvtype, i, ...).

The data sent from process j is placed in the jth portion of the receive buffer recvbuf on process root. The jth portion of recvbuf begins at offset displs[j] elements (in terms of recvtype) into recvbuf.

The receive buffer is ignored for all non-root processes.

The type signature implied by sendcount and sendtype on process i must be equal to the type signature implied by recvcounts[i] and recvtype at the root. This implies that the amount of data sent must be equal to the amount of data received, pairwise between each process and the root. Distinct type maps between sender and receiver are still allowed, as illustrated in Example .

All arguments to the function are significant on process root, while on other processes, only arguments sendbuf, sendcount, sendtype, root, and comm are significant. The argument root must have identical values on all processes, and comm must represent the same intragroup communication domain.

The specification of counts, types, and displacements should not cause any location on the root to be written more than once. Such a call is erroneous. On the other hand, the successive displacements in the array displs need not be a monotonic sequence.