:ref:`MPI_Scatterv`, :ref:`MPI_Iscatterv`, :ref:`MPI_Scatterv_init` - Scatters a buffer in parts to all tasks in a group.
#include <mpi.h>
int MPI_Scatterv(const void *sendbuf, const int sendcounts[], const int displs[],
MPI_Datatype sendtype, void *recvbuf, int recvcount,
MPI_Datatype recvtype, int root, MPI_Comm comm)
int MPI_Iscatterv(const void *sendbuf, const int sendcounts[], const int displs[],
MPI_Datatype sendtype, void *recvbuf, int recvcount,
MPI_Datatype recvtype, int root, MPI_Comm comm, MPI_Request *request)
int MPI_Scatterv_init(const void *sendbuf, const int sendcounts[], const int displs[],
MPI_Datatype sendtype, void *recvbuf, int recvcount,
MPI_Datatype recvtype, int root, MPI_Comm comm, MPI_Info info, MPI_Request *request)USE MPI
! or the older form: INCLUDE 'mpif.h'
MPI_SCATTERV(SENDBUF, SENDCOUNTS, DISPLS, SENDTYPE, RECVBUF,
RECVCOUNT, RECVTYPE, ROOT, COMM, IERROR)
<type> SENDBUF(*), RECVBUF(*)
INTEGER SENDCOUNTS(*), DISPLS(*), SENDTYPE
INTEGER RECVCOUNT, RECVTYPE, ROOT, COMM, IERROR
MPI_ISCATTERV(SENDBUF, SENDCOUNTS, DISPLS, SENDTYPE, RECVBUF,
RECVCOUNT, RECVTYPE, ROOT, COMM, REQUEST, IERROR)
<type> SENDBUF(*), RECVBUF(*)
INTEGER SENDCOUNTS(*), DISPLS(*), SENDTYPE
INTEGER RECVCOUNT, RECVTYPE, ROOT, COMM, REQUEST, IERROR
MPI_SCATTERV_INIT(SENDBUF, SENDCOUNTS, DISPLS, SENDTYPE, RECVBUF,
RECVCOUNT, RECVTYPE, ROOT, COMM, INFO, REQUEST, IERROR)
<type> SENDBUF(*), RECVBUF(*)
INTEGER SENDCOUNTS(*), DISPLS(*), SENDTYPE
INTEGER RECVCOUNT, RECVTYPE, ROOT, COMM, INFO, REQUEST, IERRORUSE mpi_f08
MPI_Scatterv(sendbuf, sendcounts, displs, sendtype, recvbuf, recvcount,
recvtype, root, comm, ierror)
TYPE(*), DIMENSION(..), INTENT(IN) :: sendbuf
TYPE(*), DIMENSION(..) :: recvbuf
INTEGER, INTENT(IN) :: sendcounts(*), displs(*), recvcount, root
TYPE(MPI_Datatype), INTENT(IN) :: sendtype, recvtype
TYPE(MPI_Comm), INTENT(IN) :: comm
INTEGER, OPTIONAL, INTENT(OUT) :: ierror
MPI_Iscatterv(sendbuf, sendcounts, displs, sendtype, recvbuf, recvcount,
recvtype, root, comm, request, ierror)
TYPE(*), DIMENSION(..), INTENT(IN), ASYNCHRONOUS :: sendbuf
TYPE(*), DIMENSION(..), ASYNCHRONOUS :: recvbuf
INTEGER, INTENT(IN), ASYNCHRONOUS :: sendcounts(*), displs(*)
INTEGER, INTENT(IN) :: recvcount, root
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_scatterv_init(sendbuf, sendcounts, displs, sendtype, recvbuf, recvcount,
recvtype, root, comm, info, request, ierror)
TYPE(*), DIMENSION(..), INTENT(IN), ASYNCHRONOUS :: sendbuf
TYPE(*), DIMENSION(..), ASYNCHRONOUS :: recvbuf
INTEGER, INTENT(IN), ASYNCHRONOUS :: sendcounts(*), displs(*)
INTEGER, INTENT(IN) :: recvcount, root
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) :: ierrorsendbuf: Address of send buffer (choice, significant only at root).sendcounts: Integer array (of length group size) specifying the number of elements to send to each processor.displs: Integer array (of length group size). Entry i specifies the displacement (relative to sendbuf) from which to take the outgoing data to process i.sendtype: Datatype of send buffer elements (handle).recvcount: Number of elements in receive buffer (integer).recvtype: Datatype of receive buffer elements (handle).root: Rank of sending process (integer).comm: Communicator (handle).info: Info (handle, persistent only).
recvbuf: Address of receive buffer (choice).request: Request (handle, non-blocking only).ierror: Fortran only: Error status (integer).
:ref:`MPI_Scatterv` is the inverse operation to :ref:`MPI_Gatherv`.
:ref:`MPI_Scatterv` extends the functionality of :ref:`MPI_Scatter` by allowing a varying count of data to be sent to each process, since sendcounts is now an array. It also allows more flexibility as to where the data is taken from on the root, by providing the new argument, displs.
The outcome is as if the root executed n send operations,
MPI_Send(sendbuf + displs[i] * extent(sendtype),
sendcounts[i], sendtype, i, ...);
// and each process executed a receive,
MPI_Recv(recvbuf, recvcount, recvtype, root, ...)The send buffer is ignored for all nonroot processes.
The type signature implied by sendcount[i], sendtype at the root must be equal to the type signature implied by recvcount, recvtype at process i (however, the type maps may be different). 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.
All arguments to the function are significant on process root, while on other processes, only arguments recvbuf, recvcount, recvtype, root, comm are significant. The arguments root and comm must have identical values on all processes.
The specification of counts, types, and displacements should not cause any location on the root to be read more than once.
Example 1: The reverse of Example 5 in the :ref:`MPI_Gatherv` manpage. We have a varying stride between blocks at sending (root) side, at the receiving side we receive 100 - i elements into the ith column of a 100 x 150 C array at process i.
MPI_Comm comm;
int gsize,recvarray[100][150],*rptr;
int root, *sendbuf, myrank, bufsize, *stride;
MPI_Datatype rtype;
int i, *displs, *scounts, offset;
...
MPI_Comm_size( comm, &gsize);
MPI_Comm_rank( comm, &myrank );
stride = (int *)malloc(gsize*sizeof(int));
...
/* stride[i] for i = 0 to gsize-1 is set somehow
* sendbuf comes from elsewhere
*/
...
displs = (int *)malloc(gsize*sizeof(int));
scounts = (int *)malloc(gsize*sizeof(int));
offset = 0;
for (i=0; i<gsize; ++i) {
displs[i] = offset;
offset += stride[i];
scounts[i] = 100 - i;
}
/* Create datatype for the column we are receiving
*/
MPI_Type_vector( 100-myrank, 1, 150, MPI_INT, &rtype);
MPI_Type_commit( &rtype );
rptr = &recvarray[0][myrank];
MPI_Scatterv(sendbuf, scounts, displs, MPI_INT,
rptr, 1, rtype, root, comm);Example 2: The reverse of Example 1 in the MPI_Gather manpage. The root process scatters sets of 100 ints to the other processes, but the sets of 100 are stride ints apart in the sending buffer. Requires use of :ref:`MPI_Scatterv`, where stride >= 100.
MPI_Comm comm;
int gsize,*sendbuf;
int root, rbuf[100], i, *displs, *scounts;
...
MPI_Comm_size(comm, &gsize);
sendbuf = (int *)malloc(gsize*stride*sizeof(int));
...
displs = (int *)malloc(gsize*sizeof(int));
scounts = (int *)malloc(gsize*sizeof(int));
for (i=0; i<gsize; ++i) {
displs[i] = i*stride;
scounts[i] = 100;
}
MPI_Scatterv(sendbuf, scounts, displs, MPI_INT,
rbuf, 100, MPI_INT, root, comm);When the communicator is an intracommunicator, you can perform a scatter operation in-place (the output buffer is used as the input buffer). Use the variable MPI_IN_PLACE as the value of the root process recvbuf. In this case, recvcount and recvtype are ignored, and the root process sends no data to itself.
Note that MPI_IN_PLACE is a special kind of value; it has the same restrictions on its use as MPI_BOTTOM.
Because the in-place option converts the receive buffer into a send-and-receive buffer, a Fortran binding that includes INTENT must mark these as INOUT, not OUT.
When the communicator is an inter-communicator, the root process in the
first group sends data to all processes in the second group. The first
group defines the root process. That process uses MPI_ROOT as the value
of its root argument. The remaining processes use MPI_PROC_NULL as the
value of their root argument. All processes in the second group use
the rank of that root process in the first group as the value of their
root argument. The receive buffer argument of the root process in the
first group must be consistent with the receive buffer argument of the
processes in the second group.
.. seealso::
* :ref:`MPI_Gather`
* :ref:`MPI_Gatherv`
* :ref:`MPI_Scatter`