NAME¶
MPI_Scatterv - Scatters a buffer in parts to all tasks in a group.
SYNTAX¶
C Syntax¶
#include <mpi.h>
int MPI_Scatterv(void * sendbuf, int *sendcounts, int *displs,
MPI_Datatype sendtype, void *recvbuf, int recvcount,
MPI_Datatype recvtype, int root, MPI_Comm comm)
Fortran Syntax¶
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
C++ Syntax¶
#include <mpi.h>
void MPI::Comm::Scatterv(const void* sendbuf, const int sendcounts[],
const int displs[], const MPI::Datatype& sendtype,
void* recvbuf, int recvcount, const MPI::Datatype&
recvtype, int root) const
- sendbuf
- 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).
OUTPUT PARAMETERS¶
- recvbuf
- Address of receive buffer (choice).
- IERROR
- Fortran only: Error status (integer).
DESCRIPTION¶
MPI_Scatterv is the inverse operation to MPI_Gatherv.
MPI_Scatterv extends the functionality of 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 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 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);
USE OF IN-PLACE OPTION¶
When the communicator is an intracommunicator, you can perform a gather
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 COMMUNICATOR IS AN INTER-COMMUNICATOR¶
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.
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_Gather
MPI_Gatherv
MPI_Scatter