MPI_Alltoallv(3OpenMPI) MPI_Alltoallv(3OpenMPI)
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 ele-
ments 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 ele-
ments 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 pro-
cesses 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 communica-
tion 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 ele-
ment 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 dif-
ferent 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++ func-
tions 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(3), MPI_Alltoallw(3)
Open MPI 1.2 September 2006 MPI_Alltoallv(3OpenMPI)
Mac OS X 10.6 - Generated Thu Sep 17 20:12:50 CDT 2009