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MPI_Alltoallv - All processes send different amount of data to, and
receive different amount of data from, all processes
#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)
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
#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)
- sendbuf
- Starting address of send buffer.
- sendcounts
- Integer
array, where entry i specifies the number of elements 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 elements 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.
- recvbuf
- Address of receive buffer.
- IERROR
- Fortran only: Error status.
MPI_Alltoallv is a generalized collective
operation in which all processes 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 communication
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 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.
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.
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.
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.
MPI_Alltoall
MPI_Alltoallw
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