NAME¶
MPI_Alltoallw - All processes send data of different types to, and
receive data of different types from, all processes
SYNTAX¶
C Syntax¶
#include <mpi.h>
int MPI_Alltoallw(void * sendbuf, int *sendcounts,
int * sdispls, MPI_Datatype *sendtypes,
void * recvbuf, int *recvcounts,
int * rdispls, MPI_Datatype *recvtypes, MPI_Comm comm)
Fortran Syntax¶
INCLUDE 'mpif.h'
MPI_ALLTOALLW( SENDBUF, SENDCOUNTS, SDISPLS, SENDTYPES,
RECVBUF, RECVCOUNTS, RDISPLS, RECVTYPES, COMM, IERROR)
<type> SENDBUF(*), RECVBUF(*)
INTEGER SENDCOUNTS(*), SDISPLS(*), SENDTYPES(*)
INTEGER RECVCOUNTS(*), RDISPLS(*), RECVTYPES(*)
INTEGER COMM, IERROR
C++ Syntax¶
#include <mpi.h>
void MPI::Comm::Alltoallw(const void* sendbuf,
const int sendcounts[], const int sdispls[],
const MPI::Datatype sendtypes[], void* recvbuf,
const int recvcounts[], const int rdispls[],
const MPI::Datatype recvtypes[])
- 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 (in bytes, offset
from sendbuf) from which to send data to rank i.
- sendtypes
- Datatype array, where entry i specifies the datatype to use when sending
data to rank i.
- 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 (in bytes, offset
from recvbuf) to which data from rank j should be written.
- recvtypes
- Datatype array, where entry j specifies the datatype to use when receiving
data from rank j.
- comm
- Communicator over which data is to be exchanged.
OUTPUT PARAMETERS¶
- recvbuf
- Address of receive buffer.
- IERROR
- Fortran only: Error status.
DESCRIPTION¶
MPI_Alltoallw is a generalized collective operation in which all processes send
data to and receive data from all other processes. It adds flexibility to
MPI_Alltoallv by allowing the user to specify the datatype of individual data
blocks (in addition to displacement and element count). Its operation can be
thought of in the following way, 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], sendcounts[i],
sendtypes[i], i, ..., comm);
for (i = 0, i < n; i++)
MPI_Recv( recvbuf + rdispls[i], recvcounts[i],
recvtypes[i], 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
and types 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, types, 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 bytes. Compare
this to MPI_Alltoallv, where these offsets are measured in units of
sendtype and
recvtype, respectively.
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_Alltoall
MPI_Alltoallv