MPI_Allgatherv(3OpenMPI) MPI_Allgatherv(3OpenMPI)
NAME
MPI_Allgatherv - Gathers data from all processes and delivers it to all. Each process may contribute
a different amount of data.
SYNTAX
C Syntax
#include <mpi.h>
int MPI_Allgatherv(void *sendbuf, int sendcount,
MPI_Datatype sendtype, void *recvbuf, int *recvcount,
int *displs, MPI_Datatype recvtype, MPI_Comm comm)
Fortran Syntax
INCLUDE 'mpif.h'
MPI_ALLGATHERV(SENDBUF, SENDCOUNT, SENDTYPE, RECVBUF,
RECVCOUNT, DISPLS, RECVTYPE, COMM, IERROR)
<type> SENDBUF(*), RECVBUF(*)
INTEGER SENDCOUNT, SENDTYPE, RECVCOUNT(*),
INTEGER DISPLS(*), RECVTYPE, COMM, IERROR
C++ Syntax
#include <mpi.h>
void MPI::Comm::Allgatherv(const void* sendbuf, int sendcount,
const MPI::Datatype& sendtype, void* recvbuf,
const int recvcounts[], const int displs[],
const MPI::Datatype& recvtype) const = 0
INPUT PARAMETERS
sendbuf Starting address of send buffer (choice).
sendcount Number of elements in send buffer (integer).
sendtype Datatype of send buffer elements (handle).
recvcount Integer array (of length group size) containing the number of elements that are received
from each process.
displs Integer array (of length group size). Entry i specifies the displacement (relative to
recvbuf) at which to place the incoming data from process i.
recvtype Datatype of receive buffer elements (handle).
comm Communicator (handle).
OUTPUT PARAMETERS
recvbuf Address of receive buffer (choice).
IERROR Fortran only: Error status (integer).
DESCRIPTION
MPI_Allgatherv is similar to MPI_Allgather in that all processes gather data from all other pro-cesses, processes,
cesses, except that each process can send a different amount of data. The block of data sent from the
jth process is received by every process and placed in the jth block of the buffer recvbuf.
The type signature associated with sendcount, sendtype, at process j must be equal to the type signa-ture signature
ture associated with recvcounts[j], recvtype at any other process.
The outcome is as if all processes executed calls to
MPI_Gatherv(sendbuf,sendcount,sendtype,recvbuf,recvcount,
displs,recvtype,root,comm)
for root = 0 , ..., n-1. The rules for correct usage of MPI_Allgatherv are easily found from the cor-responding corresponding
responding rules for MPI_Gatherv.
USE OF IN-PLACE OPTION
When the communicator is an intracommunicator, you can perform an all-gather operation in-place (the
output buffer is used as the input buffer). Use the variable MPI_IN_PLACE as the value of both send-buf sendbuf
buf and recvbuf. In this case, sendcount and sendtype are ignored. The input data of each process
is assumed to be in the area where that process would receive its own contribution to the receive
buffer. Specifically, the outcome of a call to MPI_Allgather that used the in-place option is iden-tical identical
tical to the case in which all processes executed n calls to
MPI_GATHERV ( MPI_IN_PLACE, 0, MPI_DATATYPE_NULL, recvbuf,
recvcounts, displs, recvtype, root, comm )
for root =0, ... , n-1.
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 gather operation occurs in two phases. The data
is gathered from all the members of the first group, concatenated, and received by all the members of
the second group. Then the data is gathered from all the members of the second group, concatenated,
and received by all the members of the first. The send buffer arguments in the one group must be
consistent with the receive buffer arguments in the other group, and vice versa. The operation must
exhibit 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 sec-ond second
ond 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.
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_Gatherv
MPI_Allgather
Open MPI 1.2 March 2007 MPI_Allgatherv(3OpenMPI)
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