MPI_Allgather(3OpenMPI) MPI_Allgather(3OpenMPI)
NAME
MPI_Allgather - Gathers data from all processes and distributes it to all processes
SYNTAX
C Syntax
#include <mpi.h>
int MPI_Allgather(void *sendbuf, int sendcount,
MPI_Datatype sendtype, void *recvbuf, int recvcount,
MPI_Datatype recvtype, MPI_Comm comm)
Fortran Syntax
INCLUDE 'mpif.h'
MPI_ALLGATHER(SENDBUF, SENDCOUNT, SENDTYPE, RECVBUF, RECVCOUNT,
RECVTYPE, COMM, IERROR)
<type> SENDBUF (*), RECVBUF (*)
INTEGER SENDCOUNT, SENDTYPE, RECVCOUNT, RECVTYPE, COMM,
INTEGER IERROR
C++ Syntax
#include <mpi.h>
void MPI::Comm::Allgather(const void* sendbuf, int sendcount, const
MPI::Datatype& sendtype, void* recvbuf, int recvcount,
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 Number of elements received from any process (integer).
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_Allgather is similar to MPI_Gather, except that all processes receive the result, instead of just
the root. In other words, all processes contribute to the result, and all processes receive the
result.
The type signature associated with sendcount, sendtype at a process must be equal to the type signa-ture signature
ture associated with recvcount, recvtype at any other process.
The outcome of a call to MPI_Allgather(...) is as if all processes executed n calls to
MPI_Gather(sendbuf,sendcount,sendtype,recvbuf,recvcount,
recvtype,root,comm),
for root = 0 , ..., n-1. The rules for correct usage of MPI_Allgather are easily found from the cor-responding corresponding
responding rules for MPI_Gather.
Example: The all-gather version of Example 1 in MPI_Gather. Using MPI_Allgather, we will gather 100
ints from every process in the group to every process.
MPI_Comm comm;
int gsize,sendarray[100];
int *rbuf;
...
MPI_Comm_size( comm, &gsize);
rbuf = (int *)malloc(gsize*100*sizeof(int));
MPI_Allgather( sendarray, 100, MPI_INT, rbuf, 100, MPI_INT, comm);
After the call, every process has the group-wide concatenation of the sets of data.
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_GATHER ( MPI_IN_PLACE, 0, MPI_DATATYPE_NULL, recvbuf,
recvcount, 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 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, however, need not be symmetric. The number of items sent by
the processes in first group need not be equal to the number of items sent by the the processes in
the second group. You can move data in only one direction by giving sendcount a value of 0 for com-munication communication
munication in the reverse direction.
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_Allgatherv
MPI_Gather
Open MPI 1.2 September 2006 MPI_Allgather(3OpenMPI)
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