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
- 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 signature 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 corresponding 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
sendbuf. 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 identical 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 communication 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 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.
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