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MPI_Allgather, MPI_Iallgather - Gathers data from all processes
and distributes it to all processes
#include <mpi.h>
int MPI_Allgather(const void *sendbuf, int sendcount,
MPI_Datatype sendtype, void *recvbuf, int recvcount,
MPI_Datatype recvtype, MPI_Comm comm)
int MPI_Iallgather(const void *sendbuf, int sendcount,
MPI_Datatype sendtype, void *recvbuf, int recvcount,
MPI_Datatype recvtype, MPI_Comm comm, MPI_Request *request)
INCLUDE ’mpif.h’
MPI_ALLGATHER(SENDBUF, SENDCOUNT, SENDTYPE, RECVBUF, RECVCOUNT,
RECVTYPE, COMM, IERROR)
<type> SENDBUF (*), RECVBUF (*)
INTEGER SENDCOUNT, SENDTYPE, RECVCOUNT, RECVTYPE, COMM,
INTEGER IERROR
MPI_IALLGATHER(SENDBUF, SENDCOUNT, SENDTYPE, RECVBUF, RECVCOUNT,
RECVTYPE, COMM, REQUEST, IERROR)
<type> SENDBUF(*), RECVBUF (*)
INTEGER SENDCOUNT, SENDTYPE, RECVCOUNT, RECVTYPE, COMM
INTEGER REQUEST, IERROR
#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).
- recvbuf
- Starting address of recv buffer
(choice).
- recvcount
- Number of elements received from any process (integer).
- recvtype
- Datatype of receive buffer elements (handle).
- comm
- Communicator
(handle).
- recvbuf
- Address of receive buffer (choice).
- request
- Request (handle, non-blocking only).
- IERROR
- Fortran only: Error status (integer).
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.
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
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.
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_Allgatherv
MPI_Gather
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