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MPI_Allgatherv - Gathers data from all processes and delivers it to
all. Each process may contribute a different amount of data.
#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)
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
#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
- 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).
- recvbuf
- Address of receive buffer (choice).
- IERROR
- Fortran only: Error status (integer).
MPI_Allgatherv
is similar to MPI_Allgather in that all processes gather data from all
other processes, 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 signature
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 corresponding rules for MPI_Gatherv.
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_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
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
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_Gatherv
MPI_Allgather
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