MPI_Allgather, MPI_Iallgather, MPI_Allgather_init - 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)
int MPI_Allgather_init(const void *sendbuf, int sendcount,
MPI_Datatype sendtype, void *recvbuf, int recvcount,
MPI_Datatype recvtype, MPI_Comm comm, MPI_Info info, MPI_Request *request)USE MPI
! or the older form: 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
MPI_ALLGATHER_INIT(SENDBUF, SENDCOUNT, SENDTYPE, RECVBUF, RECVCOUNT,
RECVTYPE, COMM, INFO, REQUEST, IERROR)
<type> SENDBUF(*), RECVBUF (*)
INTEGER SENDCOUNT, SENDTYPE, RECVCOUNT, RECVTYPE, COMM
INTEGER INFO, REQUEST, IERRORUSE mpi_f08
MPI_Allgather(sendbuf, sendcount, sendtype, recvbuf, recvcount, recvtype,
comm, ierror)
TYPE(*), DIMENSION(..), INTENT(IN) :: sendbuf
TYPE(*), DIMENSION(..) :: recvbuf
INTEGER, INTENT(IN) :: sendcount, recvcount
TYPE(MPI_Datatype), INTENT(IN) :: sendtype, recvtype
TYPE(MPI_Comm), INTENT(IN) :: comm
INTEGER, OPTIONAL, INTENT(OUT) :: ierror
MPI_Iallgather(sendbuf, sendcount, sendtype, recvbuf, recvcount, recvtype,
comm, request, ierror)
TYPE(*), DIMENSION(..), INTENT(IN), ASYNCHRONOUS :: sendbuf
TYPE(*), DIMENSION(..), ASYNCHRONOUS :: recvbuf
INTEGER, INTENT(IN) :: sendcount, recvcount
TYPE(MPI_Datatype), INTENT(IN) :: sendtype, recvtype
TYPE(MPI_Comm), INTENT(IN) :: comm
TYPE(MPI_Request), INTENT(OUT) :: request
INTEGER, OPTIONAL, INTENT(OUT) :: ierror
MPI_Allgather_init(sendbuf, sendcount, sendtype, recvbuf, recvcount, recvtype,
comm, info, request, ierror)
TYPE(*), DIMENSION(..), INTENT(IN), ASYNCHRONOUS :: sendbuf
TYPE(*), DIMENSION(..), ASYNCHRONOUS :: recvbuf
INTEGER, INTENT(IN) :: sendcount, recvcount
TYPE(MPI_Datatype), INTENT(IN) :: sendtype, recvtype
TYPE(MPI_Comm), INTENT(IN) :: comm
TYPE(MPI_Info), INTENT(IN) :: info
TYPE(MPI_Request), INTENT(OUT) :: request
INTEGER, OPTIONAL, INTENT(OUT) :: ierrorsendbuf: 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).info: Info (handle, persistent only).
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_Allgather( 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.
* MPI_Allgatherv * MPI_Gather