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MPI_Gatherv(3) man page (version 5.0.0rc2)

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Name

C]MPI_GathervR], C]MPI_IgathervR], C]MPI_Gatherv_initR] - Gathers varying amounts of data from all processes to the root process

Syntax

C Syntax



C]#include <mpi.h>int MPI_Gatherv(const void *sendbuf, int sendcount, MPI_Datatype sendtype, void *recvbuf, const int recvcounts[], const int displs[], MPI_Datatype recvtype, int root, MPI_Comm comm)int MPI_Igatherv(const void *sendbuf, int sendcount, MPI_Datatype sendtype, void *recvbuf, const int recvcounts[], const int displs[], MPI_Datatype recvtype, int root, MPI_Comm comm, MPI_Request *request)int MPI_Gatherv_init(const void *sendbuf, int sendcount, MPI_Datatype sendtype, void *recvbuf, const int recvcounts[], const int displs[], MPI_Datatype recvtype, int root, MPI_Comm comm, MPI_Info info, MPI_Request *request)R]Fortran Syntax



C]USE MPI! or the older form: INCLUDE [aq]mpif.h[aq]MPI_GATHERV(SENDBUF, SENDCOUNT, SENDTYPE, RECVBUF, RECVCOUNTS, DISPLS, RECVTYPE, ROOT, COMM, IERROR) <type> SENDBUF(*), RECVBUF(*) INTEGER SENDCOUNT, SENDTYPE, RECVCOUNTS(*), DISPLS(*) INTEGER RECVTYPE, ROOT, COMM, IERRORMPI_IGATHERV(SENDBUF, SENDCOUNT, SENDTYPE, RECVBUF, RECVCOUNTS, DISPLS, RECVTYPE, ROOT, COMM, REQUEST, IERROR) <type> SENDBUF(*), RECVBUF(*) INTEGER SENDCOUNT, SENDTYPE, RECVCOUNTS(*), DISPLS(*) INTEGER RECVTYPE, ROOT, COMM, REQUEST, IERRORMPI_GATHERV_INIT(SENDBUF, SENDCOUNT, SENDTYPE, RECVBUF, RECVCOUNTS, DISPLS, RECVTYPE, ROOT, COMM, INFO, REQUEST, IERROR) <type> SENDBUF(*), RECVBUF(*) INTEGER SENDCOUNT, SENDTYPE, RECVCOUNTS(*), DISPLS(*) INTEGER RECVTYPE, ROOT, COMM, INFO, REQUEST, IERRORR]Fortran 2008 Syntax



C]USE mpi_f08MPI_Gatherv(sendbuf, sendcount, sendtype, recvbuf, recvcounts, displs, recvtype, root, comm, ierror) TYPE(*), DIMENSION(..), INTENT(IN) :: sendbuf TYPE(*), DIMENSION(..) :: recvbuf INTEGER, INTENT(IN) :: sendcount, recvcounts(*), displs(*), root TYPE(MPI_Datatype), INTENT(IN) :: sendtype, recvtype TYPE(MPI_Comm), INTENT(IN) :: comm INTEGER, OPTIONAL, INTENT(OUT) :: ierrorMPI_Igatherv(sendbuf, sendcount, sendtype, recvbuf, recvcounts, displs, recvtype, root, comm, request, ierror) TYPE(*), DIMENSION(..), INTENT(IN), ASYNCHRONOUS :: sendbuf TYPE(*), DIMENSION(..), ASYNCHRONOUS :: recvbuf INTEGER, INTENT(IN) :: sendcount, root INTEGER, INTENT(IN), ASYNCHRONOUS :: recvcounts(*), displs(*) TYPE(MPI_Datatype), INTENT(IN) :: sendtype, recvtype TYPE(MPI_Comm), INTENT(IN) :: comm TYPE(MPI_Request), INTENT(OUT) :: request INTEGER, OPTIONAL, INTENT(OUT) :: ierrorMPI_Gatherv_init(sendbuf, sendcount, sendtype, recvbuf, recvcounts, displs, recvtype, root, comm, info, request, ierror) TYPE(*), DIMENSION(..), INTENT(IN), ASYNCHRONOUS :: sendbuf TYPE(*), DIMENSION(..), ASYNCHRONOUS :: recvbuf INTEGER, INTENT(IN) :: sendcount, root INTEGER, INTENT(IN), ASYNCHRONOUS :: recvcounts(*), displs(*) 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) :: ierrorR]Input Parameters

[bu]
C]sendbufR] : Starting address of send buffer (choice).
[bu]
C]sendcountR] : Number of elements in send buffer (integer).
[bu]
C]sendtypeR] : Datatype of send buffer elements (handle).
[bu]
C]recvcountsR] : Integer array (of length group size) containing the number of elements that are received from each process (significant only at root).
[bu]
C]displsR] : Integer array (of length group size). Entry i specifies the displacement relative to recvbuf at which to place the incoming data from process i (significant only at root).
[bu]
C]recvtypeR] : Datatype of recv buffer elements (significant only at root) (handle).
[bu]
C]rootR] : Rank of receiving process (integer).
[bu]
C]commR] : Communicator (handle).
[bu]
C]infoR] : Info (handle, persistent only).

Output Parameters

[bu]
C]recvbufR] : Address of receive buffer (choice, significant only at root).
[bu]
C]requestR] : Request (handle, non-blocking only).
[bu]
C]IERRORR] : Fortran only: Error status (integer).

Description

C]MPI_GathervR] extends the functionality of C]MPI_GatherR] by allowing a varying count of data from each process, since C]recvcountsR] is now an array. It also allows more flexibility as to where the data is placed on the root, by providing the new argument, C]displsR].

The outcome is as if each process, including the root process, sends a message to the root,


C]MPI_Send(sendbuf, sendcount, sendtype, root, ...)R]and the root executes n receives,


C]MPI_Recv(recvbuf + disp[i] * extent(recvtype), recvcounts[i], recvtype, i, ...)R]Messages are placed in the receive buffer of the root process in rank
order, that is, the data sent from process j is placed in the jth portion of the receive buffer C]recvbufR] on process root. The jth portion of C]recvbufR] begins at offset displs[j] elements (in terms of C]recvtypeR]) into C]recvbufR].

The receive buffer is ignored for all nonroot processes.

The type signature implied by C]sendcountR], C]sendtypeR] on process i must be equal to the type signature implied by C]recvcounts[i]R], C]recvtypeR] at the root. This implies that the amount of data sent must be equal to the amount of data received, pairwise between each process and the root. Distinct type maps between sender and receiver are still allowed, as illustrated in Example 2, below.

All arguments to the function are significant on process C]rootR], while on other processes, only arguments C]sendbufR], C]sendcountR], C]sendtypeR], C]rootR], C]commR] are significant. The arguments C]rootR] and C]commR] must have identical values on all processes.

The specification of counts, types, and displacements should not cause any location on the C]rootR] to be written more than once. Such a call is erroneous.

Example 1: Now have each process send 100 ints to C]rootR], but place each set (of 100) stride ints apart at receiving end. Use C]MPI_GathervR] and the C]displsR] argument to achieve this effect. Assume stride >= 100.


C]MPI_Comm comm;int gsize,sendarray[100];int root, *rbuf, stride;int *displs,i,*rcounts;// ...MPI_Comm_size(comm, &gsize);rbuf = (int *)malloc(gsize*stride*sizeof(int));displs = (int *)malloc(gsize*sizeof(int));rcounts = (int *)malloc(gsize*sizeof(int));for (i=0; i<gsize; ++i) { displs[i] = i*stride; rcounts[i] = 100;}MPI_Gatherv(sendarray, 100, MPI_INT, rbuf, rcounts, displs, MPI_INT, root, comm);R]Note that the program is erroneous if stride < 100.

Example 2: Same as Example 1 on the receiving side, but send the 100 ints from the 0th column of a 100 150 int array, in C.


C]MPI_Comm comm;int gsize,sendarray[100][150];int root, *rbuf, stride;MPI_Datatype stype;int *displs,i,*rcounts;// ...MPI_Comm_size(comm, &gsize);rbuf = (int *)malloc(gsize*stride*sizeof(int));displs = (int *)malloc(gsize*sizeof(int));rcounts = (int *)malloc(gsize*sizeof(int));for (i=0; i<gsize; ++i) { displs[i] = i*stride; rcounts[i] = 100;}/* Create datatype for 1 column of array */MPI_Type_vector(100, 1, 150, MPI_INT, &stype);MPI_Type_commit( &stype );MPI_Gatherv(sendarray, 1, stype, rbuf, rcounts, displs, MPI_INT, root, comm);R]Example 3: Process i sends (100-i) ints from the ith column of a 100 x
150 int array, in C. It is received into a buffer with stride, as in the previous two examples.

C]MPI_Comm comm;int gsize,sendarray[100][150],*sptr;int root, *rbuf, stride, myrank;MPI_Datatype stype;int *displs,i,*rcounts;// ...MPI_Comm_size(comm, &gsize);MPI_Comm_rank( comm, &myrank );rbuf = (int *)malloc(gsize*stride*sizeof(int));displs = (int *)malloc(gsize*sizeof(int));rcounts = (int *)malloc(gsize*sizeof(int));for (i=0; i<gsize; ++i) { displs[i] = i*stride; rcounts[i] = 100-i; /* note change from previous example */}/* Create datatype for the column we are sending */MPI_Type_vector(100-myrank, 1, 150, MPI_INT, &stype);MPI_Type_commit( &stype );/* sptr is the address of start of [dq]myrank[dq] column */sptr = &sendarray[0][myrank];MPI_Gatherv(sptr, 1, stype, rbuf, rcounts, displs, MPI_INT, root, comm);R]Note that a different amount of data is received from each process.

Example 4: Same as Example 3, but done in a different way at the sending end. We create a datatype that causes the correct striding at the sending end so that we read a column of a C array.


C]MPI_Comm comm;int gsize,sendarray[100][150],*sptr;int root, *rbuf, stride, myrank, disp[2], blocklen[2];MPI_Datatype stype,type[2];int *displs,i,*rcounts;// ...MPI_Comm_size(comm, &gsize);MPI_Comm_rank( comm, &myrank );rbuf = (int *)alloc(gsize*stride*sizeof(int));displs = (int *)malloc(gsize*sizeof(int));rcounts = (int *)malloc(gsize*sizeof(int));for (i=0; i<gsize; ++i) { displs[i] = i*stride; rcounts[i] = 100-i;}/* Create datatype for one int, with extent of entire row */disp[0] = 0; disp[1] = 150*sizeof(int);type[0] = MPI_INT; type[1] = MPI_UB;blocklen[0] = 1; blocklen[1] = 1;MPI_Type_struct( 2, blocklen, disp, type, &stype );MPI_Type_commit( &stype );sptr = &sendarray[0][myrank];MPI_Gatherv(sptr, 100-myrank, stype, rbuf, rcounts, displs, MPI_INT, root, comm);R]Example 5: Same as Example 3 at sending side, but at receiving side we
make the stride between received blocks vary from block to block.

C]MPI_Comm comm;int gsize,sendarray[100][150],*sptr;int root, *rbuf, *stride, myrank, bufsize;MPI_Datatype stype;int *displs,i,*rcounts,offset;// ...MPI_Comm_size( comm, &gsize);MPI_Comm_rank( comm, &myrank );de = (int *)malloc(gsize*sizeof(int));// .../* stride[i] for i = 0 to gsize-1 is set somehow *//*set up displs and rcounts vectors first */displs = (int *)malloc(gsize*sizeof(int));rcounts = (int *)malloc(gsize*sizeof(int));offset = 0;for (i=0; i<gsize; ++i) { displs[i] = offset; offset += stride[i]; rcounts[i] = 100-i;}/* the required buffer size for rbuf is now easily obtained */bufsize = displs[gsize-1]+rcounts[gsize-1];rbuf = (int *)malloc(bufsize*sizeof(int));/* Create datatype for the column we are sending */MPI_Type_vector(100-myrank, 1, 150, MPI_INT, &stype);MPI_Type_commit( &stype );sptr = &sendarray[0][myrank];MPI_Gatherv(sptr, 1, stype, rbuf, rcounts, displs, MPI_INT, root, comm);R]Example 6: Process i sends num ints from the ith column of a 100 x 150
int array, in C. The complicating factor is that the various values of num are not known to C]rootR], so a separate gather must first be run to find these out. The data is placed contiguously at the receiving end.

C]MPI_Comm comm;int gsize,sendarray[100][150],*sptr;int root, *rbuf, stride, myrank, disp[2], blocklen[2];MPI_Datatype stype,types[2];int *displs,i,*rcounts,num;// ...MPI_Comm_size( comm, &gsize);MPI_Comm_rank( comm, &myrank );/*First, gather nums to root */rcounts = (int *)malloc(gsize*sizeof(int));MPI_Gather( &num, 1, MPI_INT, rcounts, 1, MPI_INT, root, comm);/* root now has correct rcounts, using these we set * displs[] so that data is placed contiguously (or * concatenated) at receive end */displs = (int *)malloc(gsize*sizeof(int));displs[0] = 0;for (i=1; i<gsize; ++i) { displs[i] = displs[i-1]+rcounts[i-1];}/* And, create receive buffer */rbuf = (int *)malloc(gsize*(displs[gsize-1]+rcounts[gsize-1]) *sizeof(int));/* Create datatype for one int, with extent of entire row */disp[0] = 0; disp[1] = 150*sizeof(int);type[0] = MPI_INT; type[1] = MPI_UB;blocklen[0] = 1; blocklen[1] = 1;MPI_Type_struct( 2, blocklen, disp, type, &stype );MPI_Type_commit( &stype );sptr = &sendarray[0][myrank];MPI_Gatherv(sptr, num, stype, rbuf, rcounts, displs, MPI_INT, root, comm);R]Use Of In-Place Option

The in-place option operates in the same way as it does for
C]MPI_Gather.R] When the communicator is an intracommunicator, you can perform a gather operation in-place (the output buffer is used as the input buffer). Use the variable C]MPI_IN_PLACER] as the value of the root process C]sendbufR]. In this case, C]sendcountR] and C]sendtypeR] are ignored, and the contribution of the C]rootR] process to the gathered vector is assumed to already be in the correct place in the receive buffer.

Note that C]MPI_IN_PLACER] is a special kind of value; it has the same restrictions on its use as C]MPI_BOTTOM.R]

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 C]rootR] process in the first group gathers data from all the processes in the second group. The first group defines the root process. That process uses C]MPI_ROOTR] as the value of its C]rootR] argument. The remaining processes use C]MPI_PROC_NULLR] as the value of their C]rootR] argument. All processes in the second group use the rank of that root process in the first group as the value of their C]rootR] argument. The send buffer argument of the processes in the first group must be consistent with the receive buffer argument of the C]rootR] process in the second group.

Errors

Almost all MPI routines return an error value; C routines as the value of the function and Fortran routines in the last argument.

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 C]MPI_Comm_set_errhandlerR]; the predefined error handler C]MPI_ERRORS_RETURNR] 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

C]MPI_GatherR](3) C]MPI_ScatterR](3) C]MPI_ScattervR](3)


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