MPI_Gatherv(3OpenMPI) MPI_Gatherv(3OpenMPI)
NAME
MPI_Gatherv - Gathers varying amounts of data from all processes to the root process
SYNTAX
C Syntax
#include <mpi.h>
int MPI_Gatherv(void *sendbuf, int sendcount, MPI_Datatype sendtype,
void *recvbuf, int *recvcounts, int *displs, MPI_Datatype recvtype,
int root, MPI_Comm comm)
Fortran Syntax
INCLUDE 'mpif.h'
MPI_GATHERV(SENDBUF, SENDCOUNT, SENDTYPE, RECVBUF, RECVCOUNTS,
DISPLS, RECVTYPE, ROOT, COMM, IERROR)
<type> SENDBUF(*), RECVBUF(*)
INTEGER SENDCOUNT, SENDTYPE, RECVCOUNTS(*), DISPLS(*)
INTEGER RECVTYPE, ROOT, COMM, IERROR
C++ Syntax
#include <mpi.h>
void MPI::Comm::Gatherv(const void* sendbuf, int sendcount,
const MPI::Datatype& sendtype, void* recvbuf,
const int recvcounts[], const int displs[],
const MPI::Datatype& recvtype, int root) const = 0
INPUT PARAMETERS
sendbuf Starting address of send buffer (choice).
sendcount Number of elements in send buffer (integer).
sendtype Datatype of send buffer elements (handle).
recvcounts
Integer array (of length group size) containing the number of elements that are received
from each process (significant only at root).
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 (significant only at root).
recvtype Datatype of recv buffer elements (significant only at root) (handle).
root Rank of receiving process (integer).
comm Communicator (handle).
OUTPUT PARAMETERS
recvbuf Address of receive buffer (choice, significant only at root).
IERROR Fortran only: Error status (integer).
DESCRIPTION
MPI_Gatherv extends the functionality of MPI_Gather by allowing a varying count of data from each
process, since recvcounts is now an array. It also allows more flexibility as to where the data is
placed on the root, by providing the new argument, displs.
The outcome is as if each process, including the root process, sends a message to the root,
MPI_Send(sendbuf, sendcount, sendtype, root, ...)
and the root executes n receives,
MPI_Recv(recvbuf + disp[i] * extent(recvtype), \
recvcounts[i], recvtype, i, ...)
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 recvbuf on process root. The jth
portion of recvbuf begins at offset displs[j] elements (in terms of recvtype) into recvbuf.
The receive buffer is ignored for all nonroot processes.
The type signature implied by sendcount, sendtype on process i must be equal to the type signature
implied by recvcounts[i], recvtype 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 root, while on other processes, only argu-ments arguments
ments sendbuf, sendcount, sendtype, root, comm are significant. The arguments root and comm must have
identical values on all processes.
The specification of counts, types, and displacements should not cause any location on the root to be
written more than once. Such a call is erroneous.
Example 1: Now have each process send 100 ints to root, but place each set (of 100) stride ints
apart at receiving end. Use MPI_Gatherv and the displs argument to achieve this effect. Assume stride
>= 100.
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);
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.
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);
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.
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 "myrank" column
*/
sptr = &sendarray[0][myrank];
MPI_Gatherv(sptr, 1, stype, rbuf, rcounts, displs, MPI_INT,
root, comm);
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.
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);
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.
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 );
stride = (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);
Example 6: Process i sends num ints from the ith column of a 100 x 150 int array, in C. The compli-
cating factor is that the various values of num are not known to root, so a separate gather must
first be run to find these out. The data is placed contiguously at the receiving end.
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);
USE OF IN-PLACE OPTION
The in-place option operates in the same way as it does for MPI_Gather. 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 MPI_IN_PLACE as the value of the root process sendbuf. In this
case, sendcount and sendtype are ignored, and the contribution of the root process to the gathered
vector is assumed to already be in the correct place in the receive buffer.
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 root 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
MPI_ROOT as the value of its root argument. The remaining processes use MPI_PROC_NULL as the value
of their root argument. All processes in the second group use the rank of that root process in the
first group as the value of their root argument. The send buffer argument of the processes in the
first group must be consistent with the receive buffer argument of the root 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. 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_Gather
MPI_Scatter
MPI_Scatterv
Open MPI 1.2 September 2006 MPI_Gatherv(3OpenMPI)
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