MPI_Gatherv(3)

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 arguments 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 complicating 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