## 7.3.5. Generalized All-to-all Function

Up: Extended Collective Operations Next: Exclusive Scan Previous: Other Operations

One of the basic data movement operations needed in parallel signal processing is the 2-D matrix transpose. This operation has motivated a generalization of the MPI_ALLTOALLV function. This new collective operation is MPI_ALLTOALLW; the ``W'' indicates that it is an extension to MPI_ALLTOALLV.

The following function is the most general form of `All-to-all`. Like MPI_TYPE_CREATE_STRUCT, the most general type constructor, MPI_ALLTOALLW allows separate specification of count, displacement and datatype. In addition, to allow maximum flexibility, the displacement of blocks within the send and receive buffers is specified in bytes.

Rationale.

The MPI_ALLTOALLW function generalizes several MPI functions by carefully selecting the input arguments. For example, by making all but one process have `sendcounts[i] = 0`, this achieves an `MPI_SCATTERW` function. ( End of rationale.)

MPI_ALLTOALLW(sendbuf, sendcounts, sdispls, sendtypes, recvbuf, recvcounts, rdispls, recvtypes, comm)
IN sendbuf | starting address of send buffer (choice)
IN sendcounts | integer array equal to the group size specifying the number of elements to send to each processor (integer)
IN sdispls | integer array (of length group size). Entry j specifies the displacement in bytes (relative to sendbuf) from which to take the outgoing data destined for process j
IN sendtypes | array of datatypes (of length group size). Entry j specifies the type of data to send to process j (handle)
OUT recvbuf | address of receive buffer (choice)
IN recvcounts | integer array equal to the group size specifying the number of elements that can be received from each processor (integer)
IN rdispls | integer array (of length group size). Entry i specifies the displacement in bytes (relative to recvbuf) at which to place the incoming data from process i
IN recvtypes | array of datatypes (of length group size). Entry i specifies the type of data received from process i (handle)
IN comm | communicator (handle)

`int MPI_Alltoallw(void *sendbuf, int sendcounts[], int sdispls[], MPI_Datatype sendtypes[], void *recvbuf, int recvcounts[], int rdispls[], MPI_Datatype recvtypes[], MPI_Comm comm)
`

`MPI_ALLTOALLW(SENDBUF, SENDCOUNTS, SDISPLS, SENDTYPES, RECVBUF, RECVCOUNTS, RDISPLS, RECVTYPES, COMM, IERROR)
<type> SENDBUF(*), RECVBUF(*)
INTEGER SENDCOUNTS(*), SDISPLS(*), SENDTYPES(*), RECVCOUNTS(*), RDISPLS(*), RECVTYPES(*), COMM, IERROR
`

`void MPI::Comm::Alltoallw(const void* sendbuf, const int sendcounts[], const int sdispls[], const MPI::Datatype sendtypes[], void* recvbuf, const int recvcounts[], const int rdispls[], const MPI::Datatype recvtypes[]) const = 0
`

No ``in place'' option is supported.

The j-th block sent from process i is received by process j and is placed in the i-th block of recvbuf. These blocks need not all have the same size.

The type signature associated with sendcounts[j], sendtypes[j] at process i must be equal to the type signature associated with recvcounts[i], recvtypes[i] at process j. This implies that the amount of data sent must be equal to the amount of data received, pairwise between every pair of processes. Distinct type maps between sender and receiver are still allowed.

The outcome is as if each process sent a message to every other process with

MPI_Send(sendbuf+sdispls[i],sendcounts[i],sendtypes[i] ,i,...),

and received a message from every other process with a call to

MPI_Recv(recvbuf+rdispls[i],recvcounts[i],recvtypes[i] ,i,...).

All arguments on all processes are significant. The argument comm must describe the same communicator on all processes.

If comm is an intercommunicator, then the outcome is as if each process in group A sends a message to each process in group B, and vice versa. The j-th send buffer of process i in group A should be consistent with the i-th receive buffer of process j in group B, and vice versa.

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MPI-2.0 of July 18, 1997
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