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Purpose:
- Extension to ALLGATHER
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- Each process sends distinct data to each receiver
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- Process i sends its jth block of data to process j
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- Process j stores data from process i in ith block
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- Varying data version (like GATHERV) called ALLTOALLV
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| Fortran Binding: |
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| MPI_ALLTOALL |
(sbuf, scount, stype, rbuf, rcount, rtype, comm, ierr) |
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| C Binding: |
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| int MPI_Alltoall |
(void* sbuf, int scount, MPI_Datatype stype, void* rbuf, int rcount, MPI_Datatype rtype, MPI_Comm comm) |
Effect:
In applications like matrix transpose and FFT, an MPI_ALLTOALL call is very helpful. This is an extension to ALLGATHER where each process sends distinct data to each receiver. The jth block from process i is received by process j and stored in ith block. A graphic representation of the MPI_ALLTOALL is shown below:
The syntax of MPI_ALLTOALL is:
C
| int MPI_Alltoall |
(void* sbuf, int scount, MPI_Datatype stype, void* rbuf, int rcount, MPI_Datatype rtype, MPI_Comm comm ) |
FORTRAN
| MPI_ALLTOALL |
(sbuf, scount, stype, rbuf, rcount, rtype, comm, ierr) |
The variables for Alltoall are:
| sbuf |
is the starting address of send buffer, |
| scount |
is the number of elements sent to each process, |
| stype |
is the data type of send buffer elements, |
| rbuf |
is the address of receive buffer, |
| rcount |
is the number of elements received from any process, |
| rtype |
is the data type of receive buffer elements, |
| comm |
is the group communicator. |
Note: Same specification as ALLGATHER, except sbuf must contain scount*NPROC elements.
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