.. _mpi_reduce_scatter_block:


MPI_Reduce_scatter_block
========================

.. include_body

:ref:`MPI_Reduce_scatter_block`, :ref:`MPI_Ireduce_scatter_block`,
:ref:`MPI_Reduce_scatter_block_init` |mdash| Combines values and scatters the
results in blocks.

.. The following directive tells the man page generation script to
   generate multiple bindings for this file.
.. mpi-bindings: MPI_Reduce_scatter_block, MPI_Ireduce_scatter_block, MPI_Reduce_scatter_block_init

.. The following file was automatically generated
.. include:: ./bindings/mpi_reduce_scatter_block.rst

INPUT PARAMETERS
----------------
* ``sendbuf``: Starting address of send buffer (choice).
* ``recvcount``: lement count per block (non-negative integer).
* ``datatype``: Datatype of elements of input buffer (handle).
* ``op``: Operation (handle).
* ``comm``: Communicator (handle).
* ``info``: Info (handle, persistent only).

OUTPUT PARAMETERS
-----------------
* ``recvbuf``: Starting address of receive buffer (choice).
* ``request``: Request (handle, non-blocking and persistent only).
* ``ierror``: Fortran only: Error status (integer).

DESCRIPTION
-----------

:ref:`MPI_Reduce_scatter_block` first does an element-wise reduction on vector
of ``count = n * recvcount`` elements in the send buffer defined by
*sendbuf*, *count*, and *datatype*, using the operation *op*, where n is
the number of processes in the group of *comm*. Next, the resulting
vector of results is split into n disjoint segments, where n is the
number of processes in the group. Each segments contains *recvcount*
elements. The ith segment is sent to process i and stored in the receive
buffer defined by *recvbuf*, *recvcount*, and *datatype*.


USE OF IN-PLACE OPTION
----------------------

When the communicator is an intracommunicator, you can perform a
reduce-scatter operation in-place (the output buffer is used as the
input buffer). Use the variable MPI_IN_PLACE as the value of the
*sendbuf*. In this case, the input data is taken from the top of the
receive buffer. The area occupied by the input data may be either longer
or shorter than the data filled by the output data.


WHEN COMMUNICATOR IS AN INTER-COMMUNICATOR
------------------------------------------

When the communicator is an inter-communicator, the reduce-scatter
operation occurs in two phases. First, the result of the reduction
performed on the data provided by the processes in the first group is
scattered among the processes in the second group. Then the reverse
occurs: the reduction performed on the data provided by the processes in
the second group is scattered among the processes in the first group.
For each group, all processes provide the same *recvcounts* argument,
and the sum of the *recvcounts* values should be the same for both
groups.


NOTES ON COLLECTIVE OPERATIONS
------------------------------

The reduction functions ( MPI_Op ) do not return an error value. As a
result, if the functions detect an error, all they can do is either call
:ref:`MPI_Abort` or silently skip the problem. Thus, if you change the error
handler from MPI_ERRORS_ARE_FATAL to something else, for example,
MPI_ERRORS_RETURN , then no error may be indicated.

The reason for this is the performance problems in ensuring that all
collective routines return the same error value.


ERRORS
------

.. include:: ./ERRORS.rst

.. seealso::
   * :ref:`MPI_Reduce_scatter`