From: "Rajeev Thakur" <thakur@XXXXXXXXXXX>
To: <mpi-core@XXXXXXXXXXX>
Subject: clarification text for MPI_Reduce_scatter
Date: Thu, 10 Oct 2002 17:59:28 -0500
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Dear MPI-Core group,
                    I would like to clarify the definition of
MPI_Reduce_scatter for the case where MPI_IN_PLACE is passed as the sendbuf
argument. The current definition is ambiguous about where in the recvbuf is
the reduce-scattered data placed on each process when sendbuf=MPI_IN_PLACE.
Is it stored at the beginning of the recvbuf or at the location
corresponding to that process's segment in the recvbuf? I propose that we
specify the latter, which follows the same model as defined for MPI_Scatter.
The text for MPI_Scatter clearly says that, in the case of MPI_IN_PLACE, the
segment that the root should send to itself is not moved.

Therefore, I propose to replace lines 16-20, pg. 163, MPI-2 Standard, with
the following text:

-----
The "in place" option for intracommunicators is specified by passing
MPI_IN_PLACE in the sendbuf argument. In this case, on each process,
the input data is taken from recvbuf. Process i gets the ith segment
of the result, and it is stored at the location corresponding to
segment i in recvbuf.
-----

I also note that line 19, pg 163, in the existing text makes no sense, and I
have not included it above. The line says "Note that the area occupied by
the input data may be either longer or shorter than the data filled by the
output data."

Rajeev