content-class: urn:content-classes:message
Subject: RE: clarification text for MPI_Reduce_scatter
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Date: Thu, 10 Oct 2002 18:55:01 -0500
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Thread-Topic: clarification text for MPI_Reduce_scatter
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From: "Marc Snir" <snir@XXXXXXXX>
To: "Rajeev Thakur" <thakur@XXXXXXXXXXX>,<mpi-core@XXXXXXXXXXX>
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What you propose is not a clarification, it is a change. The current
definition says that the receiving data is stored in "the receive buffer
defined by recvbuf, recvcounts[i] and datatype". This means the first
recvcounts[i] entries of the buffer that starts at address recvbuf.
I.e., the output should be stored at the top of the input buffer.
Following the same model as for MPI_SCATTER is not a compelling
argument: for MPI_SCATTER the input data provided by the root is not
modified. Here, the data provided by each process is modified. I do
remember that we discussed this issue and decided on the current
definition.

I agree that the last sentence does not make sense.  

-----Original Message-----
From: Rajeev Thakur [mailto:thakur@mcs.anl.gov] 
Sent: Thursday, October 10, 2002 5:59 PM
To: mpi-core@XXXXXXXXXXX
Subject: clarification text for MPI_Reduce_scatter


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