From: "Rajeev Thakur" <thakur@XXXXXXXXXXX>
Date: January 14, 2008 11:45:43 PM CST
To: "'William Gropp'" <gropp@XXXXXXXXXXX>
Subject: RE: Reduce_scatter

The reduction operations have only one datatype argument that  
describes both
send and receive buffers. Hence the type signature is the same type  
and the
counts have to be the same. Therefore for intercommunicator version of
MPI_Allreduce, the statement "Both groups should provide the same count
value." is also correct I think.

Rajeev


> -----Original Message-----
> From: Rajeev Thakur [mailto:thakur@XXXXXXXXXXXX
> Sent: Monday, January 14, 2008 10:49 PM
> To: 'William Gropp'
> Subject: Reduce_scatter
>
> Bill,
>      The definition of MPI_Reduce_scatter for
> intra-communicators says that the recvcounts array must be
> identical on all processes
> (http://www.mpi-forum.org/docs/mpi-11-html/node83.html). If
> we go by that definition, then the current defintion of
> reduce-scatter for inter-communicators is correct as it is.
> It says "Within each group, all processes provide the same
> recvcounts argument, and the sum of the recvcounts entries
> should be the same for the two groups."
> http://www.mpi-forum.org/docs/mpi-20-html/node150.htm
>
> Rajeev
>