On May 24, 2012, at 10:34 PM CDT, George Bosilca wrote:
> On May 24, 2012, at 23:18, Dave Goodell <[email protected]> wrote:
>
>> So I take back my prior "right". Upon further inspection of the text and
>> the MPICH2 code I believe it to be true that the number of the elements in
>> the recvcounts array must be equal to the size of the LOCAL group.
>
> This is quite illogical, but it will not be the first time the standard is
> lacking some. So, if I understand you correctly, in the case of an
> intercommunicator a process doesn't know how much data it has to reduce, at
> least not until it receives the array of recvcounts from the remote group.
> Weird!
No, it knows because of the restriction that $sum_i^n{recvcounts[i]}$ yields
the same sum in each group.
The way it's implemented in MPICH2, and the way that makes this make a lot more
sense to me, is that you first do intercommunicator reductions to temporary
buffers on rank 0 in each group. Then rank 0 scatters within the local group.
The way I had been thinking about it was to do a local reduction followed by an
intercomm scatter, but that isn't what the standard is saying, AFAICS.
-Dave
