On May 24, 2012, at 10:34 PM CDT, George Bosilca wrote: > On May 24, 2012, at 23:18, Dave Goodell <good...@mcs.anl.gov> 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