[mpi3-coll] Question on reduction operation in intercommunicators

[Adam Moody]

Hi everyone,
The spec is a bit fuzzy regarding the op parameter for reduction-type 
collectives on intercommunicators.  There are a few lines in the 
intracommunicator text under MPI_REDUCE, which says:

page 161, lines 11-15:

    "Furthermore, the datatype and op given for predefined operators 
must be the same on all processes."
    "Note, that it is possible for users to supply different 
user-defined operations to MPI_REDUCE in each process.  MPI does not 
define which operations are used on which operands in this case."

However, nothing is explicitly said along such lines for 
intercommunicators.  I assume by default the above statements must 
extend to intercommunicators as well.

I have two questions.

    1)  For intracommunicators, I understand the restriction on having 
the same op specified by all processes for predefined operators  -- 
performance.  I could see a similar argument being made that all 
processes (from both groups) in an intercomm also specify the same op 
for predefined ops.  However, I also see utility in forcing all 
processes within a group to specify the same op, but allowing each group 
to specify different ops (e.g., one may want the maximum of values from 
group A and the minimum of values from group B).  The downside in this 
case is that processes of one group don't know the operation used by 
processes in the other group without some communication.  Also, we'd 
need to be precise in specifying what data the two ops apply to (i.e., 
do I apply the op specified in group A to the data from group A or the 
data from group B?).

    2)  A similar question arises for user-defined operators, since here 
the current spec says that different processes may specify different 
ops.  It says MPI can apply which ever ops to which ever operands in 
such a case for intracommunicators.  Again, I could see value in 
enabling the user to specify the same user-defined op within a group, 
but different ops across groups.

Any thoughts?
-Adam