[Mpi3-subsetting] MPI_ANY_SOURCE

[Richard Barrett]

>> Now, to one of your questions. An MPI_ANY_SOURCE

Although I appreciate the discussion, my intent (uh-oh!) in bring this up to
let you know I "accept" the problem, yet ask for the capability anyway, but
in a manner that keeps it from presenting problems everywhere. Or maybe I'm
under-estimating what I was once told: the use of MPI_ANY_SOURCE anywhere
means it is a problem everywhere, ie in _every_ function involved in
transmitting data?

If that is the case, but I still _really_ wanted to use -lmpi_subset, I
could do this: suppose a pe knows it will receive data from m pes. It could
post numpe non-blocking receives, complete m, discover who they're from,
then cancel the rest. Now I'm thinking I've created a bigger problem: when
running acros numpes=100k cores, but m is say 10. True?

Barring some sort of workaround, excluding codes that "need" MPI_ANY_SOURCE
seems to meaningfully reduce the number of codes that could use
-lmpi_subset.

> 3. I (basically) understand the adverse performance effects of allowing
promiscuous receives (MPI_ANY_SOURCE). However, this is a powerful capability
for many codes, and used only in moderation, eg for setting up communication
requirements (such as communication partners in unstructured, semi-structured,
and dynamic mesh computations). In this case the sender knows its partner, but
the receiver does not. A reduction(sum) is used to let each process know the
number of communication partners from which it will receive data, the process
posts that many promiscuous receives, which when satisfied lets it from then on
specify the sender. So would it be possible to include this capability in a
separate function, say the blocking send/recv, but not allow it in the
non-blocking version?

Richard

-- 
  Richard Barrett
  Future Technologies Group, Computer Science and Mathematics Division, and
  Scientific Computing Group, National Center for Computational Science
  Oak Ridge National Laboratory
  http://ft.ornl.gov/~rbarrett