[Mpi3-rma] Use cases for RMA

[William Gropp]

I went through the mail that was sent out in response to our request  
for use cases, and I must say it was underwhelming.  I've included a  
short summary below; based on this, we aren't looking at the correct  
needs.  I don't think that these are representative of *all* of the  
needs of RMA, but without good use cases, I don't see how we can  
justify any but the most limited extensions/changes to the current  
design.  Please (a) let me know if I overlooked something and (b) send  
me (and the list) additional use cases.  For example, we haven't  
included any of the issues needed to implement PGAS languages, nor  
have we addressed the existing SHMEM codes.  Do we simply say that a  
high-quality implementation will permit interoperation with whatever  
OpenSHMEM is?  And what do we do about the RMI issue that one of the  
two use cases that we have raises?

Basically, I received two detailed notes for the area of Quantum  
Chemistry.  In brief:

MPI-2 RMA is already adequate for most parts, as long as the
implementation makes progress (as it is required to) on passive
updates.  Requires get or accumulate; rarely requires put.
Dynamic load balancing (a related but separate issue) needs some sort
of RMW.  (Thanks to Jeff Hammond)

More complex algorithms and data structures appear to require remote
method invocation (RMI).  Sparse tree updates provide one example.
(Thanks to Robert Harrison)

Bill


William Gropp
Deputy Director for Research
Institute for Advanced Computing Applications and Technologies
Paul and Cynthia Saylor Professor of Computer Science
University of Illinois Urbana-Champaign