Subject: dynamic processes and dynamic groups

From: L J Clarke <[email-masked]>
Date: Sat, 6 Mar 93 20:36:37 GMT

Hi again

Well, we certainly are having some fun now in this discussion of static
vs dynamic processes in MPI.  This is as much about dynamic groups as
dynamic processes, and the latter part is really about MPI as a
standards committee.  I have a lot of sympathy with John Flower's
concerns. 

There appears to be reasonable consensus among those contributing to the
discussion that in a world process model where processes dynamically
appear and dissapear in an asynchronous fashion an enumeration of
processes is not a useful way of identifying processes - what sense
could we make of the enumeration? An opaque type process (or task)
identifier is better. 

The same considerations surely lead us to the observation that in a
world group model where processes dynamically join and leave groups in
an asynchronous fashion an enumeration of members of a group is not a
useful way of identifying members - what sense could we make of the
enumeration? Rolf previously pointed this out on mpi-comm - March 4,
subject "Process ids". 

What are asynchronously created (terminated) processes to do? I guess
they either join (leave) an existing group, or create (destroy) a group
for their own purposes.  The primitives for these two are really process
group resizing (or destruction/recreation) and process group creation
(termination).  I guess their must be heavyweight PVM users who have
other experiences of usage of dynamic process creation/termination -
Al?. 

These thoughts/questions, however interesting and discussion provoking
they may or may not be, do not take us any further on answering some of
the BIG questions about what MPI will be, regarding for example:

a) Process model; static v dynamic; ...
b) Group model; static v dynamic; ...

I've previously stated that I'd vote for static process model in (a). 
This "seemed to be" the intent of MPI (although I couldn't find it
written down and recorded as agreed), would be useful, and could be
agreed upon within the MPI time schedule.  This is not to say that I
think limitation to a static process model would be the most useful
thing upon which to agree to describe a 'de facto' standard.  I have
pretty well the same opinions regarding process groups. 

I'd like us all agree on how we're going to move forward and get the
details of the standard described, as I'm concerned that at the current
rate we may not.  I'll broadly support John Flower's suggestions,
primarily because I don't see any others, and I'll do my best to
implement whatever strategy we can agree on.  Final thought, perhaps we
should move the discussion of MPI strategy to mpi-comm. 

Best Wishes
Lyndon

         /--------------------------------------------------------\
    e||) | Lyndon J Clarke    Edinburgh Parallel Computing Centre | e||) 
    c||c | Tel: 031 650 5021  Email: [email-masked]  | c||c 
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