Subject: Thoughts from Europe

From: [email-masked]
Date: Wed, 15 Jul 1992 08:14:48 -0600

While at the 3rd Wrkshop on Compiling for Parallel Computers in Vienna, 
several of us took advantage of the opportunity for a face-to-face 
meeting of European and American HPF "enthusiasts".  This note includes 
some of the questions, concerns, and other issues raised there; since no 
firm conclusions were reached, I didn't include any of them.  I haven't 
caught up on the HPFF mail yet, so some of this may have already been 
discussed.

Meeting participants:
Chuck Koelbel
David Loveman
Hans Zima
Barbara Chapman
Piyush Mehrotra
Henk Sips
Tom Lake
John Merlin
Rob Schreiber

The meeting started with a rundown from Henk of the last HPFF-Europe 
workshop.  They had decided to focus on the issues that were not active 
in the US group at the moment, including MIMD support, irregular 
distributions, and parallel I/O.  The intent was to make the groups 
complementary rather than competitive; the Americans were very supportive 
of this approach.

Clemens Thle is starting an electronic discussion group on the topic of 
MIMD support.  Several Americans indicated they would be interested in 
joining this discussion.  It was clear to all that these features would 
be important in the future, but could not be finalized in time for the 
January HPF draft.

There is apparently some discussion in Europe regarding parallel I/O; the 
US group is stagnant.  There is some hope that discussion can be revived 
here.  Hopefully the discussions will not diverge.

Hans Zima reopened the question of the TEMPLATE construct and whether it 
is needed in HPF.  Some discussion ensued about the complexity of 
distributions with and without TEMPLATEs (they are the same) and 
distributions over subsets of the processor array or replicated 
distributions.  TEMPLATE is still controversial in Europe, but it is now 
accepted into HPF.

The global naming issue was raised again.  Without F90 modules, global 
naming produces too many suprises.  The group at this meeting agreed to 
keep local names; now if only the rest of the discussion groups can agree 
to this...

Subroutine interfaces were a hot topic.  The group recognized three 
features they wanted:
	Redistribute any actual to match the dummy's distribution
	Declare (assert) the actual to have the dummy's distribution
	Declare the dummy to inherit the actual's distribution
The first two are possible in HPF as it stands, the last is not.  Some of 
the meeting attendees thought this should be the default behavior (i.e. 
when there are no distributions), but this is not practical on some 
machines.  We recommend that the proposal be expanded to allow this 
possibility.

Some discussion centered on allocated arrays.  The current proposal talks 
about all arrays being "created with an alignment"; it is unclear whether 
arrays are created when they are declared or when they are allocated.  
Either choice causes some problems.  Clarification is sought.

There needs to be some defined relation between multiple processor 
arrays.  This is most useful for arrays of different dimensions but with 
the same number of elements, and may be covered by the proposals already 
circulating around the group.

More control over partitioning of computation and load balancing is 
needed.  Some form of the ON clause may be enough, maybe not.  Local 
subroutines may also provide this.  Some question how this is related to 
MIMD support.

OK, that should start the discussions rolling (as if they needed much help).

	Chuck