[Mpi-forum] MPI Count proposal from today's meeting

N.M. Maclaren nmm1 at cam.ac.uk
Fri Jun 18 04:18:25 CDT 2010

On Jun 18 2010, Supalov, Alexander wrote:

> Thanks. I agree that use of the MPI_UNDEFINED by the MPI_GET_COUNT may 
> need additional review. Why do you think the ranks are "short int" 
> entities?

Eh?  If anyone is proposing to support processor numbers greater than
can be stored in a default integer in either Fortran or C, they need
professional help, badly!

MPI's basic design won't stand up to such scaling, for a start - indeed,
I don't know of any current or proposed design that would.  As a rather
rusty mathematician, I can think of a few plausible ones, but they are
so radical that most computer scientists would have hysterics at the
very concepts.

Also, the way that most people write MPI code won't stand up to that.
You had better not have any automatic or static arrays of that size, for
a start, because almost every compiler will crash or generate bad code
if you do.  ALL such arrays must be dynamic.

On this matter, are you SERIOUSLY thinking of making MPI_COunt unsigned?
For the other reasons I mentioned, it's a crazy idea.  I don't know how
well you know C, C++ or Fortran, but I know the first extremely well,
the last well, and the other tolerably.  The portability problems and
other gotchas are foul beyond most people's imagination - and, God help
us, they really do cause trouble in practice :-(

Nick Maclaren.

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