[Mpi3-tools] One more suggested change to MPI_T / moving count from get_info to alloc_handle

[Martin Schulz]

Hi all,

Christof has found a potential problem in the API design that
I think is worth discussing and (at least IMHO) worth changing:

The current design returns the "count" of a variable, i.e., how many
data elements are used to return or set the data, during the
Get_info calls. For some variables, though, it may return information
in multiple elements, e.g., once for each rank. The number of
these elements, in this case, would depend on the MPI object
we are binding the variable to and hence is not known until
we allocate the matching handle.

Examples for these kind of variables are eager limits that can
be set on individual rank pairs or number of packets received
or sent, again on individual communication pairs. In both cases
the variable would be bound to / associated with a Comm
object.

The only way to support this in the current interface proposal
is to return a count that is the biggest possible. In the example
that would be the size of MPI_COMM_WORLD, which wouldn't
scale.

One (easy) way to solve this (and the credit for this goes to
Christof) is to move returning the count from the Get_info
calls to the Handle_allocate calls. This way, the count could
be set based on the MPI object we are associating the
variable with.

Anybody see any problems with this change? Note, that valid 
implementations can still use scalar variables only by always
return one for each allocated handle.

Comments?

Thanks,

Martin


________________________________________________________________________
Martin Schulz, schulzm at llnl.gov, http://people.llnl.gov/schulzm
CASC @ Lawrence Livermore National Laboratory, Livermore, USA