From: Rolf Rabenseifner <Rabenseifner@XXXXXXXXXXXXXXXXXXXX>
Subject: Re: MPI-2 standard issues - MPI_Type_create_f90_...
To: Nicholas.Nevin@XXXXXXXXXXXX
Date: Fri, 9 Feb 2001 18:01:39 +0100 (CET)
Cc: gropp@XXXXXXXXXXX,In-Reply-To: <20010209111839.F20056@XXXX>, 2001 11:18:40 AM
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[I'm splitting the stream by using a new subject]
 
Nick: 
> > >2. Why the restriction that types obtained from calls to
> > >MPI_Type_create_f90_{real,complex,integer} cannot be freed? This
> > >restriction doesn't appear to serve any useful purpose and the
> > >requirement that MPI_Type_get_envelope return the precision/range used
> > >to create such a type seems to either require an implementation to
> > >maintain a potentially very large table or allocate dynamic storage
> > >(which cannot be freed) to store the precision/range.

Bill: 
> > This looks like a mistake.  A correction could say that the type returned 
> > is effectively a dup of the predefined type, it can (and should be) freed 
> > by the user, and implementer could use internal attributes to store the 
> > info needed for MPI_Type_get_envelope.  Note that for layered libraries on 
> > heterogeneous system, it is necessary to provide the features specified for 
> > MPI_Type_get_envelope.

Nick:
> I'm happy with such a correction.

Sorry, but this would really change the MPI-2 standard.
MPI-2 allows to return always the same handle, and not a dup.
And to save memory, the best is that
 
 - MPI_Type_get_envelope returns the original (predefined) handle 
   and not a dup (no costs!) 
   (--> therefore no reason for freeing it!) 
 
 - and MPI_Type_create_f90_{real,complex,integer} should check 
   whether it has already returned a predefined handle with
   the same semantics. May cost typically a small search,
   because application will not use dozens of such datatypes.  
   For this purpose, you need to link your predefined dataypes
   in a list or to setup a small hash table. Not more. 

With this, you have no problem with 
- the MPI-2 standard (p.295, line 32 "They cannot be freed;") and
- the MPI-1 standard (p.8, line 36 "Such objects [predefined opaque 
  objects] may not be destroyed.")

Is this okay or have I make a mistake?
Rolf 
 

Dr. Rolf Rabenseifner                      High Performance Computing
Parallel Computing                         Center Stuttgart    (HLRS)
Rechenzentrum Universitaet Stuttgart (RUS) Phone:    ++49 711 6855530
Allmandring 30                             FAX:      ++49 711 6787626
D-70550 Stuttgart                   rabenseifner@XXXXXXXXXXXXXXXXXXXX
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