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Date: Fri, 09 Feb 2001 09:19:17 -0600
To: Nicholas Nevin - Sun HPC High Performance Computing <Nicholas.Nevin@XXXXXXXXXXXX>
From: William Gropp <gropp@XXXXXXXXXXX>
Subject: Re: MPI-2 standard issues
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At 09:56 AM 2/9/2001 -0500, Nicholas Nevin - Sun HPC High Performance Computing wrote:
A have a couple of questions regarding the MPI-2 standard.

1.  In section 8.5 is the following

        "Instead, a predefined attribute key MPI_LASTUSEDCODE is
associated with MPI_COMM_WORLD. The attribute value corresponding to
this key is the current maximum error class including the user-defined
ones."

Shouldn't "maximum error class" read "maximum error code"?

This reflects the statement from the MPI-1 standard that MPI_ERR_LASTCODE is greater or equal to the value of every error *class*.  The text of the description says "last standard error code", not "last error code".  One reading of this is that MPI_ERR_LASTCODE (and the MPI-2 attribute MPI_LASTUSEDCODE) defines the maximum defined error class.  Error codes can be much larger; we encode information about the specific error case into the code.  Our maximum error code is effectively MAX_INT.  But our maximum error class is much smaller.



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.

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.

Bill