[petsc-dev] MPI_Exscan

[William Gropp]

Yes, in this case, you'd miss the availability of MPI_Exscan.  The  
issue is partly one of trading flexibilty/ubiquity against reasonable  
expectations and code clarity.  I believe distinguishing between MPI 1  
and MPI 2 (or MPI 3) makes sense; historically, checking for MPI-IO  
also made sense (and checking for Fortran might also).

The MPI Forum wants to avoid the situation where there is a good  
reason to check for each individual routine; that's one reason why  
various subset approaches have not received much support.  I'll also  
note that the GNU autoconf tests for libraries are designed around  
testing for a single function that the library should contain, rather  
than the full set of routines that are expected; this is an implicit  
acceptance of the library and its API as the testable unit, rather  
than the individual collection of routines, and I believe represents a  
compromise between maximum flexibility and code and design clarity.

Bill

On Aug 1, 2011, at 8:56 PM, Lisandro Dalcin wrote:

> On 31 July 2011 14:34, William Gropp <wgropp at illinois.edu> wrote:
>> I'm referring to the MPI_VERSION and MPI_SUBVERSION.
>> I'm in general a supporter of feature checks, but in the case of  
>> MPI, the
>> documentation is very precise about the content of each version and  
>> there
>> are comprehensive tests for MPI implementations that have ensured  
>> compliance
>> with the spec.  The MPI Forum has repeatedly rejected any MPI subset,
>> ensuring that MPI_VERSION and MPI_SUBVERSION can be used.
>
> So, suppose an MPI implementation advertises 1.3 compliance (by
> defining  MPI_VERSION as 1 and MPI_SUBVERSION as 3), and implements
> some calls from MPI 2.0 like MPI_Exscan() ? Using the version macros
> would prevent PETSc to use the MPI_Exscan() available in the
> implementation as an extension... Or does "The MPI Forum has
> repeatedly rejected any MPI subset" means that such implementations
> are not conforming with the MPI std?
>
>
> -- 
> Lisandro Dalcin
> ---------------
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William Gropp
Director, Parallel Computing Institute
Deputy Director for Research
Institute for Advanced Computing Applications and Technologies
Paul and Cynthia Saylor Professor of Computer Science
University of Illinois Urbana-Champaign