[Mpi-22] determine if running in a heterogenous environment

[Jeff Squyres]

I think that coming up with a precise definition for "heterogeneous"  
could be problematic...

On Mar 2, 2008, at 7:01 AM, Dries Kimpe wrote:

> Below: proposal for MPI-2.2 that adds support for querying if a
> communicator is heterogenous.
>
> ----------------------------------------------------------------------------
> Proposal:
> ----------------------------------------------------------------------------
>
> Provide some method to determine if a communicator is heterogenous.
>
> There are a number of different possibilities to provide this  
> capability:
>
> 1) Provide a predefined integer valued attribute which can be used  
> to query a
> communicator.  (for example MPI_HETEROGENOUS)
>
> -- or --
>
> 2) Create a seperate function: MPI_Comm_is_heterogenous (MPI_Comm  
> comm,
> int * flag);
>
> ----------------------------------------------------------------------------
> Rationale:
> ----------------------------------------------------------------------------
>
> Currently, MPI-2.0 does not provide a portable way for an  
> application to
> determine if it is running in a heterogenous environment.
>
> Some applications are not written with heterogenous environments in  
> mind;
> They do not (always) use correct datatype descriptions when sending or
> receiving data, but instead treat the data as an array of bytes,
> relying on all datatypes having the same memory representation on both
> sender and receiver. Most often, this is done to avoid the added  
> complexity
> of creating the correct datatypes.
>
> Although the pack/unpack functions provide an alternative, the do  
> not have
> the same memory requirements (need an extra buffer to receive the  
> data) or
> performance characteristics (need an extra copy).
>
> Adding a way for an application to test if it is currently running  
> in a
> heterogenous environment enables it take appropriate action (aborting,
> switching to type-safe functions, ...)
>
> The MPI implementation -- if it supports heterogenous environments --
> already needs to determine this information because it is  
> responsible for
> performing type conversions in heterogenous communicators.
>
> Providing this information on a per-communicator base instead of  
> returning
> it for the whole MPI_UNIVERSE / MPI_COMM_WORLD enables both the MPI
> implementation and the user application to avoid overhead
> in case of a homogenous communicator that is a subset of an  
> inhomogenous
> MPI_COMM_WORLD.
>
> ----------------------------------------------------------------------------
> Alternative ways to get the same information without modifying the  
> standard:
> ----------------------------------------------------------------------------
>
> * Store information about the architecture when compiling the  
> application;
> Compare this information at runtime with all other members of the
> communicator.
>
> * (not 100% correct): calculate at run time the size of a number of
> elementary datatypes, compare information with the other ranks in the
> communicator.
>
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-- 
Jeff Squyres
Cisco Systems