Portable Programming with MPI

  portable programming The MPI specification was designed to make it possible to write portable message passing programs while avoiding unacceptable performance degradation. Within the context of MPI, ``portable'' is synonymous with ``safe.'' Unsafe programs may exhibit a different behavior on different systems because they are non-deterministic: Several outcomes are consistent with the MPI specification, and the actual outcome to occur depends on the precise timing of events. Unsafe programs may require resources that are not always guaranteed by MPI, in order to complete successfully. On systems where such resources are unavailable, the program will encounter a resource error. Such an error will manifest itself as an actual program error, or will result in deadlock.

There are three main issues relating to the portability of MPI programs (and, indeed, message passing programs in general).

1. The program should not depend on the buffering of messages by MPI or lower levels of the communication system. A valid MPI implementation may, or may not, buffer messages of a given size (in standard mode).
2. The program should not depend upon whether collective communication routines, such as MPI_Bcast(), act as barrier synchronizations. In a valid MPI implementation collective communication routines may, or may not, have the side effect of performing a barrier synchronization.
3. The program should ensure that messages are matched by the intended receive call. Ambiguities in the specification of communication can lead to incorrect or non-deterministic programs since race conditions may arise. MPI provides message tags and communicators to help avoid these types of problem.

If proper attention is not paid to these factors a message passing code may fail intermittently on a given computer, or may work correctly on one machine but not on another. Clearly such a program is not portable. We shall now consider each of the above factors in more detail.

• Dependency on Buffering
• Collective Communication and Synchronization
• Ambiguous Communications and Portability