Building Library Components that Can Use Any MPI Implementation

The Message Passing Interface (MPI) standard for programming parallel computers is widely used for building both programs and libraries. Two of the strengths of MPI are its support for libraries and the existence of multiple implementations on many platforms. These two strengths are in conflict, however, when an application wants to use libraries built with different MPI implementations. This paper describes several solutions to this problem, based on minor changes to the API. These solutions also suggest design considerations for other standards, particularly those that expect to have multiple implementatations and to be used in concert with other libraries. The MPI standard [1, 2] has been very successful. There are multiple implementations of MPI for most parallel computers [3], including vendor-optimized versions and several freely-available versions. In addition, MPI provides support for constructing parallel libraries. However, when an application wants to use routines from several different parallel libraries, it must ensure that each library was built with the same implementation of MPI. This is due to the contents of the header files ‘mpi.h’ (for C and C++) and ‘mpif.h’ for Fortran or the MPI module for Fortran 90. Because the goals of MPI included high performance, the MPI standard gives the implementor wide latitude in the specification of many of the datatypes and constants used in an MPI program. For each individual MPI program, this lack of detailed specification of the header files causes no problems; few users are even aware that the specific value of, for example, MPI ANY SOURCE is not specified by the standard. Only the names are specified. Because the values of the items defined in the header files are not defined by the standard, software components that use different MPI implementations may not be mixed in a single application. Users currently faced with building an application from multiple libraries must either mandate that a specific MPI implementation be used for all components or that all libraries be built with all MPI implementations. Neither approach is adequate; third-party MPI libraries may only be available for specific MPI implementations and the building and testing each library for each MPI implementation is both time-consuming and difficult to manage; in addition, the risk of picking the wrong version of a library