Verifying Collective MPI Calls

The collective communication operations of MPI, and in general MPI operations with non-local semantics, require the processes participating in the calls to provide consistent parameters, eg. a unique root process, matching type signatures and amounts for data to be exchanged, or same operator. Exhaustive consistency checks are typically too expensive to perform under normal use of MPI and would compromise optimizations for high performance in the collective routines, but confusing and hard-to-find errors (deadlocks, wrong results, or program crash) can happen by inconsistent calls to collective operations. We suggest to use the MPI profiling interface to provide for more extensive semantic checking of calls to MPI routines with collective (non-local) semantics. With this, exhaustive semantic checks can be enabled during application development, and disabled for production runs. We discuss what can reasonably be checked by such an interface, and mention some inherent limitations of MPI to making a fully portable interface for semantic checking. The proposed collective semantics verification interface for the full MPI-2 standard has been implemented for the NEC proprietary MPI/SX and MPI/EX implementations.