MPI Finally Needs to Deal with Threads

The Message Passing Interface (MPI) standard has long been a cornerstone of parallel computing, enabling multi-threaded processes to communicate effectively. However, integrating multi-threading with MPI is not as straightforward as it might seem. MPI must function optimally in a multi-threaded environment, which requires robust thread support. The description provided by MPI about how multiple application threads may interact with MPI forms a contract for guarantees that applications can rely on and that implementations must provide, and vice versa. We find that the definitions regarding multi-threaded behavior provided by MPI are imprecise, which leads to differing interpretations and potential over- or undercommitment of computational resources in order to provide their perceived semantics. We provide an analysis of relevant parts of the MPI standard dealing with multi-threading and provide proposals for definitions of concurrency, conflicts, and race conditions, which we hope will clarify the guarantees that are part of the contract between MPI multi-threaded applications and implementations.