Type-Safe Multithreaded Programming with Priorities

Many multithreaded programs are naturally interactive and use priorities to ensure responsiveness. Programming with priorities, however, is challenging because existing techniques using a fixed set of priorities do not suffice to express rich priority relations and the lack of language support for the correct usage of priorities can lead to delays due to priority inversion. In this paper, we take an important step toward providing language support for multithreaded programming with priorities. We consider a language that provides the classic spawn-sync (a.k.a., fork-join) interface for threads and that allows the programmer to define priorities and their relationship by defining a partial order relation between them. We then equip the language with a modal type system analogous to S4 modal logic, where the accessibility relation between worlds is assumed to be symmetric and transitive. The type system ensures that priorities are used correctly without priority inversions, thus precluding a whole class of important run-time problems statically. We show that our proposed techniques are realistic by extending the Standard ML language with the necessary primitives to support multithreading and priorities and providing a prototype compiler for the extended language.