CAREER: Towards Practical Deterministic Parallel Languages

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Title: CAREER: Towards Practical Deterministic Parallel LanguagesParallel, multicore processors have become ubiquitous, but parallel programming has not. This gap implies that many everyday programs do not fully use the hardware on which they run. The problem persists because traditional parallel programming approaches are high-risk: a parallel program can yield inconsistent answers, or even crash, due to unpredictable interactions between simultaneous tasks. Certain classes of programs, however, admit strong mathematical guarantees that they will behave the same in spite of parallel execution. That is, they enable deterministic parallel programming. Functional programming, extended with "LVars" --shared-state data structures that support commutating operations-- is one such model. While this theoretical model has been proven deterministic, significant questions remain regarding practical aspects such as efficiency and scalability. This research addresses those questions by developing new LVar data structures and scaling them to larger distributed memory machines. The intellectual merits are in the development of novel algorithms that support parallel programming. Further, the LVar model provides a new lens through which to view problems in parallel programming, which can lead to downstream discoveries. The project's broader significance and importance are (1) its potential to lower the cost and risk of parallel programming and (2) its educational goal: to employ deterministic parallel programming in the introductory programming course at both a university level, and in K-12 education. Changing how programming is taught may be necessary for leveraging hardware parallelism to become a normal and unexceptional part of writing software.Three specific technical challenges are addressed in this research. First, LVars traditionally require more storage space over time, because "delete" operations do not commute with others. Semantically, the state-space of each LVar forms a join semi-lattice and all modifications must move the state "upwards" monotonically. Nevertheless, this project investigates new ways that LVars can free memory, using a concept of Saturating LVars. Second, this research seeks to formalize the relationship of LVar-based parallel programs to their purely functional counterparts, characterizing asymptotic performance advantages. Finally, this project explores the scalability of LVar-based programming abstractions in a distributed memory setting, where they share similarities with recent distributed programming constructs such as concurrent replicated data structures.