Observationally Cooperative Multithreading

TL;DR

Observationally Cooperative Multithreading (OCM) enables parallel execution of threads while maintaining the consistency of cooperative multithreading, simplifying reasoning, debugging, and allowing flexible concurrency-control strategies.

Contribution

This paper introduces OCM, a new shared-memory parallelism model that preserves cooperative semantics in parallel execution, facilitating easier programming and flexible concurrency control.

Findings

Threads can run in parallel without violating cooperative semantics

Programmers can defer concurrency-control decisions until after implementation

OCM provides a flexible abstraction for exploring various concurrency-control techniques

Abstract

Despite widespread interest in multicore computing, concur- rency models in mainstream languages often lead to subtle, error-prone code. Observationally Cooperative Multithreading (OCM) is a new approach to shared-memory parallelism. Programmers write code using the well-understood cooperative (i.e., nonpreemptive) multithreading model for uniprocessors. OCM then allows threads to run in parallel, so long as results remain consistent with the cooperative model. Programmers benefit because they can reason largely sequentially. Remaining interthread interactions are far less chaotic than in other models, permitting easier reasoning and debugging. Programmers can also defer the choice of concurrency-control mechanism (e.g., locks or transactions) until after they have written their programs, at which point they can compare concurrency-control strategies and choose the one that offers the best performance. Implementers and researchers also benefit from the agnostic nature of OCM -- it provides a level of abstraction to investigate, compare, and combine a variety of interesting concurrency-control techniques.