Making asynchronous parallelism safe for the world

Abstract

We need a programming model that combines the advantages of the synchronous and asynchronous parallel styles. Synchronous programs are determinate (thus easier to reason about) and avoid synchronization overheads. Asynchronous programs are more flexible and handle conditionals more efficiently.Here we propose a programming model with the benefits of both styles. We allow asynchronous threads of control but restrict shared-memory accesses and other side effects so as to prevent the behavior of the program from depending on any accidents of execution order that can arise from the indeterminacy of the asynchronous process model.These restrictions may be enforced either dynamically (at run time) or statically (at compile time). In this paper we concentrate on dynamic enforcement, and exhibit an implementation of a parallel dialect of Scheme based on these ideas. A single successful execution of a parallel program in this model constitutes a proof that the program is free of race conditions (for that particular set of input data).We also speculate on a design for a programming language using static enforcement. The notion of distinctness is important to proofs of noninterference. An appropriately designed programming language must support such concepts as “all the elements of this array are distinct,” perhaps through its type system.This parallel programming model does not support all styles of parallel programming, but we argue that it can support a large class of interesting algorithms with considerably greater efficiency (in some cases) than a strict SIMD approach and considerably greater safety (in all cases) than a full-blown MIMD approach.