Semantics of Barriers in a Non-strict, Implicitly-parallel Language Semantics of Barriers in a Non-strict, Implicitly-parallel Language

Barriers in parallel languages may be used to schedule parallel activities, control memory usage and ensure proper sequentialization of side-eeects. In this paper we present operational semantics of barriers in Id and pH, which are non-strict, implicitly-parallel, functional languages extended with side-eeects. The semantics are presented as a translation from a source language with barriers into a kernel language without barriers where the termination properties of an expression are made explicit in the form of signals. These signals are generated using a monotonic, primitive operator (W) that detects weak head-normal forms of expressions and are used to control the execution of expressions via a strict application operator (Sap). We present two versions of the semantics | the rst uses purely data-driven, eager evaluation and the second mixes eager evaluation with a demand-driven identiier dereference mechanism. We compare and contrast the two for their ability to do resource management and preserve useful semantic properties. 1 Introduction Purely functional languages have the advantage of being non-procedural yet determinate: the result of a functional program is always the same regardless of the order in which its sub-expressions are evaluated. The introduction of side-eeects in a language usually forces a strict, sequential order of evaluation in order to guarantee determinacy (e.g., Scheme and ML). However, it is possible to specify only a partial order on side-eeects and still retain an overall consistent picture of the computation. This paper studies the use and the semantics of barriers as a partial sequentializa-tion construct in Id 12] and pH 13], which are non-strict, implicitly-parallel, functional languages extended with side-eeects. The traditional view of barriers comes from data-parallel languages such as *Lisp and CM-Fortran, where concurrently executing copies of the same program periodically synchronize using a global barrier mechanism. Some parallel machines like the CM5 and Cray T3D even provide such global barrier support in hardware. However, in this paper we study the semantics of barriers under a more general,