To take advantage of distributed-memory parallel machines it is essential to have good control of task granularity. This paper describes a fairly accurate parallel simulator for Haskell, based on the Glasgow compiler, and complementary tools for visualising task granularities. Together these tools allow us to study the effects of various annotations on task granularity on a variety of simulated parallel architectures. They also provide a more precise tool for the study of parallel execution than has previously been available for Haskell programs. These tools have already confirmed that thread migration is essential in parallel systems, demonstrated a close correlation between thread execution times and total heap allocations, and shown that fetching data synchronously normally gives better overall performance than asynchronous fetching, if data is fetched on demand. 1 Introduction Our aim is to produce fast, cost-effective implementations of lazy functional languages. One way to impro...

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Declarative programming languages can play an important role in the process of designing and implementing parallel systems. They bridge the gap between a high-level specification, with proven properties of the overall system, and the execution of the system on real hardware. Efficiently exploiting parallelism on a wide range of architectures is a challenging task and should in our view be handled by a sophisticated runtime environment. Based on this design philosophy we have developed and formalised Glasgow parallel Haskell (GpH), and implemented it as a conservative extension of the Glasgow Haskell Compiler. The high-level nature of declarative languages eases the task of mapping an algebraic specification down to executable code. In fact, the operational components of the specification can already be considered an implementation, with the associated properties acting as assertions in the program. Based on a formal model of the declarative language, the validity of these properties can be established by manual proof, which works on a level of detail similar to the specification language itself. Many operational aspects, usually complicating