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Show 2 1.1 The Advanta&es of Implementation Until very recently, most work in the area of parallel logic programming has been from the Viewpoint of developing execution models or appropriate machine architectures. Several different execution models and language paradigms have been offered, each with a different perspective on parallel execution and applications. Several different architectures have been proposed to execute parallel languages efficiently. Now that parallel computers are available, these kinds of studies are no longer sufficient. It seems appropriate that effort be directed to determJn.ing just how well existing declarative programming models translate to implementations on genuine multiprocessors. Implementation on genuine multiprocessors forces attention to detail. A real machine has limitations and architectural quirks which never burden "paper" machines. Execution models which permit the easy or elegant specification of certain kinds of problems are seldom asked to be efficient. When efficiency is claimed, it is generally supported by appeals to reason and sometimes by simulation, but almost never by direct measurement of an implementation. While simulation of hardware is well-founded in the mathematics of circuit design and semiconductor physics, software simulation technology is embryonic in comparison. The level of detail at which software simulations are performed is usually only deep enough to capture first-order charactertstics of selected performance features. Direct measurement of an implementation, on the other hand, yields performance measurements which, while possibly less dramatic, are true reflections of performance. 1.2 Machines of the Future Parallel computers are now available in a wide variety of architectures. It is unreasonable to suppose that the same style of execution will perform well across a wide spectrum of parallel architectures. Each kind of parallel machine demands a design which is well-suited to its strengths and weaknesses. It seems likely that the parallel computers of the future will continue the current trend. If this is true, they |
