Deenition of the Earth Model

Parallel processing has tremendous potential, but has been disappointing in practice. One reason is that parallel computer designers cannot a ord to put the same resources into building processors as the makers of commodity chips, who command a much larger market. Therefore, most parallel architects have turned to o -the-shelf microprocessors, which do not support parallel processing well. Multithreaded systems based on data ow principles promise a solution to the problems inherent in many of today's parallel machines. Acceptance of such systems in a world ruled by commodity systems requires taking an evolutionary approach, in which the multithreaded machine is initially emulated on an existing multiprocessor based on o -the-shelf microprocessors, while still achieving good performance, and then transformed into more powerful versions by gradually replacing the stock components with custom hardware. EARTH (Efcient Architecture for Running Threads) is a multithreading model suitable for such an evolutionary approach. EARTH can be de ned at several layers of abstraction, including a program execution model, an architecture model, and a language de nition. This paper de nes the program execution model, which describes the most fundamental characteristics of EARTH which are independent of any particular implementation. Separate sections are devoted to the EARTH thread model, the architecture model, and a set of abstract EARTH operations. A nal section discusses possible extensions to the existing model. The material in this report mostly comes from Chapter 4 of the author's doctoral dissertation [24]. The text has been edited slightly for use as a standalone report, and some introductory text from the dissertation's rst chapter has been included. Refer to the dissertation for additional background material and experimental work. i