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NOW (Network of Workstations) |
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Or JPOW (Just a Pile of Workstations)an alternative to shared memory multiprocessors? |
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As architects consider various ways of facilitating interprocessor communication in a shared memory multiprocessor, they must be constantly aware of the cost required to improve interprocessor communications. In a typical shared memory multiprocessor, the cost does not scale linearlyeach additional processor requires additional network services and facilities. Depending on the type of interconnection, the cost for an additional processor may increase at a greater than linear rate. For those applications that require rapid communications and have a great deal of interprocessor communications traffic, this added cost is quite acceptable. It is readily justified on a cost-performance basis. However, many other applications, including many ''naturally parallel" applications, may have limited interprocessor communications. In many simulation applications, the various cases to be simulated can be broken down and treated as independent tasks to be run on separate processors with minimum interprocessor communication. For these applications, simple networked systems of workstations provide perfectly adequate communications services. After all, typical networked message communication times are on the order of 13 milliseconds. For applications whose program execution time greatly exceeds its interprocessor communication time, this is a quite acceptable message passing time. Indeed, as they move to fiberoptics, networks will improve interprocessor communications delay. |
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The problem for the shared memory multiprocessor systems architect is to create a system that can generally satisfy a broad spectrum of applications. This requires a system whose costs scale linearly with the number of processors and whose overall cost effectively competes with the NOWthe simple network of workstationson the one hand, and satisfies the more aggressive communications requirement for those applications that demand it on the other. |
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As with any systems design, it is impossible to satisfy the requirements of all applications. The designer simply must choose a broad enough set of applications and design a system robust enough to satisfy those applications. |
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