Expected Architectures of Future will be:
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Physically distributed but hardware support of shared memory for tightly coupled MPP's such as future IBM SP-X, Convex Exemplar, SGI (combined with Cray)
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Physically distributed but without hardware support -- NOW's and COW's -- The World Wide Web as a Metacomputer
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Essentially all problems run efficiently on a distributed memory BUT
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Software is easier to develop on a shared memory machine
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Some Shared Memory Issues:
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Cost - Performance : additional hardware (functionality, network bandwidth) to support shared memory
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Scaling. Can you build very big shared memory machines?
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Yes for NUMA distributed shared memory
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Compiler challenges for distributed shared memory are difficult and major focus of academic and commercial work
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This is not practically important now as 32 node KSR-2 (from past) or SGI Power Challenge (cost ~< $2m) is already at high end of important commercial market
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