Architectural approach to the role of optics in monoprocessor and multiprocessor machines

The relevance of introducing optical interconnects (OI's) in monoprocessors and multiprocessors is studied from an architectural point of view. We sho w that perhaps the major explanation for why optical technologies have near ly been unable to penetrate into computers is that OI's generally do not sh orten the memory-access time, which is the most critical issue for today's stored-program machines. In monoprocessors the memory-access time is domina ted by the electronic latency of the memory itself. Thus implementing OI's inside the memory hierarchy without changing the memory architecture cannot dramatically improve the global performance. In strongly coupled multiproc essors the node-bypass latency dominates. Therefore the higher the connecti vity (possibly with optics), the shorter the path to another node, but the more expensive the network and the more complex the structure of electronic nodes. This relation leaves the choice of the the best network open in ter ms of simplicity and latency reduction. The bottlenecks resulting from and the benefits of implementing OI's are discussed with respect to symmetric m ultiprocessors, rings,: and distributed shared-memory supercomputers. (C) 2 000 Optical Society of America.