Photonic Architectures for Distributed Shared Memory Multiprocessors

This paper studies the interaction between the access protocol used to provide arbitration for a wavelengthdivision multiple access photonic network and the cache coherence protocol required to support a distributed shared memory environment. The architecture is based on wavelength division multiplexing which enables multiple multi-access channels to be realized on a single optical fiber. Larger blocks are supported to reduce the per bit overhead and increase the exploitation of spatial locality, while false sharing is reduced through a mechanism to provide a finer granularity of invalidation. Two main approaches have been considered to harness the enormous available bandwidth of WDMA optical networks: reservation (control-channel based) or pre-allocation media access protocols. This paper extends the function of a control channel to include broadcast support of cache-level control information, in addition to its primary role of data channel reservation, thereby enabling a snooping based coherence protocol to be considered. Larger snooping-based multiprocessors may be possible with this approach. Two major scenarios are considered through trace-based discrete-event simulation in this paper: a system with a directory based cache coherence protocol and a pre-allocation based WDMA access protocol is compared to a system with a snooping based cache coherence protocol and a reservation based WDMA access protocol.