Tomohiko Shimoyama

An Optical Bus Computer Cluster with a deferred cache coherence protocol

In this paper, we first propose a class of workstation cluster which utilizes optical wavelength-division multiplexing (WDM) technology to connect nodes (work-stations) of the cluster. The Optical Bus Computer Cluster (OBCC) falls in the class of cache coherent non-uniform memory access (CC-NUMA) multiprocessors. The basic topology of the cluster is star-shaped with an optical star-coupler in the center to enable one-hop simultaneous broadcasting of information packets from one node to all other cluster nodes. WDM technology not only multiplies by N times the network bandwidth using a single optical fiber, where N is the degree of wavelength multiplexing, but also provides independent communication paths between pairs of cluster nodes by properly assigning wavelengths to inter-node communication. Then we identify the cache subsystem requirements for the OBCC and propose or deferred cache coherence protocol suitable for the OBCC. The basic coherence maintenance scheme is to lazy-evaluate the cache coherence transactions among cluster nodes, utilizing the weak consistency memory model. By deferring the transactions, it is possible to combine multiple transaction issues into one transaction by accumulating modified status bits in the enhanced cache status fields. Since not only the remote memory access but also coherence transaction are costly operations in CC-NUMA systems, the deferred invocation of coherence transactions is particularly useful in CC-NUMA systems such as OBCC. We then give a performance evaluation by simulation that the coherence protocol effectively reduces coherence transactions, particularly in situations where false sharing of longer cache lines becomes noticeable.