Analysis of bus hierarchies for multiprocessors

Abstract

In order to build large shared-memory multiprocessor systems that take advantage of current hardware-enforced cache coherence protocols, an interconnection network is needed that acts logically as a single bus while avoiding the electrical loading problems of a large bus. This paper develops models of bus delay and bus throughput to aid in optimizing the design of such a network. These models are used to derive a method for determining the maximum number of processors that can be supported by each of several bus organizations including conventional single-level buses, two-level bus hierarchies, and binary tree interconnections. An example based on a TTL bus is presented to illustrate the methods and to show that shared-memory multiprocessors with several dozen processors are feasible using a simple two-level bus hierarchy.