Communication cost in bus structured distributed systems

Four bus connection schemes are explored with respect to the attribute of system communication cost. Single and multiple buses are examined for two methods of bus protocol. One method involves holding the bus throughout the time needed to access and transfer the data, and the other involves using tagged requests and replies. A means of quantifying communication cost for these distributed systems is derived using elements of queuing theory. The expressions for communication cost are compared to see how communication cost differs under the two protocols and how it varies as a function of processor speed. This comparison shows the two protocols to have equal communication cost for infinitely fast processors and to experience increased communication cost as the processors slow down. Communication cost increases more rapidly for the bus-hold method than it does for the tagged request-reply method (as processor speed decreases). Last, the effect of varying (nonuniform) reference patterns on communication cost is analyzed for the request-reply bus system. This provides a quantitative measure of performance degradation as the distribution of reference becomes less uniform