Distributed round-robin and first-come first-serve protocols and their applications to multiprocessor bus arbitration

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Two new distributed protocols for fair and efficient bus arbitration are presented. The protocols implement round-robin (RR) and first-come first-serve (FCFS) scheduling, respectively. Both protocols use relatively few control lines on the bus, and their logic is simple. The round-robin protocol, which uses statically assigned arbitration numbers to resolve conflict during an arbitration, is more robust and simpler to implement than previous distributed RR protocols that are based on rotating agent priorities. The proposed FCFS protocol uses partly static arbitration numbers, and is the first practical proposal for a FCFS arbiter known to the authors. The proposed protocols thus have a better combination of efficiency, cost, and fairness characteristics than existing multiprocessor bus arbitration algorithms. Three implementations of our RR protocol, and two implementations of our FCFS protocol, are discussed. Simulation results are presented that address: 1) the practical potential for unfairness in the simpler implementation of the FCFS protocol, 2) the practical implications of the higher waiting time variance in the RR protocol, and 3) the allocation of bus bandwidth among agents with unequal request rates in each protocol. The simulation results indicate that there is very little practical difference in the performance of the two protocols.