Reducing snoop-energy in shared bus-based mpsocs by filtering useless broadcasts

Abstract

In shared bus-based multiprocessor system-on-a-chips (MPSoCs), snoop-based schemes are widely used to maintain cache coherency. However, many of broadcasts are useless because remote caches seldom have the matching blocks and their tag lookups do not supply data. From the energy perspective, such tag lookups consume unnecessary energy and make the system energy wasteful. In this paper, we propose a broadcast filtering technique to reduce snoop-energy in both of cache and bus. Broadcast filtering is achieved by help of snooping cache and split-bus. The snooping cache checks if matching blocks exist in remote caches before broad casting a coherency request. If no remote cache has the matching block, it eliminates the broadcast. If broadcasting is necessary, only a part of split-bus is used so that the request is selectively broadcasted only to the remote caches that have matching blocks. Simulation results show that our technique reduces 90%, 50%, and 30% of cache lookups, bus usage, and snoop-energy, respectively, with only 2% of degradation in performance. Our technique reduces more energy than other state-of-the-art techniques.