Reducing Snoop-Induced Power Consumption in Small-Scale , Bus-Based SMP Systems

We propose methods for reducing the power required for handling snoop requests in small-scale, snoop-coherence, bus-based SMP systems. Observing that a large fraction of snoops do not find copies on all other caches, we introduce JETTY, which is a small, cache-like structure. A JETTY is placed in-between the bus and the L2 backside of each processor where it acts as a filter for snoop requests. In particular, it filters the vast majority of snoops that would not find a locally cached copy. We propose a number of alternative JETTY methods that operate by identifying either a subset of non-locally-cached blocks or a superset of locally cached blocks. We demonstrate that for a set of parallel applications and a 4-way SMP system, relatively small JETTY structures can filter up to 77% of all snoops that would miss on the average. This resulted in average power reduction of 41% measured as a fraction of the power required for all snoop-induced tag-array

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