Design and Implementation of the Blue Gene/P Snoop Filter

As multi-core processors evolve, coherence traffic between cores is becoming problematic, both in terms of performance and power. The negative effects of coherence (snoop) traffic can be significantly mitigated through snoop filtering. Shielding each cache with a device that can squash snoop requests for addresses known not to be in cache improves performance significantly for caches that cannot perform normal load and snoop lookups simultaneously. In addition, reducing snoop lookups yields power savings. This paper describes the design of the Blue Gene/P snoop filters, and presents hardware measurements to demonstrate their effectiveness.