PROSA: Protocol-Driven Network on Chip Architecture

Nowadays chip multiprocessors (CMPs) tend to increase the number of cores, usually implementing a distributed shared last level cache (LLC). The network on chip (NoC) is in charge of interconnecting the cores, memory controller(s) and cache banks, largely impacting memory access latency. Packet switching (PS) is typically used in NoCs but circuit switching (CS) may complement PS achieving higher performance if the circuit is established before its need. In this paper we propose PROSA, an architecture to improve memory access latency by using CS. In PROSA, the coherence protocol steers the circuit setup logic in order to configure circuits before they are needed and only for the time they are required. PROSA uses a clustered router approach where groups of four routers are clustered and their circuit control logic is combined. Based on key design decisions, we present different PROSA versions, analyzing their impact on applications and NoC performance. PROSA is able to reduce applications' execution time by 35 percent while it significantly reduces average network fllt latency by 54 percent, leading to a reduction of miss load (and store) latency of 21 percent (in CMP systems with 64 processors). PROSA needs 8.4 percent more area, but reduces power consumption by 7 percent.