Distance-based recent use (DRU): an enhancement to instruction cache replacement policies for transition energy reduction

According to the International Technology Roadmap for Semiconductors (ITRS), the minimum feature size for microprocessors will shrink to 40 nm by 2010. Leakage currents in devices fabricated at these dimensions have been shown to be so dominant that design methodologies driven by power budgets will face challenges in reducing static power in addition to active power. An effective solution to tackle static power is to transition devices to a low-static-power sleep mode using special circuit-level techniques. However, these transitions come with energy costs, and as these techniques are perfected, and devices transition more often to sleep state, the relative contribution of transition energy to total energy will increase. To deal with the transition overhead, often used techniques are history-based and concentrate only on recognizing when to transition, but do not provide for reducing total transitions without adversely effecting the total sleep time of the devices. In this paper, we study transition-overhead reduction in associative instruction caches. We take advantage of the fact that many programs, particularly those for multimedia applications, spend most of their time in loops and most execution is near-sequential (high spatial locality). We present a technique called DRU (Distance-based Recent Use), which constrains near-sequential fetches to a single bank from the set of associative banks. Evaluation of DRU for different replacement policies in a system-level environment using Mediabench's applications and with various processor architectures (including SPARC and MIPS) have shown energy savings between 20%-28% with negligible hardware and timing overheads.