Microprocessor performance has been improved by increasing the capacity of on-chip caches. However, the performance gain comes at the price of static energy consumption due to subthreshold leakage current in cache memory arrays. This paper compares three techniques for reducing static energy consumption in on-chip level-1 and level-2 caches. One technique employs low-leakage transistors in the memory cell. Another technique, power supply switching, can be used to turn off memory cells and discard their contents. A third alternative is dynamic threshold modulation, which places memory cells in a standby state that preserves cell contents. In our experiments, we explore the energy and performance tradeoffs of these techniques. We also investigate the sensitivity of microprocessor performance and energy consumption to additional cache latency caused by leakage-reduction techniques.

Microprocessor clock frequency has improved by nearly 40% annually over the past decade. This improvement has been provided, in equal measure, by smaller technologies and deeper pipelines. From our study of the SPEC 2000 benchmarks, we find that for a high-performance architecture implemented in 100 nm technology, the optimal clock period is approximately 8 fan-out-of-four (FO4) inverter delays for integer benchmarks, comprised of 6 FO4 of useful work and an overhead of about 2 FO4. The optimal clock period for floating-point benchmarks is 6 FO4. We find these optimal points to be insensitive to latch and clock skew overheads. Our study indicates that further pipelining can at best improve performance of integer programs by a factor of 2 over current designs. At these high clock frequencies it will be difficult to design the instruction issue window to operate in a single cycle. Consequently, we propose and evaluate a high-frequency design called a segmented instruction window.

Microprocessor performance has been improved by increasing the capacity of on-chip caches. However, the performance gain comes at the price of increased static energy consumption due to sub-threshold leakage current. This paper compares three techniques for reducing static energy consumption in on-chip level-1 and level-2 caches. One technique employs low-leakage transistors in the memory cell. Another technique, power supply switching can be used to turn off the memory cells and discard their contents. A third alternative is dynamic threshold modulation, which places the memory cells in a standby state that preserves cell contents. In our experiments, we explore the energy/performance trade-offs of these techniques and find that the dynamic threshold modulation achieves the best results for level-1 caches, improving the energy-delay product by 2% in a level-1 instruction cache and 7% in a level-1 data cache. Low-leakage transistors perform best for the level-2 cache as they reduce the static energy by up to 98% and improve the energy-delay product by more than a factor of 50.

The doubling microprocessor performance every three years has been the result of two factors: more transistors per chip and superlinear scaling of the processor dock with technology generation. Our results show that, due to both diminishing improvements in clock rates and poor wire scaling as semiconductor devices shrink, the achievable performance growth of conventional microarchitectures will slow substantially. In this paper, we describe technology-driven models for wire capacitance wire delay, and microarchitectural component delay. Using the results of these models, we measure the simulated performance-estimating both clock rate and IPC-of an aggressive out-of-order microarchitecture as it is scaled from a 250 nm technology to a 35 nm technology. We perform this analysis for three clock scaling targets and two microarchitecture scaling strategies: pipeline scaling and capacity scaling. We find that no scaling strategy permits annual performance improvements of better than 12.5% which is far worse than the annual 50-60% to which we have grown accustomed.