Power-Efficient Issue Queue Design

Increasing levels of power dissipation threaten to limit the performance gains of future high-end, out-of-order issue microprocessors. Therefore, it is imperative that designers devise techniques that significantly reduce the power dissipation of the key hardware structures on the chip without unduly compromising performance. Such a key structure in out-of-order designs is the issue queue., Although crucial in achieving high performance, the issue queues are often a major contributor to the overall power consumption of the chip, potentially affecting both thermal issues related to hot spots and energy issues related to battery life. In this chapter, we present two techniques that significantly reduce issue queue power while maintaining high performance operation., First, we evaluate the power savings achieved by implementing a CAM/RAM structure for the issue queue as an alternative to the more power-hungry latch-based issue queue used in may designs. We then present the microarchitecture and circuit design of an adaptive issue queue that leverages transmission gate insertion to provide dynamic low-cost configurability of size and speed. We compare two different dynamic adaptation algorithms that use issue queue utilization and parallelism metrics in order to size the issue queue on-the-fly during execution. Together, these two techniques provide over a 70% average reduction in issue queue power dissipation for a collection of the SPEC CPU2000 integer benchmarks, with only a 3% overall performance degradation.