OS-aware tuning: improving instruction cache energy efficiency on system workloads

Low power has been considered as an important issue in instruction cache (I-cache) designs. Several studies have shown that the I-cache can be tuned to reduce power. These techniques, however, exclusively focus on user-level applications, even though there is evidence that many commercial and emerging workloads often involve heavy use of the operating system (OS). This study goes beyond previous work to explore the opportunities to design energy-efficient I-cache for system workloads. Employing a full-system experimental framework and a wide range of workloads, we characterize user and OS I-cache accesses and motivate OS-aware I-cache tuning to save power. We then present two techniques (OS-aware cache way lookup and OS-aware cache set drowsy mode) to reduce the dynamic and the static power consumption of I-cache. The proposed OS-aware cache way lookup reduces the number of parallel tag comparisons and data array read-outs for cache accesses to save dynamic I-cache power in a given operation mode. The proposed OS-aware cache set drowsy mode puts I-cache regions that are only heavily used by another operation mode to reduce leakage power. The proposed mechanisms require minimal hardware modification and addition. Simulation based experiments show that with no or negligible impact on performance, applying OS-aware tuning techniques yields significant dynamic and static power savings across the experimented applications. To our knowledge, this is the first work to explore cache power optimization by considering the interactions of application-OS-hardware. It is our belief that the proposed techniques can be applied to improve the I-cache energy efficiency on server processors mostly targeting on modern and commercial applications that heavily invoke OS activities