Symbiotic Cache Resizing for CMPs with Shared LLC

Abstract

This paper investigates the problem of finding the optimal sizes of private caches and a shared LLC in CMPs. Resizing private and shared caches in modern CMPs is one way to squeeze wasteful power consumption out of architectures to improve power efficiency. However, shrinking each private/shared cache has different impact on the performance loss and the power savings to the CMPs because each cache contributes differently to performance and power. It is beneficial for both performance and power to shrink the LRU way of the private/shared cache which saves power most and increases data traffic least. This paper presents Symbiotic Cache Resizing (SCR), a runtime technique that reduces the total power consumption of the on-chip cache hierarchy in CMPs with a shared LLC. SCR turnoffs private/shared-cache ways in an inter-core and inter-level manner so that each disabling achieves best power saving while maintaining high performance. SCR finds such optimal cache sizes by utilizing greedy algorithms that we develop in this study. In particular, Prioritized Way Selection picks the most power-inefficient way. LLC-Partitioning-aware Prioritized Way Selection finds optimal cache sizes from the multi-level perspective. Lastly, Weighted Threshold Throttling finds optimal threshold per cache level. We evaluate SCR in two-core, four-core and eight-core systems. Results show that SCR saves 13% power in the on-chip cache hierarchy and 4.2% power in the system compared to an even LLC partitioning technique. SCR saves 2.7X more power in the cache hierarchy than the state-of-the-art LLC resizing technique while achieving better performance.