On Multi-level Exclusive Caching: Offline Optimality and Why Promotions Are Better Than Demotions

Multi-level cache hierarchies have become very common; however, most cache management policies result in duplicating the same data redundantly on multiple levels. The state-of-the-art exclusive caching techniques used to mitigate this wastage in multi-level cache hierarchies are the DEMOTE technique and its variants. While these achieve good hit ratios, they suffer from significant I/O and computational overheads, making them unsuitable for deployment in real-life systems.

We propose a dramatically better performing alternative called PROMOTE, which provides exclusive caching in multi-level cache hierarchies without demotions or any of the overheads inherent in DEMOTE. PROMOTE uses an adaptive probabilistic filtering technique to decide which pages to ``promote" to caches closer to the application. While both DEMOTE and PROMOTE provide the same aggregate hit ratios, PROMOTE achieves more hits in the highest cache levels leading to better response times. When inter-cache bandwidth is limited, PROMOTE convincingly outperforms DEMOTE by being x more efficient in bandwidth usage. For example, in a trace from a real-life scenario, PROMOTE provided an average response time of ms as compared to ms for DEMOTE on a two-level hierarchy of LRU caches, and ms as compared to ms on a three-level cache hierarchy.

We also discover theoretical bounds for optimal multi-level cache performance. We devise two offline policies, called OPT-UB and OPT-LB, that provably serve as upper and lower bounds on the theoretically optimal performance of multi-level cache hierarchies. For a series of experiments on a wide gamut of traces and cache sizes OPT-UB and OPT-LB ran within % and % of each other for two-cache and three-cache hierarchies, respectively. These close bounds will help evaluate algorithms and guide future improvements in the field of multi-level caching.