Distance associativity for high-performance energy-efficient non-uniform cache architectures

Wire delays continue to grow as the dominant component of latency for large caches. A recent work proposed adaptive, non-uniform cache architecture (NUCA) to manage large, on-chip caches. By exploiting the variation in access time across widely-spaced subarrays, NUCA allows fast access to close subarrays while retaining slow access to far subarrays. While the idea of NUCA is attractive, NUCA does not employ design choices commonly used in large caches, such as sequential tag-data access for low power. Moreover, NUCA couples data placement with tag placement that is possible in a non-uniform access cache. Consequently, NUCA can place only a few blocks within a given cache set in the fastest subarrays, and must employ a high-bandwidth switched network to swap blocks within the cache for high performance. In this paper, we propose the non-uniform access with replacement and placement using distance associativity cache or NuRAPID, which leverages sequential tag-data access to decouple data placement from tag placement. Distance associativity, the placement of data at a certain distance (and latency), is separated from set associativity, the placement of tags within a set. This decoupling enables NuRAPID to place flexibly the vast majority of frequently-accessed data in the fastest subarrays, with fewer swaps than NUCA. Distance associativity fundamentally changes the trade-offs made by NUCA´s best-performing design, resulting in higher performance and substantially lower cache energy. A one-ported, non-banked NuRAPID cache improves performance by 3% on average and up to 15% compared to a multi-banked NUCA with an infinite-bandwidth switched network, while reducing L2 cache energy by 77%.