Distance Associativity for High-Performance Energy-Efficient Non-Uniform Cache Architectures

Wire delays continue to grow as the dominant component oflatency for large caches.A recent work proposed an adaptive,non-uniform cache architecture (NUCA) to manage large, on-chipcaches.By exploiting the variation in access time acrosswidely-spaced subarrays, NUCA allows fast access to closesubarrays while retaining slow access to far subarrays.Whilethe idea of NUCA is attractive, NUCA does not employ designchoices commonly used in large caches, such as sequential tag-dataaccess for low power.Moreover, NUCA couples dataplacement with tag placement foregoing the flexibility of dataplacement and replacement that is possible in a non-uniformaccess cache.Consequently, NUCA can place only a few blockswithin a given cache set in the fastest subarrays, and mustemploy a high-bandwidth switched network to swap blockswithin the cache for high performance.In this paper, we proposethe Non-uniform access with Replacement And PlacementusIng Distance associativity" cache, or NuRAPID, whichleverages sequential tag-data access to decouple data placementfrom tag placement.Distance associativity, the placementof data at a certain distance (and latency), is separated from setassociativity, the placement of tags within a set.This decouplingenables NuRAPID to place flexibly the vast majority offrequently-accessed data in the fastest subarrays, with fewerswaps than NUCA.Distance associativity fundamentallychanges the trade-offs made by NUCA's best-performingdesign, resulting in higher performance and substantiallylower cache energy.A one-ported, non-banked NuRAPIDcache improves performance by 3% on average and up to 15%compared to a multi-banked NUCA with an infinite-bandwidthswitched network, while reducing L2 cache energy by 77%.