Author retrospective for a NUCA substrate for flexible CMP cache sharing.

ABSTRACTIn 2005, as chip multiprocessors started to appear widely, it became possible for the on-chip cores to share the last-level cache. At the time, architects either considered the last-level cache to be divided into per-core private segments, or wholly shared. The shared cache utilized the capacity more efficiency but suffered from high, uniform latencies. This paper proposed a new direction: allowing the caches to be non-uniform, with a varying number of processors sharing each section of the cache. Sharing degree, the number of cores sharing a last-level cache, determines the level of replication in on-chip caches and also affects the capacity and latency for each shared cache. Building on our previous work that introduced non-uniform cache architectures (NUCA), this study explored the design space for shared multi-core caches, focusing on the effect of sharing degree. Our observation of a per-application optimal sharing degree led to a static NUCA design with a reconfigurable sharing degree. This work in multicore NUCA cache architectures has been influential in contemporary systems, including the level-3 cache in the IBM Power 7 and Power 8 processors.