Revisiting virtual L1 caches: A practical design using dynamic synonym remapping

Abstract

Virtual caches have potentially lower access latency and energy consumption than physical caches because they do not consult the TLB prior to cache access. However, they have not been popular in commercial designs. The crux of the problem is the possibility of synonyms. This paper makes several empirical observations about the temporal characteristics of synonyms, especially in caches of sizes that are typical of L1 caches. By leveraging these observations, the paper proposes a practical design of an L1 virtual cache that (1) dynamically decides a unique virtual page number for all the synonymous virtual pages that map to the same physical page and (2) uses this unique page number to place and look up data in the virtual caches. Accesses to this unique page number proceed without any intervention. Accesses to other synonymous pages are dynamically detected, and remapped to the corresponding unique virtual page number to correctly access data in the cache. Such remapping operations are rare, due to the temporal properties of synonyms, allowing a Virtual Cache with Dynamic Synonym Remapping (VC-DSR) to achieve most of the benefits of virtual caches but without software involvement. Experimental results based on real world applications show that VC-DSR can achieve about 92% of the dynamic energy savings for TLB lookups, and 99.4% of the latency benefits of ideal (but impractical) virtual caches for the configurations considered.