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 due not to consulting the TLB prior to every 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, we propose a practical design of an L1 virtual cache that (1) dynamically decides a unique virtual page for all the synonymous virtual pages that map to the same physical page and (2) uses this unique page to place and look up data in the virtual caches. Accesses with this unique page proceed without any intervention. Accesses to other synonymous pages are dynamically detected, and remapping to the corresponding unique virtual page is performed to correctly look up the data in the cache.

Such remapping operations are rare, due to the temporal properties of synonyms. This new Virtual Cache with Dynamic Synonym Remapping (VC-DSR) can handle them in an energy and latency efficient manner, which achieves most of the benefits of virtual caches without software involvement. Experimental results based on real world applications show over 96% dynamic energy savings for TLB lookups without significant impact on performance compared to the system with ideal virtual caches (less than 0.4% slowdown).