A fully associative software-managed cache design

Abstract

As DRAM access latencies approach a thousand instruction-execution times and on-chip caches grow to multiple megabytes, it is not clear that conventional cache structures continue to be appropriate. Two key features—full associativity and software management—have been used successfully in the virtual-memory domain to cope with disk access latencies. Future systems will need to employ similar techniques to deal with DRAM latencies. This paper presents a practical, fully associative, software-managed secondary cache system that provides performance competitive with or superior to traditional caches without OS or application involvement. We see this structure as the first step toward OS- and application-aware management of large on-chip caches. This paper has two primary contributions: a practical design for a fully associative memory structure, the indirect index cache (IIC), and a novel replacement algorithm, generational replacement , that is specifically designed to work with the IIC. We analyze the behavior of an IIC with generational replacement as a drop-in, transparent substitute for a conventional secondary cache. We achieve miss rate reductions from 8% to 85% relative to a 4-way associative LRU organization, matching or beating a (practically infeasible) fully associative true LRU cache. Incorporating these miss rates into a rudimentary timing model indicates that the IIC/generational replacement cache could be competitive with a conventional cache at today's DRAM latencies, and will outperform a conventional cache as these CPU-relative latencies grow.