Semantic prefetching using forecast slices

Abstract

Modern prefetchers identify memory access patterns in order to predict future accesses. However, many applications exhibit irregular access patterns that do not manifest spatio-temporal locality in the memory address space. Such applications usually do not fall under the scope of existing prefetching techniques, which observe only the stream of addresses dispatched by the memory unit but not the code flows that produce them. Similarly, temporal correlation prefetchers detect recurring relations between accesses, but do not track the chain of causality in program code that manifested the memory locality. Conversely, techniques that are code-aware are limited to the basic program functionality and are bounded by the machine depth. In this paper we show that contextual analysis of the code flows that generate memory accesses can detect recurring code patterns and expose their underlying semantics even for irregular access patterns. Moreover, program locality artifacts can be used to enhance the memory traversal code and predict future accesses. We present the semantic prefetcher that analyzes programs at run-time and learns their memory dependency chains and address calculation flows. The prefetcher then constructs forecast slices and injects them at key points to trigger timely prefetching of future contextually-related iterations. We show how this approach takes the best of both worlds, augmenting code injection with forecast functionality and relying on context-based temporal correlation of code slices. This combination allows us to overcome critical memory latencies that are currently not covered by any other prefetcher. Our evaluation of the semantic prefetcher using an industrial-grade, cycle-accurate x86 simulator shows that it improves performance by 24% on average over SPEC 2006 (outliers up to 3.7x), and 16% on average over SPEC 2017 (outliers up to 1.85x), using only ~6KB.