Temporal Prefetching Without the Off-Chip Metadata

Temporal prefetching offers great potential, but this potential is difficult to achieve because of the need to store large amounts of prefetcher metadata off chip. To reduce the latency and traffic of off-chip metadata accesses, recent advances in temporal prefetching have proposed increasingly complex mechanisms that cache and prefetch this off-chip metadata. This paper suggests a return to simplicity: We present a temporal prefetcher whose metadata resides entirely on chip. The key insights are (1) only a small portion of prefetcher metadata is important, and (2) for most workloads with irregular accesses, the benefits of an effective prefetcher outweigh the marginal benefits of a larger data cache. Thus, our solution, the Triage prefetcher, identifies important metadata and uses a portion of the LLC to store this metadata, and it dynamically partitions the LLC between data and metadata. Our empirical results show that when compared against spatial prefetchers that use only on-chip metadata, Triage performs well, achieving speedups on irregular subset of SPEC2006 of 23.5% compared to 5.8% for the previous state-of-the-art. When compared against state-of-the-art temporal prefetchers that use off-chip metadata, Triage sacrifices performance on single-core systems (23.5% speedup vs. 34.7% speedup), but its 62% lower traffic overhead translates to better performance in bandwidth-constrained 16-core systems (6.2% speedup vs. 4.3% speedup).