Bundling: Reducing the overhead of multiprocessor prefetchers

Prefetching has proven to be a useful technique for re-ducing cache misses in multiprocessors at the cost of in-creased coherence traffic. This is especially troublesome for snoop-based systems, where the available coherence band-width often is the scalability bottleneck. The bundling technique presented in this paper reduces the overhead caused by prefetching in two ways: piggy-backing prefetches with normal requests, and requiring only one device to perform the snoop lookup for each prefetch transaction. This can reduce both the address bandwidth and the number of snoop lookups compared with a non-prefetching system. We describe bundling implementations for two important transaction types: reads and upgrades. While bundling could reduce the overhead of most exist-ing prefetch schemes, the evaluation of bundling performed in this paper has been limited to two of them: sequential prefetching and Dahlgren’s adaptive sequential prefetch-ing. Both schemes have their snoop bandwidth halved for all commercial and scientific benchmarks in the study. The combined effect of bundling applied to these prefetch schemes lowers the cache miss rate, the address bandwidth and the snoop bandwidth, compared with a system with no prefetching, for all applications. Bundling, will not reduce the data bandwidth introduced by a prefetch scheme. However, we argue that the data bandwidth is more easily scaled than the snoop bandwidth for snoop-based coherence systems.