Citation : James Dundas and Trevor Mudge . Improving data cache performance by pre - exe

In this paper we propose and evaluate a technique that improves first level data cache performance by pre-executing future instructions under a data cache miss. We show that these preexecuted instructions can generate highly accurate data prefetches, particularly when the first level cache is small. The technique is referred to as runahead processing. The hardware required to implement runahead is modest, because, when a miss occurs, it makes use of an otherwise idle resource, the execution logic. The principal hardware cost is an extra register file. To measure the impact of runahead, we simulated a processor executing five integer Spec95 benchmarks. Our results show that runahead was able to significantly reduce data cache CPI for four of the five benchmarks. We also compared runahead to a simple form of prefetching, sequential prefetching, which would seem to be suitable for scientific benchmarks. We confirm this by enlarging the scope of our experiments to include a scientific benchmark. However, we show that runahead was also able to outperform sequential prefetching on the scientific benchmark. We also conduct studies that demonstrate that runahead can generate many useful prefetches for lines that show little spatial locality with the misses that initiate runahead episodes. Finally, we discuss some further enhancements of our baseline runahead prefetching scheme.