The FNL+MMA Instruction Cache Prefetcher

International audienceWhen designing a prefetcher, the computer architect has to define which event should trigger a prefetch action and which blocks should be prefetched. We propose to trigger prefetch requests on I-Shadow cache misses. The I-Shadow cache is a small tag-only cache that monitors only demand misses. FNL+MMA combines two prefetchers that exploit two characteristics of the I-cache usage. In many cases, the next line is used by the application in the near future. But systematic next-line prefetching leads to overfetching and cache pollution. The Footprint Next Line prefetcher, FNL, overcomes this difficulty through predicting if the next line will be used in the "not so long" future. Prefetching up to 5 next lines, FNL achieves a 16.5% speed-up on the championship public traces. If no prefetching is used, the sequence of I-cache misses is partially predictable and in advance. That is, when block B is missing, the nth next miss after the miss on block B is often on the same block B (n). This property holds for relatively large n up to 30. The Multiple Miss Ahead prefetcher, MMA, leverages the property. We predict the nth next miss on the I-Shadow cache and predict if it might miss the overall I-cache. A 96KB FNL+MMA achieves a 28.7% speed-up and decreases the I-cache miss rate by 91.8%