Multi-Lookahead Offset Prefetching in Data Prefetching Championship
Learn about Offset Prefetching, MLOP Offset Selection, Access Map Table, and MLOP Score Update in the context of the Third Data Prefetching Championship. Discover how MLOPs DPC3 Settings optimize prefetching strategies with detailed examples and insights
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Presentation Transcript
Multi-Lookahead Offset Prefetching Mehran Shakerinava Mohammad Bakhshalipour Pejman Lotfi-Kamran Hamid Sarbazi-Azad Presenter: Farid Samandi (Stony Brook University) The Third Data Prefetching Championship (DPC3), in conjunction with ISCA 2019
Offset Prefetching Maintains a set of prefetching offsets. Adds offsets to memory access addresses to produce prefetching targets. Periodically updates prefetching offsets based on observed memory access patterns. Offset prefetchers differ in how they implement this part. 2
Example Prefetching Offsets = {+3, +6} 3
MLOP Offset Selection MLOP scores candidate offsets based on their coverage potential. There is an offset score vector associated with each lookahead level. Lookahead level N only counts covered misses if they occur at least N timesteps later. Select highest scoring offset from each lookahead level. (Ignore repeated offsets) 4
Example Prefetching Offsets = {+3, +6} Prefetching Offsets = (+3, +6) 5
Access Map Table MLOP uses an Access Map Table to track the state of memory blocks. Each entry is a vector of states pertaining to a contiguous segment of memory. We ve added a queue to each entry that stores the M - 1 most recent accesses, where M is the highest lookahead level. 6
Example 7
MLOPs DPC3 Settings Sits in L1 and prefetches into L1 and L2. A simple next-line prefetcher sits in L2 and L3. Access Map Table divides memory into 4KB segments (64 blocks/1 page). Evaluates all possible offsets ([-63, +63]). Uses lookahead levels 1 to 16. 9
MLOPs DPC3 Settings Updates scores 500 times in each round. Selected offsets with score above 200 prefetch into L1. And selected offsets with score above 150 (but not above 200) prefetch into L2. Does not prefetch into L3. Large Access Map Table. 10
Thank you! 12
