Main Idea

1. Dead Block Replacement and Bypass with a Sampling PredictorDaniel A. JiménezDepartment of Computer ScienceThe University of Texas at San Antonio

2. Main Idea • Dead blocks • Will not be used before they are evicted • Can by identified through prediction • Dead block replacement and bypass • Replace predicted dead blocks • Quicker than waiting for them to become LRU • Bypass “dead on arrival” blocks

3. Improves Cache Efficiency – LRU replacement – Dead block replacement and bypass Dead block replacement and bypass improves cache efficiency from 22% to 87% for 456.hmmer

4. PC-Based Prediction • Memory instruction PC indexes counters • Like a branch predictor • Does this PC lead to block death? • Yes, increment counter • No, decrement counter • Predict this PC will lead to block death? • Yes, if counter exceeds threshold • Based on reference trace predictor

5. Sampling • Sampler: A few sets of partial tags • Managed by LRU replacement • Keep track of PCs that lead to block death • Generalize predictions to entire cache • In the cache • Only one bit of storage needed per block • Keeps track of latest prediction • Previous schemes need a lot more metadata • Previous predictors would not fit in budget

6. Tricks • Sampler has lower associativity • 12 in the sampler, 16 in the cache • Sampler uses dead block replacement, too • Learns to replace its own tags more quickly • But the sampler doesn’t bypass itself • Predictor uses skewed indexing • Improves accuracy over using a single table • One table uses resetting counters

7. Skewed Predictor

8. Results – Single Thread

9. Results – Multi-Core

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