Dead Block Replacement and Bypass with a Sampling Predictor
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Dead Block Replacement and Bypass with a Sampling Predictor Daniel A. Jiménez Department of Computer Science The University of Texas at San Antonio. Main Idea. Dead blocks Will not be used before they are evicted Can by identified through prediction Dead block replacement and bypass

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Main idea

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


Main idea
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


Improves cache efficiency
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


Pc based prediction
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


Sampling
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


Tricks
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