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Pipeline Gating: Speculation Control For Energy Reduction

This paper explores pipeline gating as a method to reduce unnecessary speculative work in high-performance microprocessors, which often leads to increased energy demands. By implementing an effective control mechanism for speculation, the study demonstrates how to minimize wasted work while maintaining performance. Key parameters include branch predictors and confidence estimators. Results indicate significant reductions in unnecessary workloads, achieving substantial energy savings with negligible impact on processor performance. This research offers valuable insights for enhancing the energy efficiency of modern processors.

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Pipeline Gating: Speculation Control For Energy Reduction

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  1. Pipeline Gating: Speculation Control For Energy Reduction Srilatha Manne, Artur Klauser, Dirk Grunwald ISCA '98 Presented by Maria Zeniou 21/06/2005

  2. Motivation • Although speculation and out-of-order execution on microprocessors have increased IPC, the have come at the cost of wasted work • Wasted work increases power demands of the processor as a whole

  3. Goal • Control speculation and reduce the amount of unnecessary work in high-performance, wide-issue, super-scalar processors

  4. Pipeline gating for work reduction • Extra work(of a given pipeline stage): • Energy = Power x Time • Simply reducing the power may not decrease the energy demands if the task now takes longer to execute • Reduce work while retaining performance

  5. Pipeline Gating

  6. Parameters • Branch predictor • Confidence estimator • Stage at which a gating decision is made • Which stage to gate • Low-confident branches needed to engage gating

  7. Confidence Estimaton Metrics Metrics to characterize the performance of confidence estimators: • Specificity (SPEC): the fraction of all mispredicted branches actually detected by the confidence estimator as being low confident • Predictive value of a negative test (PVN): the probability of a low-confidence branch being incorrectly predicted

  8. Confidence Estimators • Perfect • Static: associates a confidence estimate with each conditional branch instruction • JRS • Saturating: High confidence only if saturating counters for both gshare and bimodal are in a strong state and have the same predicted direction • Distance

  9. Results

  10. Contributions • Pipeline gating method: to reduce the number of speculatively issued instructions • Compare the effectiveness and cost of this design – Show how to increase the effectiveness of confidence estimation mechanisms for pipeline gating • Present results that show a significant reduction in unnecessary work with a negligible performance loss

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