Pipeline Gating: Speculation Control For Energy Reduction

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

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Goal

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

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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

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Pipeline Gating
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Parameters

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

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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

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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

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Results
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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