A case for dynamic pipeline scaling

ABSTRACT

Energy consumption can be reduced by scaling down frequency when peak performance is not needed. A lower frequency permits slower circuits, and hence a lower supply voltage. Energy reduc¿tion comes from voltage reduction, a technique called Dynamic Voltage Scaling (DVS).This paper makes the case that the useful frequency range of DVS is limited because there is a lower bound on voltage. Lowering fre¿quency permits voltage reduction until the lowest voltage is reached. Beyond that point, lowering frequency further does not save energy because voltage is constant.However, there is still opportunity for energy reduction outside the influence of DVS. If frequency is lowered enough, pairs of pipe¿line stages can be merged to form a shallower pipeline. The shal¿low pipeline has better instructions-per-cycle (IPC) than the deep pipeline. Since energy also depends on IPC, energy is reduced for a given frequency. Accordingly, we propose Dynamic Pipeline Scaling (DPS). A DPS-enabled deep pipeline can merge adjacent pairs of stages by making the intermediate latches transparent and disabling corresponding feedback paths. Thus, a DPS-enabled pipeline has a deep mode for higher frequencies within the influ¿ence of DVS, and a shallow mode for lower frequencies. Shallow mode extends the frequency range for which energy reduction is possible. For frequencies outside the influence of DVS, a DPS-enabled deep pipeline consumes from 23% to 40% less energy than a rigid deep pipeline.

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INDEX TERMS

Primary Classification:
  C. Computer Systems Organization
  C.1 PROCESSOR ARCHITECTURES
      C.1.3 Other Architecture Styles
          Subjects: Pipeline processors

Additional Classification:
  C. Computer Systems Organization
  C.1 PROCESSOR ARCHITECTURES
      C.1.3 Other Architecture Styles
          Subjects: Adaptable architectures
  C.3 SPECIAL-PURPOSE AND APPLICATION-BASED SYSTEMS
      Subjects: Real-time and embedded systems
  C.5 COMPUTER SYSTEM IMPLEMENTATION
      C.5.3 Microcomputers
          Subjects: Workstations; Microprocessors; Personal computers; Portable devices (e.g., laptops, personal digital assistants)

General Terms:
Design, Performance

Keywords:
clock gating, configurable pipeline, dynamic voltage scaling, fetch gating, power and energy management, shallow and deep pipelines, variable-depth pipeline