Softwarecontrolled Processor Speed Setting for

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• Software-controlled Processor Speed Setting for
• Low-power Streaming Multimedia

Andrea Acquaviva, Luca Benini, Bruno Ricco SCOPES
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Introduction

• Modern H/W components
• Dynamically adjust frequency and voltage for
  power saving
• Previous work
• Assume linear relationship between clock
  frequency of the processor and execution time of
  a certain task
• There is no reduction in energy
• In real systems, it is not linear relationship
  between clock frequency and execution time
• Recent work
• On real systems, running at less than the maximum
  frequency can be advantageous
• Provide a theoretical explanation for this fact
  in this paper

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Multimedia System model

• System model
• Data come into systems from environment through a
  wireless or wired link from a host computer
• Main processing unit general-purpose
  microprocessor, integrated in a SOC architecture
• StrongARM 1100 core
• Can adjust the clock frequency via software by
  writing a control word in a special register
• Input stream structured in frames
• Real time constraints

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Variable Frequency and Energy Optimization

• Energy reduction can be obtained by setting clock
  frequency without voltage regulation

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Variable Frequency and Energy Optimization

• Where
• Varies with frequency because memory access time
  is fixed
• With small f, it reduces memory latency in terms
  of CPU wait states

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Impact of Clock Frequency on Performance

• System performance as a function of clock
  frequency
• In order to choose speed which guarantees the
  required performance
• Multimedia processing carried out frame by frame
• FR(f,s,d) frame processing rate at frequency f,
  characteristics of entire stream s,
  characteristics of a single frame d
• For MP3, FR is a strong function of streams bit
  rate (br) and sample rate (sr)
• For a fixed br and sr
• Monitor worst-case , best-case, average-case
  frame processing time at various frequencies
• And define FRB(f), FRW(f), FRA(f) curves
• Normalized with FRA(fmax)
• Repeat with various br and sr
• Define FROW(f), FROB(f), FROA(f) performance
  model of system

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Impact of Clock Frequency on Performance

• Frequency setting algorithm
• Know FROW(f), FROB(f), FROA(f), and FRA(fmax)
  for each br and sr
• Algorithm
• stream decoding begin
• extract br and sr from the header
• look up corresponding FRA(fmax)
• determine frame rate FR
• compute FRreq FR / FRA(fmax)
• with FRreq , compute fmin,fmax,fav

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Impact of Clock Frequency on Performance
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Experimental Result

• Target system
• HP SmartBadge ? prototype hand-held device
• Using simulation tool
• Embedded application MPEG-layer? audio decoding

? Energy per frame monotonically increase with
frequency - contradicting the simplistic model
where energy is constant with variable
frequency gt Because the processor wastes energy
waiting for slow memories during cache misses
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Experimental Result
?Can be used any MP3 stream
?More than 40 of energy savings are obtained
with respect to trivial policy
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Conclusion and Future Work

• Introduce automatic run-time setting for the
  optimum processor frequency that minimize the
  energy for MP3 decoding
• Future work
• Speed setting policies for rapidly changing clock
  speed requirement
• Take into account the impact of input and output
  buffering