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Power Reduction Techniques For Microprocessor Systems. Presented by William Lane. Material and graphics presented here are from: Power Reduction Techniques For Microprocessor Systems Vasanth Venkatachalam and Michael Franz ACM Computing Surveys, Vol. 37, No3, September 2005, pp. 195 - 237.

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power reduction techniques for microprocessor systems

Power Reduction Techniques For Microprocessor Systems

Presented by William Lane

Material and graphics presented here are from:

Power Reduction Techniques For Microprocessor Systems

Vasanth Venkatachalam and Michael Franz

ACM Computing Surveys, Vol. 37, No3, September 2005, pp. 195 - 237

slide2
“Power consumption is a major factor that limits the performance of computer systems.”

Most of the power a computer consumes ends up as heat.

The above diagram shows the power density of a few microprocessors.

In some cases heat generation and power consumption may not be an issue.

For example: Home computer used for gaming vs. Cell phone

forms of power consumption
Forms of power consumption:
  • Dynamic
    • Switched capacitance
    • Short circuit current
  • Static
    • Leakage

Static power loss seems to be where most of the total power is ending up.

dynamic power loss
Dynamic Power Loss
  • Switched capacitance:
    • 85-90% of dynamic power loss
    • Energy used in charging or dissipating capacitors at outputs of circuits
  • Short Circuit Current:
    • 10-15% of dynamic power loss
    • Energy used for pairs of transistors that switch current with opposite polarity cause a momentary short circuit
equation for switched capacitance power loss
Equation for Switched Capacitance power loss

P = aCV2f

P = power loss a = activity factor

V = supply voltage f = clock frequency

C = capacitance

methods of reducing switched capacitance power loss
Methods of reducing Switched Capacitance power loss
  • Decreasing the capacitance
    • This can worsen the performance.
  • Decreasing activity level
    • This can be difficult because generally we try to fit more on to computer chips. Ex. Clock Gating
  • Reduce clock frequency
    • Reduces the performance and may not end up reducing the energy used.
  • Decrease supply voltage
    • Lower supply voltage can increase time delay of the circuit which would then need a lower clock frequency.
dynamic voltage scaling
Dynamic Voltage Scaling
  • A combination of adjusting the supply voltage and clock frequency are used to increase or decrease the power of the processor. Adjusting these two together gives the best results.
  • Many modern processors use such a method to control the speed of the processor.
  • DVS can be used for consuming less power while also keeping the temperature of the chip cooler.
static power loss
Static Power Loss
  • Reduce supply voltage
    • Uses less power
  • Reduce the size of the circuit
    • Clock gating
    • Technology Mapping (tool for circuit design)
  • Cool computer
    • Recent study showed that cooling the processor by 50 degrees reduced leakage by up to a factor of 5
  • Change threshold voltage of transistors
    • Increasing the threshold voltage lowers the amount of leakage through the transistor, but it then takes more voltage to switch the transistor. Ex: Adaptive Body Biasing
further studies in power consumption
Further studies in power consumption
  • Buses
    • Reduce the amount of switching in a bus. Invert signal to decrease number of bits that change.

11111 to 00001 4 bits change

11111 to 11110 1 bit change

    • Low Swing Bus
      • Use lower voltage for the bus
    • Bus segmentation
      • Shut down parts of bus not used
further studies in power consumption10
Further studies in power consumption
  • Memory
    • Each memory access costs energy
      • Decrease number of memory access
      • Decrease amount of energy needed to access memory
    • Scratch pad memory
      • Used mostly in embedded application; programmer defines what data to store in the cache.
    • Instruction level cache
      • Stores the next few instructions only. Any memory references are removed. Located between L1 cache and processor
    • Adaptive Cache
      • Turns off part off cache to save energy. Can be used with drowsy cache to save data in cache when cache is turned off.
a few open questions
A few open questions
  • Is it more energy efficient to run a program using a more powerful processor which runs for less time, or is it better to run a processor that is less powerful but finishes in more time? Which processor uses more energy?
    • The speedup of a program is not linear to the speedup of the processor.
  • Is it better to use multiple slow processors or one fast processor? Which uses more energy?
  • For a laptop computer would you pick a processor that would burn through your battery quickly but gives you more computing power?
in conclusion
In conclusion
  • The energy efficiency of a processor needs to be weighed against the application of the processor.
  • Most often it is a choice between a more powerful processor and a more energy efficient one.
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