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# Power Reduction Techniques For Microprocessor Systems Presented by William Lane Material and graphics presented here are from: Power Reduction Techniques.

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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

“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: 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 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 P = aCV 2 f P = power lossa = activity factor V = supply voltagef = clock frequency C = capacitance

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 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 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 Buses  Reduce the amount of switching in a bus. Invert signal to decrease number of bits that change. 11111 to 000014 bits change 11111 to 111101 bit change  Low Swing Bus Use lower voltage for the bus  Bus segmentation Shut down parts of bus not used

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 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 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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