Power Reduction Techniques For Microprocessor Systems

1. 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

2. “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

3. 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.

4. 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

5. Equation for Switched Capacitance power loss P = aCV2f P = power loss a = activity factor V = supply voltage f = clock frequency C = capacitance

6. 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.

7. 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.

8. 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

9. 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

10. 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.

11. 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?

12. 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.