1. Energy Management Uichin Lee KAIST KSE

2. Mobile Processing Power – Changing the Mobile Device From http://www.ieee-infocom.org/2010/docs/Infocom2010_keynote.pdf

3. Chipset Business Evolving to System Business Integration is key to driving advanced functionality to mass market From http://www.ieee-infocom.org/2010/docs/Infocom2010_keynote.pdf

4. Creating New Mobile, Computing and CE Device Categories From http://www.ieee-infocom.org/2010/docs/Infocom2010_keynote.pdf

5. But Major Gaps Exist

6. Battery Technology is Falling Behind How do we balance battery life with performance and cost?

7. Marc A. Viredaz, Lawrence S. Brakmo, William R. Hamburgen HP Labs ACM Queue Oct. 2003 Energy Management on Handheld Devices

8. Itsy Platform Itsy goals (YR 2000): – Small, powerful, flexible h/w platform – Flexible, extensible, advanced s/w environment Base system – StrongArm SA-1100 microprocessor – 32MB DRAM & Flash Drive – LCD display and touch screen Passive matrix gray scale – Li-ion battery (charges from USB) – 2-axis accelerometer – Microphone – Jacks (headset, docking) – Daughtercard connecter Software – Linux 2.0.30 w/ modified memory/flash-based file systems – Power management capability

9. Ohm’s law, Power, Energy Ohm’s law: V=IR (=current*resistance) Power: watt (W) = 1 joule/second (J/s) – Power (W) = VI= I 2 R Energy: (Ws, or Joule) – Energy (Ws) = power (w) * time (s) I=V/R? 2.2Ω9v Voltage drop = 16v 2kΩ I=V/R?

10. Power Measurement Voltages are directly measured. Currents are calculated from the corresponding sense-resistor voltage drop. Elementary power domains are delimited by dashed lines.

11. Power Consumption

12. Energy Saving Techniques Simple approach: if a unit is not used, turn off or put into sleep mode But requires well structured h/w and s/w design Inter-connected building blocks must independently function and be independently powered on/off Operating systems (or applications) utilize measured power values to balance performance and battery life Major power draws: processor, memory, display, audio system, wireless networking

13. Processor StrongARM SA-1100

14. Processor Frequency or/and voltage scaling: Power ∝ f·v 2 – f: operating frequency, v: voltage J.Pouwelse, K.Langendoen, and H. Sips, “Dynamic Voltage Scaling on a Low-Power Microprocessor”, MOBICOM2001 Voltage Scaling in Strong Arm SA-1100 59Mhz at 0.79v  251Mhz at 1.65v

15. Processor: How to Scale? How to scale voltage/frequency?... power time Watts Low frequencyHigh frequency ? Critical Power Slope: Understanding the Runtime Effects of Frequency Scaling, ICS2002

16. Processor: Critical Power Slope Frequency Power : energy efficient to run at lower freq : energy efficient to run at higher freq f min : min operating frequency P fmin : power consumption at freq f min P idle : idle power consumption Critical Power Slope: Understanding the Runtime Effects of Frequency Scaling, ICS2002

17. Memory DRAM typically has four states: – Activate/pre-charge: read/write happen (most energy consuming) – Fast lower-power: short-term sleeping (w/ fast wake-up time: ~10ns, and consumes only half of the active power) – Self-refresh: only refreshing is happening (much less power consumption, requires several 100 cycles) – Deep power-down: refreshing stopped (lost data) Figure from: http://research.microsoft.com/pubs/102932/flicker-tr-2009.pdf

18. Display LCD itself consumes minimal energy, yet display front- and back-light dominates.. – Possible to dim lights of “light” pixels (for energy saving) Organic light-emitting diode (OLED) – Better quality than LCD (fast response), but it’s emissive and can’t make use of ambient light (energy consuming..)

19. Display: Chameleon (MobiSys 2011) Chameleon: A Color-Adaptive Web Browser for Mobile OLED Displays, Mian Dong and Lin Zhong, MobiSys 2011

20. Display: Chameleon (MobiSys 2011) Changing Web Content for OLED friendly? – Generate Device Specific OLED Power Model – Color Transformation of GUI Objects and Images – Keep Color Consistency for Each Website – Calculate Color Maps Offline – Give User Options

21. Wireless Networking Technologies – WiFi, Bluetooth – 2G/3G/4G cellular communications Power consumption: BT < WiFi < 2/3/4G Caveats: – bit/joule must be considered – bit/joule varies with data rate Cool-Tether: Energy Efficient On-the-fly WiFi Hot-spots using Mobile Phones, CoNext 2009

22. Energy Profile of Samsung Galaxy S2 Accelerometer, compass << gyroscope, GPS << WLAN, WAN(3G/4G)