Energy Management

Uichin LeeKAIST KSE

Mobile Processing Power –Changing the Mobile Device

From http://www.ieee-infocom.org/2010/docs/Infocom2010_keynote.pdf

Chipset Business Evolving to System Business

Integration is key to driving advanced functionality to mass marketFrom http://www.ieee-infocom.org/2010/docs/Infocom2010_keynote.pdf

Creating New Mobile, Computing and CE Device Categories

From http://www.ieee-infocom.org/2010/docs/Infocom2010_keynote.pdf

But Major Gaps Exist

Battery Technology is Falling Behind

How do we balance battery life with performance and cost?

Marc A. Viredaz, Lawrence S. Brakmo, William R. HamburgenHP Labs ACM Queue Oct. 2003

Energy Management on Handheld Devices

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

Ohm’s law, Power, Energy

• Ohm’s law: V=IR (=current*resistance)

• Power: watt (W) = 1 joule/second (J/s)– Power (W) = VI= I2R

• Energy: (Ws, or Joule)– Energy (Ws) = power (w) * time (s)

I=V/R?

2.2Ω9v Voltage drop = 16v 2kΩ

I=V/R?

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

Power Consumption

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

Processor

StrongARM SA-1100

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

Processor: How to Scale?

• How to scale voltage/frequency?

...

power

time

activeE

t

Watts

activeE

idleE

t

Watts

Low frequency High frequency?

Critical Power Slope: Understanding the Runtime Effects of Frequency Scaling, ICS2002

Processor: Critical Power Slope

Frequency

Powercriticalm

minf

minfPidleP

criticalmm

criticalmm : energy efficient to run at lower freq

: energy efficient to run at higher freq

fmin: min operating frequencyPfmin: power consumption at freq fmin

Pidle: idle power consumption

min

min

fPP

criticalidlefm

Critical Power Slope: Understanding the Runtime Effects of Frequency Scaling, ICS2002

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

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..)

Display: Chameleon (MobiSys 2011)

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

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

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

Energy Profile of Samsung Galaxy S2

• Accelerometer, compass << gyroscope, GPS << WLAN, WAN(3G/4G)