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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)