Energy Management: Part I

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..)

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

Andrew Rice and Simon Hay Percom 2010

Decomposing power measurementsfor mobile devices

We want to know how much energy a particular action will consume

Example: joining the wirelessnetwork consumes 6 Joules

HTC G1 (or Magic), Android 1.1, 194 trials

We measure energy consumptionby intercepting the power supply

Both voltagesare sampledat 250 kHz

Power V1 x V2∝

V1

V20.02Ω

Trace of the G1 boot process

HTC G1 (or Magic), Android 1.1

Joining a wireless network: DHCP• Dynamic Host Configuration Protocol (DHCP)

– Provides automatic configuration of the host connected to network – Provides hosts with initial configuration information upon bootup:

• IP address with subnet mask, default gateway, IP address of the DNS serverserver A

clientserver B

determineconfiguration

determineconfiguration

DHCP discoverDHCP discover

DHCP offerDHCP offer

select configuration

DHCP requestDHCP request

DHCP ack

Initialization completes

graceful shutdownDHCP release

discard lease

using the allocated configuration

(selected)(not selected)

Joining a wireless network: ARP

• Address Resolution Protocol (ARP) and Reverse ARP (RARP): translation between IP and MAC addresses

RARP

Ethernet MACaddress(48 bit)

ARPIP address(32 bit) Network

Layer

Link Layer

IP

ARP NetworkAccess RARP

Media

ICMP IGMP

TransportLayer

TCP UDP

Joining a wireless network: ARP

Argon128.143.137.144

00:a0:24:71:e4:44

Router137128.143.137.1

00:e0:f9:23:a8:20

ARP Reply:The MAC address of 128.143.71.1is 00:e0:f9:23:a8:20

Argon128.143.137.144

00:a0:24:71:e4:44

Router137128.143.137.1

00:e0:f9:23:a8:20

ARP Request:What is the MAC addressof 128.143.71.1?

Access point beacons correlate withspikes in the power trace

HTC G1 (or Magic), Android 1.1

Timestamped events from thephone must be aligned with the

appropriate sample points

The synchronization information isembedded in power trace

HTC G1 (or Magic), Android 1.1

Bright screen

Dimmed screen

Hypothesis matching pulses

HTC G1 (or Magic), Android 1.1

g(t)

Find alignment from autocorrelationwith a hypothesized signal

HTC G1 (or Magic), Android 1.1

f(t)

Cross-correlation: a measure of similarity of two waveforms as a function of a time-lag applied to one of them. Autocorrelation: cross-correlation of a signal with itself; there will always be a peak at a lag of zero, unless the signal is a trivial zero signal.

autocorrelation (f*f)(t) – cross-correlation (f*g)(t)

HTC G1 (or Magic), Android 1.1

Remove the DHCP overhead byusing static addressing

HTC G1 (or Magic), Android 1.1

Static addressing reduces theconnection cost to 1.5 Joules

Static Addressing Dynamic Addressing

HTC G1 (or Magic), Android 1.1, Static = 143 trials, Dynamic = 194 trials

We could remove the ARP probesfrom our client implementation

• “ARP probe”: ARP probe is broadcast to see if the address is already in use

• RFC2131 “...the client SHOULD probe the newly received address, e.g., with ARP.”

• RFC2119 – SHOULD “...there may exist valid reasons in particular circumstances to ignore a particular item”

Enter the

Android 2.1 doesn't ARP probe inour tests

Dynamic addressing now costs 1.5JDynamic Addressing N1

Google N1, Android 2.1, 100 trials / HTC G1 (or Magic), Android 1.1, 194 trials

Dynamic Addressing G1

The G1 histogram peaks are due todiscontinuities in connection time

HTC G1 (or Magic), Android 1.1, Dynamic

Joul

es c

onsu

med

Time to connect (seconds)

Caused by power control in radio?

HTC G1 (or Magic), Android 1.1, Dynamic

Pow

er (W

atts)

Pow

er (W

atts)

DH

CP S

tart

DH

CP S

tart

DH

CP F

inis

hD

HCP

Fin

ish

This power control is evident whensending data too

Send 7K of data over TCP Send 8K of data over TCP

HTC G1 (or Magic), Android 1.1

This effect has a big impact onenergy cost

HTC G1 (or Magic), Android 1.1, 1120 Trials (HTC Hero, Android 1.5 is the same)

N1 energy performanceBest case: same Worst case: much better

Google N1, Android 2.1, 900 Trials