Self-Tuning Wireless Network Power Management
Manish Anand
Edmund B. Nightingale
Jason Flinn
Department of Electrical Engineering and Computer Science
University of Michigan
Manish Anand2MobiCom 2003
Motivation
Wireless connectivity is vital to mobile computing•But, taxes limited battery capacity of a mobile device
Power management can extend battery lifetime -However, it can negatively impact performance
Manish Anand3MobiCom 2003
802.11 Network Power Management
Network interface may be continuously-active (CAM)– Large power cost (~1.5 Watts)– May halve battery lifetime of a handheld
Alternatively, can use power-saving mode (PSM)– If no packets at access point, client interface sleeps– Wakes up periodically (beacon every 100 ms)– Reduces network power usage 70-80%
Manish Anand4MobiCom 2003
Effect of Power Management on NFS
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PSM-static
PSM-adaptive
CAM
PSM-static:• 16-32x slower• 17x more energy
PSM-adaptive:• up to 26x slower• 12x more energy
Time to list a directory on handheld with Cisco 350 card
Manish Anand5MobiCom 2003
What’s Going On?
NFS issues RPCs one at a time …..
NFS Server
Access Point
Mobile Client
50ms 100ms 100msBeacons
RPC requests RPC responses
Each RPC delayed 100ms – cumulative delay is large– Affects apps with sequential request/response pairs– Examples: file systems, remote X, CORBA, Java RMI…
Manish Anand6MobiCom 2003
Outline
• Motivation• Self Tuning Power Management
– Design Principles– Implementation– Evaluation
• Related Work and Summary
Manish Anand7MobiCom 2003
Know Application Intent
Application: NFS File access
Best Policy: Use CAM during activity period
CAMPSM
Beacon Period
• Not enough Not enough network traffic to network traffic to switch to CAMswitch to CAM
•Data rate is Data rate is dependent on the dependent on the power mgt.power mgt.
Manish Anand8MobiCom 2003
Know Application Intent
Application: Stock Ticker that is receiving 10 packets per second
Best policy: Use PSM
• Data rate is not Data rate is not dependent on dependent on power mgmt.power mgmt.
STPM allows applications to disclose hints about:- When data transfer are occurring - How much data will be transferred (optional) - Max delay on incoming packets
PSM
Beacon Period
CAM
Manish Anand9MobiCom 2003
Be Proactive
Transition cost of changing power mode: 200-600 ms.
Large transfers: use a reactive strategy
- If transfer large enough, should switch to CAM
- Break-even point depends on card characteristics
- STPM calculates this dynamically
Many applications (like NFS) only make short transfers: be proactive
- Benefit of being in CAM small for each transfer
- But if many transfers, can amortize transition cost
- STPM builds empirical distribution of network transfers
- Switches to CAM when it predicts many transfers likely in future
Manish Anand10MobiCom 2003
Respect the Critical Path
Many applications are latency sensitive
- NFS file accesses
- Interactive applications
- Performance and Energy critical
Other applications are less sensitive to latency
- Prefetching, asynchronous write-back (Coda DFS)
- Multimedia applications (with client buffering)
- Only energy conservation critical
Applications disclose the nature of transfer: foreground or background
Manish Anand11MobiCom 2003
Embrace Performance/Energy Tradeoff
Inherent tradeoff exists between
performance and energy
conservation
STPM lets user specify relative priorities using a tunable knob
E N E R G Y
T IM E
b attery life is n o t a con sid era tion
lon ger b a ttery life is n eed ed
Manish Anand12MobiCom 2003
Adapt to the operating environment
Must consider base power of the mobile computer
Consider mode that reduces network power from 2W to 1W- Delays interactive application by 10%
On handheld with base power of 2 Watts:- Reduces power 25% (from 4W to 3W)- Energy reduced 17.5% (still pretty good)
On laptop with base power of 15 Watts:- Reduces power by only 5.9%- Increases energy usage by 3.5%- Battery lasts longer, user gets less work done
Manish Anand13MobiCom 2003
Outline
• Motivation• Self Tuning Power Management
– Design Principles– Implementation– Evaluation
• Related Work and Summary
Manish Anand14MobiCom 2003
STPM Architecture
STPMModule
Applications
NetworkDevice Driver
DeviceCharacteristics
ModeTransitions
Energy-AwareOS
Hints
BasePower
Energy/Perf.Tradeoff
Operating System
User or Energy Aware OS
Manish Anand15MobiCom 2003
Transition to CAM
STPM switches from PSM to CAM when:
1. Application specifies max delay < beacon period
2. Disclosed transfer size > break-even size
3. Many forthcoming transfers are likely
To predict forthcoming transfers STPM generates an empirical distribution of run lengths
>150 ms >150 ms>150 ms
Transfers
Run Run Run Run
Manish Anand16MobiCom 2003
Intuition: Using the Run-Length History
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Run Length
Num
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f Run
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A good time to switch
Switch when expected # of transfers remaining in run is high
Manish Anand17MobiCom 2003
Expected Time to complete a Run
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)(
1024
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nr
irL
irL
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ni
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n
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PSMn
Expected time to execute transfers in PSM mode
Expected to execute rest of the transfers in CAM
mode
Time penalty for making a PSM to CAM switch
Manish Anand18MobiCom 2003
Expected Energy to complete a Run
)())((
)()(
)()(
1024
)(
1
1
)(
nrP
irPPL
irPPL
baseTCTCni
baseidleCAMCAM
n
i
baseidlePSMPSMn
• Energy calculation includes base power
Manish Anand19MobiCom 2003
Performance and Energy Tradeoff
Calculate expected time and energy to switch after each # of transfers– What if these goals conflict?– Refer to knob value for relative priority of each goal!
)100()/()/( knobknobC meannmeannn
Manish Anand20MobiCom 2003
Outline
• Motivation• Self Tuning Power Management
– Design Principles– Implementation– Evaluation
• Related Work and Summary
Manish Anand21MobiCom 2003
Evaluation
Client: iPAQ handheld with Cisco 350 wireless card
Evaluate STPM vs. CAM, PSM-static, and PSM-adaptive:– NFS distributed file system– Coda distributed file system– XMMS streaming audio– Remote X (thin-client display)
Run DFS workload to generate access stats for STPM– Use Mummert’s file system trace (SOSP ’95)– File system operations (e.g. create, open, close)– Captures interactive software development
Manish Anand22MobiCom 2003
Results for Coda Distributed File System
STPM: 21% less energy, 80% less time than 802.11b power mgmt.
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CAM PSM-static PSM-adaptive STPM
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CAM PSM-static PSM-adaptive STPM
Energy (Joules) Time (Minutes)
Workload: 45 minute interactive software development activity
Manish Anand23MobiCom 2003
Results for Coda on IBM T20 Laptop
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CAM PSM-static PSM-adaptive STPM
PSM-Static and PSM-Adaptive use more energy than CAM!
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CAM PSM-static PSM-adaptive STPM
Energy (Joules) Time (Minutes)
Same workload as before: effect of base power on power mgmt strategies
Manish Anand24MobiCom 2003
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Results for XMMS Streaming Audio
STPM: 2% more power usage than PSM-static – no dropped pkts
XMMS buffers data on client:• App not latency sensitive• PSM uses least power
Power (Watts)
Workload: 128Kb/s streaming MP3 audio from an Internet server
Effect of knowing application intent
Manish Anand25MobiCom 2003
Related Work– Lu, Y.H., Benini, L., AND Micheli, G.D. Power-aware operating
systems for interactive systems. IEEE Trans. on VLSI (April 2002)
– Simunic, T., Benini, L., Glynn, P. and Micheli, G.D. Dynamic
Power Management for Portable Systems. Mobile Computing and Networking (2000)
– Kravets, R., and Krishnan, P. Application-driven power management for mobile communication. ACM Wireless Nets. (2000)
– Shih’s Wake on wireless: (MOBICOM '02)
– Krashinsky’s BSD Protocol: (MOBICOM '02)
Manish Anand26MobiCom 2003
Summary
STPM adapts to:
– Base power of mobile computer
– Application network access patterns
– Relative priority of performance and energy conservation
– Characteristics of network interface
Compared to previous power management policies, we perform better and conserve more energy
Self-Tuning Wireless Network Power Management
Manish Anand
Edmund B. Nightingale
Jason Flinn
Department of Electrical Engineering and Computer Science
University of Michigan
Manish Anand28MobiCom 2003
Expected Time to complete a Run
)(
)(
)(
1024
1
1
nr
irL
irL
TC
ni
CAM
n
i
PSMn
Expected time to execute transfers in PSM mode
Expected to execute rest of the transfers in CAM
mode
Time penalty for making a PSM to CAM switch
343213 ...)()( TCCAMPSM LLConsider the case of switching before the 3rd transfer:
Manish Anand29MobiCom 2003
Results for tuning performance/energy
• Decreasing the knob value never yields increased energy usage
• Increasing the knob value never yields reduced performance
Same workload as before: effect of tuning relative priorities
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Time (minutes)
En
erg
y (J
ou
les)
CAM
PSM-adaptive
PSM-static
knob=100
knob=95knob=90
knob=80knob=0-70
Manish Anand30MobiCom 2003
Self Tuning Power Management
STPM adapts to:
– Base power of mobile computer
– Application network access patterns
– Relative priority of performance and energy conservation
– Characteristics of network interface
Compared to previous power management policies, we perform better and conserve more energy
Manish Anand31MobiCom 2003
Results for Non Hinting Applications
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STPM-non-hint
STPM
Running Mummert’s purcell trace on Coda
Energy (Joules)
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Time (Minutes)
STPM without hints: 16% less energy, 72% less time than 802.11b Power Management
Manish Anand32MobiCom 2003
Results for executing a web trace
Result of executing a 45 minute BU web trace
• CAM performs only 0.8% better than PSM-static while expending 62% more energy
• STPM behaves like PSM-static when conserving energy and like CAM in presence of abundant energy
ENERGYENERGYTIMETIME
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rgy(
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CAM PSM-static PSM-adaptive
STPM
Tim
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inut
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Manish Anand33MobiCom 2003
Results for Remote X (No Think Time)
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CAM PSM-static PSM-adaptive STPM
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CAM PSM-static PSM-adaptive STPM
STPM uses less energy than CAM if think time > 6.5 seconds
Energy (Joules) Time (Minutes)
Manish Anand34MobiCom 2003
Managing Other Devices with STPM
STPM well-suited for power management when:– Performance / energy conservation tradeoff exists– Transition costs are substantial
Consider disk power management:– Web browser, DFS, mobile DB cache data locally– Hard drive spins down for power saving– Significant transition cost to resume rot. latency– Faster, less energy to read small object from server– But, if many accesses, want to spin-up disk
For what other devices can STPM be applied?
Manish Anand35MobiCom 2003
Expected Cost Calculation
Manish Anand36MobiCom 2003
STPM as a wireless power management strategy
• Holistic solution
– Application intent through hints
– Proactive solution using run histogram
– Nature of network transfer : foreground or background
– Performance/Energy tradeoff with a tunable knob
– Operating Environment: base power