1. Self-Tuning Wireless Network Power Management Manish Anand Edmund B. Nightingale Jason Flinn Department of Electrical Engineering and Computer Science University of Michigan

2. MobiCom 2003 Manish Anand 2 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

3. MobiCom 2003 Manish Anand 3 802.11 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%

4. MobiCom 2003 Manish Anand 4 Effect of Power Management on NFS 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

5. MobiCom 2003 Manish Anand 5 Whats Going On? NFS issues RPCs one at a time ….. NFS Server Access Point Mobile Client 50ms100ms Beacons RPC requestsRPC responses Each RPC delayed 100ms – cumulative delay is large – Affects apps with sequential request/response pairs – Examples: file systems, remote X, CORBA, Java RMI…

6. MobiCom 2003 Manish Anand 6 Outline Motivation Self Tuning Power Management –Design Principles –Implementation –Evaluation Related Work and Summary

7. MobiCom 2003 Manish Anand 7 Know Application Intent Application: NFS File access Best Policy: Use CAM during activity period CAM PSM Beacon Period Not enough network traffic to switch to CAM Not enough network traffic to switch to CAM Data rate is dependent on the power mgt.Data rate is dependent on the power mgt.

8. MobiCom 2003 Manish Anand 8 Know Application Intent Application: Stock Ticker that is receiving 10 packets per second Best policy: Use PSM Data rate is not dependent on power mgmt. Data rate is not dependent on 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

9. MobiCom 2003 Manish Anand 9 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

10. MobiCom 2003 Manish Anand 10 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

11. MobiCom 2003 Manish Anand 11 Embrace Performance/Energy Tradeoff Inherent tradeoff exists between performance and energy conservation STPM lets user specify relative priorities using a tunable knob

12. MobiCom 2003 Manish Anand 12 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

13. MobiCom 2003 Manish Anand 13 Outline Motivation Self Tuning Power Management –Design Principles –Implementation –Evaluation Related Work and Summary

14. MobiCom 2003 Manish Anand 14 STPM Architecture User or Energy Aware OS

15. MobiCom 2003 Manish Anand 15 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 Transfers Run

16. MobiCom 2003 Manish Anand 16 Intuition: Using the Run-Length History A good time to switch Switch when expected # of transfers remaining in run is high

17. MobiCom 2003 Manish Anand 17 Expected Time to complete a Run 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

18. MobiCom 2003 Manish Anand 18 Expected Energy to complete a Run Energy calculation includes base power

19. MobiCom 2003 Manish Anand 19 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!

20. MobiCom 2003 Manish Anand 20 Outline Motivation Self Tuning Power Management –Design Principles –Implementation –Evaluation Related Work and Summary

21. MobiCom 2003 Manish Anand 21 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 Mummerts file system trace (SOSP 95) –File system operations (e.g. create, open, close) –Captures interactive software development

22. MobiCom 2003 Manish Anand 22 Results for Coda Distributed File System STPM: 21% less energy, 80% less time than 802.11b power mgmt. Energy (Joules) Time (Minutes) Workload: 45 minute interactive software development activity

23. MobiCom 2003 Manish Anand 23 Results for Coda on IBM T20 Laptop PSM-Static and PSM-Adaptive use more energy than CAM! Energy (Joules) Time (Minutes) Same workload as before: effect of base power on power mgmt strategies

24. MobiCom 2003 Manish Anand 24 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

25. MobiCom 2003 Manish Anand 25 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) –Shihs Wake on wireless: (MOBICOM '02) –Krashinskys BSD Protocol: (MOBICOM '02)

26. MobiCom 2003 Manish Anand 26 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

27. Self-Tuning Wireless Network Power Management Manish Anand Edmund B. Nightingale Jason Flinn Department of Electrical Engineering and Computer Science University of Michigan

28. MobiCom 2003 Manish Anand 28 Expected Time to complete a Run 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 Consider the case of switching before the 3 rd transfer:

29. MobiCom 2003 Manish Anand 29 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 CAM PSM-adaptive PSM-static knob=100 knob=95 knob=90 knob=80 knob=0-70

30. MobiCom 2003 Manish Anand 30 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

31. MobiCom 2003 Manish Anand 31 Results for Non Hinting Applications Running Mummerts purcell trace on Coda Energy (Joules) Time (Minutes) STPM without hints: 16% less energy, 72% less time than 802.11b Power Management

32. MobiCom 2003 Manish Anand 32 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 ENERGY TIME

33. MobiCom 2003 Manish Anand 33 Results for Remote X (No Think Time) STPM uses less energy than CAM if think time > 6.5 seconds Energy (Joules)Time (Minutes)

34. MobiCom 2003 Manish Anand 34 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?

35. MobiCom 2003 Manish Anand 35 Expected Cost Calculation

36. MobiCom 2003 Manish Anand 36 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