Turducken: Hierarchical Power Management for Mobile Devices

1. Turducken: Hierarchical Power Management for Mobile Devices Jacob Sorber, Nilanjan Banerjee, Mark Corner, Sami Rollins† University of Massachusetts, Amherst http://prisms.cs.umass.edu/hpm

2. Batteries and Wireless: Two Achilles Heels • Imagine using a laptop like it was a desktop: • your mail is always fresh • your file system is always consistent • your web is always prefetched • instant messages are instantaneous • Why isn’t this a reality? • Mobility leads to variable connectivity • Limited energy necessitates shutting laptop off • The result is we don’t even try!

3. Tension in Maintaining Availability • The laptop can remain active—available for communication • System lifetime is severely limited • The laptop can be suspended—extending lifetime • System is not available • Compromise: wake periodically to communicate • More frequent = more available, shorter lifetime • Less frequent = less available, longer lifetime • The tension remains despite current methods • Don’t know when to wake up • Systems lean toward longer lifetime • Goal: A system that is always available with extended lifetime

4. Solution: Hierarchical Power Management • Hierarchical Power Management • Combine a range of embedded platforms into a single device • Perform tasks on the tier that best fits the task • Download email using PDA • Download large movies with laptop • Sleep high-power tiers when not needed • Focus on non-interactive use • Provide power-efficiency when NOT in use • Remove the human bottleneck • Allow new laptop functionality

5. Outline • HPM Design Principles: • Combine the Right Platforms • Use the right device for the job • Decompose Tasks • Design of an HPM Laptop • Implementation of Turducken • Hardware • Software • Experimental Results • Related Work and Conclusions

6. Combine the Right Platforms • More platforms = more overhead • Size, weight, and complexity • Suspend power • Transition cost between platforms • Still need to support a large range of power modes • Use few devices separated by orders-of-magnitude in power

7. Use the Right Device for the Job 40W Laptop Tasks requiring large memory or storage space Very complex computation 2W PDA Power Needs Download Web Pages Send/Recv Mail 0.04W Sensor Simple computation and storage

8. Decompose Tasks • Not only separate platforms, but subdivide tasks • Tasks include common subtasks: • Checking for network connectivity • Service Discovery • Check for work to be done • Interact with user • Example: • Mote checks for a network, PDA checks for mail server • Laptop remains asleep until user reads the mail

9. Outline • HPM Design Principles: • Combine the Right Platforms • Use the right device for the job • Decompose Tasks • Design of an HPM Laptop • Implementation of Turducken • Hardware • Software • Experimental Results • Related Work and Conclusions

10. HPM Hardware Design

11. HPM Software Design • Take advantage of transparent proxies • Proxies periodically perform service discovery and fetch content • Provide it on demand to higher tiers in system • Many applications are already distributed • Lower tiers act as proxy for higher tiers • Little or no application recoding and invisible to users

12. Outline • HPM Design Principles: • Combine the Right Platforms • Use the right device for the job • Use the right number of devices • Design of an HPM Laptop • Implementation of Turducken • Hardware • Software • Experimental Results • Related Work and Conclusions

13. Hardware Implementation • Combines Laptop, PDA, and mote into a laptop • Laptop and PDA communicate via USB • PDA and mote via RS-232 • Wakeup using relays • WiFi detector allows mote to detect 802.11 beacons

14. Hardware Implementation

15. Applications • Time Synchronization • Can be completely performed by the mote • Lowest tier periodically synchronizes with remote time server • Web Prefetching • More complicated application which requires at least PDA • Mote detects wireless networks • PDA periodically fetches updates to frequently accessed pages • E-Mail • Involves user interaction • Both incoming and outgoing updates • Laptop accesses mail via proxy on the PDA

16. Evaluating Turducken • How does HPM affect battery lifetime? • Show results for a variety of workloads • Vary the consistency of the application • Vary the availability of the network • Consider three configurations • Laptop • Laptop+Mote • Laptop+PDA+Mote • Experiments take a long time • Extrapolate from a set of measured data points

17. Effect of Consistency • Measure the impact of maintaining consistency • System wakes periodically—vary frequency • Network is always present • Time Synchronization (please see paper) • Web Prefetching • Laptop routes network traffic through PDA • PDA periodically refreshes expired pages • E-Mail • System wakes with constant rate to send mail (2 min/hr) • PDA periodically checks for new mail and sends pending mail

18. Results: Web Prefetching Lifetime (hours) Wake Interval (minutes) • PDA is the right device for the job • For always-on operation, Turducken lasts up to 10 times longer

19. Results: E-Mail Lifetime (hours) Wake Interval (minutes) • More interactive use reduces opportunities for Turducken to save power • Turducken still lasts up to 6 times longer

20. Power-By-Tier: Web Caching • Laptop+Mote shows cost of adding unnecessary platforms • Turducken replaces 400mW of laptop active power with 25mW on PDA • Laptop suspend power is the greatest limiting factor: try hibernation Laptop Laptop+Mote Full Turducken

21. Effect of Network Availability • Measure the impact of intermittent connectivity • Network is present with varying probability • Mote wakes at a fixed interval • Only wakes PDA when network is available • Only show results for Web Prefetching

22. Results: Web Prefetching Lifetime (hours) Probability of WiFi • With no network mote prevents laptop from waking • Turducken performs well over a variety of connectivity patterns

23. Related Work • Wake-On-Wireless [Shih, et al] • Low-power radio receives wakeup messages • Requires in-network support • Focus on when to wake up • Wireless optimizations [Anand, et al], Disk Spindown [Helmbold, et al], Multiple radios [Pering, et al], Multiple processor cores [Mayo & Ranganathan] • none provide orders-of-magnitude scaling of power

24. Conclusions • Always-on availability and extended battery life • Hierarchical Power Management • Integrate multiple independent platforms • Pass control between tiers • Suspend unneeded tiers • Use proxy-based approach • Little or no application recoding • Always-on operation for up to 10x as long

25. Turducken: Hierarchical Power Management for Mobile Devices Jacob Sorber, Nilanjan Banerjee, Mark Corner, Sami Rollins† University of Massachusetts, Amherst http://prisms.cs.umass.edu/hpm

26. Turducken: What’s In The Name?