1. Rethinking OS Design Metrics Workload Hardware Resources Services & API Internal Structure Policies / Mechanisms Energy efficiency Processor, Memory, Disks (?), Wireless & IR, Keyboard(?), Display(?), Mic & Speaker, Motors & Sensors, GPS, Camera, Batteries Productivity applications Process control Personal (PDAs), Embedded You are here

2. Energy Efficiency Metrics Power consumption in watts (mW). Battery lifetime in hours (seconds, months). Energy consumption in Joules (mJ). Energy  Delay Watt per megabyte

3. Physics Basics Voltage is amount of energy transferred from a power source (e.g., battery) by the charge flowing through the circuit. Power is the rate of energy transfer Current is the rate of flow of charge in circuit

4. Relationships Power (watts) = Voltage (volts) * Current (amps) Power (watts) = Energy (Joules) / Time (sec) Energy (Joules) = Power (watts) * Time (sec) Energy (Joules) = Voltage (volts) * Charge (coulombs) Current (amps) = Voltage (volts) / Resistance (ohms)

5. Terminology and Symbols ConceptSymbolUnitAbbrev. Current Iamperes A Charge Qcoulombs C Voltage Vvolts V Energy Ejoules J Power Pwatts W Resistance Rohms 

6. Relationships Energy (Joules) = Power (watts) * Time (sec) E = P * t Power (watts) = Voltage (volts) * Current (amps) P = V * I Current (amps) = Voltage (volts) / Resistance (ohms) I = V / R

7. Battery Terminology Primary (non-reusable) and Secondary (rechargable) Voltages: V oc (initial no-load) V (operating voltage under load) V cut (cut-off when cell is considered discharged - 80% of V oc ) Capacity expressed in amp-hours theoretical - based on amount of material in cell nominal - based on amp-hours obtained when discharged at constant current until V cut

8. Battery Terminology Discharge time - elapsed time until a fully charged cell reaches V cut C rate - discharge current expressed in amps relative to nominal capacity –example: for a lead acid battery with nominal capacity of 5Ah, a discharge rate of C/20 means 250mA of current. Specific energy - Watt-hours per kilogram delivered at constant discharge Energy density of cell - Watt-hours per liter

9. Battery Technology

10. Discharge behavior of lithium-ion cell with V oc = 3V and V cut = 1V Discharge Behavior

11. Battery Stuff Diffusion: At non-zero current, active material at electrode-electrolyte interface are consumed and replaced by new stuff moving in Polarization as current increases; At high enough current, diffusion is unable to compensate for depletion at electrode and cell voltage drops Recovery (due to diffusion) when current decreased

12. Ragone plot for different chemistries

13. Pulsed Discharge Exploiting recovery ability to get more out of a battery Delivered specific energy can be increased by pulsed instead of constant discharge for a fixed power level. [Chiasserini and Rao 99] - model & analysis Is bursty better for battery lifetimes? –Can durations of idle and busy states be optimized?

14. Pulsed Discharge Bipolar lead acid cell Pulse = 3ms Rest = 22ms

15. Smart Batteries Part of Intel Power Initiative Embedded battery controller that can be controlled by OS. Interface –Battery reports designed capacity, latest full charged capacity, remaining capacity. –Warning levels can be set. User notifications

16. Rethinking OS Design Metrics Workload Hardware Resources Services & API Internal Structure Policies / Mechanisms Energy efficiency Processor, Memory, Disks (?), Wireless & IR, Keyboard(?), Display(?), Mic & Speaker, Motors & Sensors, GPS, Camera, Batteries Productivity applications Process control Personal (PDAs), Embedded You are here

17. I/O Bus Memory Bus Processor Cache Main Memory Disk Controller Disk Graphics Controller Network Interface Graphics Network interrupts System Organization I/O Bridge

18. I/O Bus Memory Bus Processor Cache Main Memory Disk Controller Disk Graphics Controller Network Interface Graphics Network interrupts Power Budgets I/O Bridge [Lorch95] 4-17% ave. 9% +backlight 23% 4-12% ave. 8% ave 18% appox 20%

19. Typical Notebook Power Budgets [Harris 95] Watts 2 4 6 8 CPU mem video HDD DC-DC B/W 1993 notebook full power (Color to 21W)

20. What are the Costs? Measured Power Consumption (PalmPilot Pro - 1997 model) Sleeping in cradle CPU Idle Hotsync Memoryintensive CPU Event Loop (nilevents) Backlight

21. CPU/Memory [Tiwari94] 486DX2 Instr current (mA) NOP276 Load428 Store522 Register add314 cache miss216 Memory op current (mA) no access5-77 page hit123 page miss248

22. Intel Power Initiative Targets

23. I/O Bus Memory Bus Processor Cache Main Memory Disk Controller Disk Graphics Controller Network Interface Graphics Network interrupts Power Budget Targets I/O Bridge [Intel targets] 4% 33% 2- 3% 13% 10% 8%

24. Itsy Measurement Methodology I sense = V sense /.02 Sampling rate: 5000 per second

25. Itsy Results

26. PowerScope [Flinn] Statistical sampling approach –Program counter/process (PC/PID) + correlated current readings. –Off-line analysis to generate profile Causality –Goal is to assign energy costs to specific application events / program structure –Mapped down to procedure level –System-wide. Includes all processes, including kernel

27. Experimental Setup Data Gathering Multimeter’s clock drives sampling at period of 1.6ms  Trigger Profiling computer Takes current sample  Interrupt causes PC/PID sample to be buffered User-level daemon writes to disk when buffer 7/8 full  Trigger next sample

28. System Monitor Kernel Mods NetBSD recording of PC and PID fork(), exec(), exit() instrumented to record pathname associated with process new system calls to control profiling pscope_init(), pscope_start(), pscope_stop(), pscope_read() (user-level daemon, to disk)

29. Voltage essentially constant, only current recorded. Each sample is binned into process bucket and procedure within process bucket. Energy calculated by summing each bucket E  V meas S I t D t Energy Analyzer t=0 n

31. Case Study Video application original 12.1MB Step 1 lossy compression B: 7MB, C: 2.8MB Step 2: display size reduced from 320x240 to 160x120 A small : 4.9MB, C small : 1MB Step 3: WaveLAN put into standby mode when not used Step 4: Disk powered off

32. Base case Every optimization

33. Energy = S Power i x Time i To reduce energy used for task: –Reduce power cost of power state i through better technology. How to Reduce Energy Consumption? i e power states

34. Opportunities for Lower Power through Technology Circuits Gated Clocks - disable functional units that are not in use for particular instruction –Compile for Voltage Scaling For given circuit: –E is related to V 2 and time, f(clockrate) –Linear relationship between V and clockrate –Ability of software to dynamically change?

35. Displays Active Matrix LCDs –90% of backlight gets transmitted through the layers of display Possible future technologies –Reflective displays use ambient light 1/50th energy of active matrix –Field-emission displays uses an array of cathodes for each pixel instead of one gun as in CRT displays. Selective activation possible.

36. Energy = S Power i x Time i To reduce energy used for task: –Reduce power cost of power state i through better technology. –Reduce time spent in the higher cost power states. How to Reduce Energy Consumption? i e power states

37. Power Modes of HW Devices Busy transition Idle transition Low power cost High power cost ? ?

38. Energy = S Power i x Time i To reduce energy used for task: –Reduce power cost of power state i through better technology. –Reduce time spent in the higher cost power states. –Amortize transition states, if significant. How to Reduce Energy Consumption? i e power states

39. StrongARM Processor Power Modes 160MHz microprocessor, 2 16kB caches on chip Normal active mode: 450mW Idle mode: 20mW, return to normal, no delay. –Internal clocking stopped Sleep mode: 150  W, return to normal 140  s –Internal power to chip off, I/O circuitry remains powered, no state saved

40. Standby 0.6x mW Active 1.0x mW Nap 0.1x mW PwrDown.01x mW 0.1x ns 100x ns 1.0x ns Read/write transaction Rambus RDRAM Power Modes

41. spinup

42. Wireless LAN Power Modes

43. Bluetooth Freq Hop Radio nominal range 10 meters –augmentable to 100 meters with power amplifier 721 kbits/sec Adaptive range- RSSI (received signal strength indicator) Listen every 1.28 sec.

44. Rethinking OS Design Metrics Workload Hardware Resources Services & API Internal Structure Policies / Mechanisms Energy efficiency Processor, Memory, Disks (?), Wireless & IR, Keyboard(?), Display(?), Mic & Speaker, Motors & Sensors, GPS, Camera, Batteries Productivity applications Process control Personal (PDAs), Embedded You are here