WHY APPLICATIONS ARE STILL DRAINING OUR BATTERIES and how we can help. Aaron Schulman & Sachin Katti

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Charla Moore
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1 WHY APPLICATIONS ARE STILL DRAINING OUR BATTERIES and how we can help. Aaron Schulman & Sachin Katti

2 We cannot rely on improvements in battery technology In past 30 years: only 4x improvement in energy density Source: Panasonic 2

3 We can no longer rely on shrinking transistors End of Dennard Scaling 2005 Transistors (thousands) Source: Horowitz et al. 3

Drivers Camera GPS Storage Uses hardware Power & Thermal Management Manages system-level power and

4 We now rely on heterogeneous design and power management logic However, this makes it difficult for developers to reason about energy App Operating system Hardware CPU RAM Battery PCU App Kernel Drivers Camera GPS Storage Uses hardware Power & Thermal Management Manages system-level power and thermals Bluetooth WiFi Manages IC-level power and thermals Cellular Source: MediaTek PCU PCU PCU 4

5 Application energy regressions often appear in the wild

6 My work: Empower application developers with tools that enable them to write efficient software Measurement App Operating system Hardware CPU RAM Battery PCU App Kernel Drivers Camera GPS Storage Power & Thermal Management Cellular WiFi Bluetooth PCU PCU PCU Accurately measure system-level power and attribute to apps. 6

7 BattOr: Power measurement for application developers Insight: The designs of battery powered computing devices are amenable to power measurement 1. Power monitor circuit 2. Power supply interceptors Power (W) Time (ms) 3. Clock synchronization 7

8 Application developer requirements for power measurement Accurate Power measurements must be actionable Attributed Power must be associated with application actions Practical Doesn t require hardware mods, portable, automatable 8

device NEAT - Requires modification of an external battery and soldering

9 Today s power monitors trade off ease-of-use for accuracy and attribution Monsoon - Requires AC power and battery wires must be soldered onto device NEAT - Requires modification of an external battery and soldering a trigger wire Accurate Accurate Attributed Also, neither can monitor laptops 9

for embedded systems USB Power supply connection

10 A portable power monitor for smartphones & laptops Insight: Power measurement is an ideal application for embedded systems USB Power supply connection Current & Voltage Measurement Accurate Storage Practical 10

11 Intercepting a smartphone s battery connection without soldering Practical Insight: Many smartphones have the same battery connectors Phone Battery 11

12 Camera flash-based clock sync Attributed Insight: Clock sync markers can be modulated into power measurements 12

13 Demo: BattOr is built into Google Chrome 13

14 Can BattOr find energy bugs in real world applications? 14

15 Diagnosing an energy bug in Chrome-for-Mac s graphics stack Power (W) Apr Time (msec) Power (W) Apr Power (W) API fix Power (W) Partial swap Time (msec) Time (msec) Time (msec) 133% 125% 114% 15

Teaching Stanford EE students about embedded systems by building BattOrs Instrumentation Amplifier - Analog circuits Digitally controlled potentiometers - Synchronous serial - GPIO LEDs 16 MHz

16 Teaching Stanford EE students about embedded systems by building BattOrs Instrumentation Amplifier - Analog circuits Digitally controlled potentiometers - Synchronous serial - GPIO LEDs 16 MHz Crystal - Clocks - Timing Atmel 8-bit microcontroller (integrated ADC) - Microcontroller arch. - Firmware development 2 x3 PCB - PCB Design - SMD soldering 1 Mbit/s UART - Asynchronous serial 16

17 Stanford s EE107 students used their BattOrs to compare similar Android apps Podcast & Radio Addict 1.34x Podcast Republic Player FM A student s BattOr connected to a Nexus 5 smartphone 17

18 Why is their energy consumption different? Player FM decodes and add effects to the audio in software Podcast & Radio Addict Player FM 18

19 Datacenters need energy debugging too Efficiency gains from improving cooling and power distribution are diminishing Source: Google PUE - ratio of total data center energy to computing energy 19

20 Future work: BattOr in the data center Vision: Use BattOrs to develop efficient cluster computing applications Likely many energy bugs in cluster computing applications Can we produce a global trace of the energy consumed across all servers? How do we design cluster applications that are optimized for energy? 20

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