WHY APPLICATIONS ARE STILL DRAINING OUR BATTERIES

and how we can help.

Aaron Schulman & Sachin Katti

We cannot rely on improvements in battery technology

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In past 30 years: only 4x improvement in energy density

Source: Panasonic

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We can no longer rely on shrinking transistors

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End of Dennard Scaling2005

Transistors (thousands)

Source: Horowitz et al.

Storage

We now rely on heterogeneous design and power management logic

App

Cellular

CPU

WiFi

GPSCamera

RAM Battery

Operating systemApp Hardware

Kernel Drivers

Bluetooth

Manages system-levelpower and thermals

Manages IC-levelpower and thermals

Uses hardware

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However, this makes it difficult for developers to reason about energy

Power & Thermal Management

PCU

PCU PCU PCUSource: MediaTek

Application energy regressions often appear in the wild2015

2016

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Accurately measure system-level power and attribute to apps.

Empower application developers with tools that enable them to write efficient software

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StorageApp

Bluetooth

CPU

WiFi

GPSCamera

RAM Battery

Operating systemApp Hardware

Kernel Drivers

CellularPower & Thermal

Management

PCU

PCU PCU PCU

Measurement

My work:

BattOr: Power measurement for application developers

1. Power monitor circuit

2. Power supply interceptors

3. Clock synchronization

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1011011010010000110011001100100011110011100010100011001111110000

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Insight: The designs of battery powered computing devices are amenable to power measurement

Application developer requirements for power measurement

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Power measurements must be actionable

Power must be associated with application actions

Doesn’t require hardware mods, portable, automatable

Accurate

Attributed

Practical

Today’s power monitors trade off ease-of-use for accuracy and attribution

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Monsoon - Requires AC power and battery wires must

be soldered onto device

NEAT - Requires modification of an external battery

and soldering a trigger wire

Also, neither can monitor laptops

Accurate AttributedAccurate

A portable power monitor for smartphones & laptops

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Current & Voltage Measurement

Storage

USB

Power supply connection

AccuratePractical

Insight: Power measurement is an ideal application for embedded systems

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Intercepting a smartphone’s battery connection without soldering

Insight: Many smartphones have the same battery connectorsPhoneBattery

Practical

Camera flash-based clock sync

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Attributed

Insight: Clock sync markers can be modulated into power measurements

Demo: BattOr is built into Google Chrome

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Can BattOr find energy bugs in real world applications?

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Diagnosing an energy bug in Chrome-for-Mac’s graphics stack

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Apr. 2015

133%

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Apr. 2015

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125%

API fix

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114%

Partial swap

Teaching Stanford EE students about embedded systems by building BattOrs

Atmel 8-bit microcontroller (integrated ADC)

Digitally controlled potentiometers

1 Mbit/s UART

Instrumentation Amplifier

LEDs

16 MHz Crystal

- Synchronous serial

- Microcontroller arch. - Firmware development

- Analog circuits

- GPIO

- Clocks- Timing

- Asynchronous serial2”x3” PCB- PCB Design - SMD soldering

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Stanford’s EE107 students used their BattOrs to compare similar Android apps

A student’s BattOr connected to a Nexus 5 smartphone

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Podcast

Republic

Player FM

Podcast &

Radio Addict

1.34x

Why is their energy consumption different?

Podcast & Radio Addict

Player FM

Player FM decodes and add effects to the audio in software

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Datacenters need energy debugging tooEfficiency gains from improving cooling and power distribution are diminishing

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PUE - ratio of total data center energy to computing energy

Source: Google

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?

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