104: Power Struggles

iOhYes

Discussion - Energy Efficiency for iOS Apps

• Apple’s Energy Efficiency Guide

Are you telling me I have to worry about my app’s power consumption?

• This is iOS. I thought Apple was taking care of that for me...right??

What is Energy?

• Power is an instantaneous measurement of energy at any given point in time
• Energy is power used over time (Joules measured over watt-hours)
• Low power used over a long(er) period of time can amount to the same energy expenditure as short bursts of high power (more on this later)

Major sources of energy consumption/power draw

• Device wake
  • Aside from being powered off, a device in its sleep state is consuming the lowest amount of energy possible
  • Whenever possible, avoid preventing the device from sleeping or forcing it to awaken
  • Use technologies like push and background tasks judiciously
• CPU Usage
  • An idle CPU uses ~10x the power of a sleeping CPU
  • Just 1% CPU use costs 10% more than idle
  • 10% CPU use costs 200% that of an idle CPU
  • 100% CPU can result in 1000% (10x) power draw compared to idle
• Networking / Bluetooth
• Graphics/animation/video
• Sensors
  • Location (Wi-Fi/GPS)
  • Accelerometer
  • Gyroscope
  • Magnetometer
• Disk IO. Use batch operation whenever possible

Mitigating energy costs

• Batching, Batching, Batching
  • Operations have a dynamic and fixed energy cost
    • Fixed cost represents the energy used while the device is waiting to enter an idle state
    • The same amount of work, performed across multiple threads, can have a significantly lower total energy cost compared against single-threaded work requiring a longer time to execute
  • Network and other inter-device (BT, for example) operations require radios to be powered up. Avoid continuous communications and state polling, batching operations whenever possible
  • Defer any operations that aren’t time-sensitive to time that the app will be otherwise active (take advantage of fixed cost you’re already having to pay)
• Prioritize operations using Quality of Service Classes (iOS 8+)
  • Classes
    • User-interactive
    • User-initiated
    • Utility
    • Background
  • Can be set on both NSOperationQueue and individual NSOperation objects
  • GCD queues can be created with QOS class attributes
• Use timers efficiently, or better yet, avoid them
  • GCD provides mechanisms you can use instead of timers, for example dispatch_block_wait()
  • If you must use timers (not just NSTimer: basically anything that takes a time interval as a deadline), take advantage of APIs that allow for timer coalescing using tolerances.
• Minimize I/O
• React to Low Power Mode (iOS 9)
  • NSProcessInfoPowerStateDidChangeNotification
  • [[NSProcessInfo processInfo] isLowPowerModeEnabled]

Instruments to the Rescue

• Energy Diagnostics Logging

Other Resources

• Performance Tips from Apple

Picks

John

• ‘Operator’ font created by Hoefler & Co., a font design company

Darryl

• Achieving All-day Battery Life

Alternative show title suggestions

• Power Draw
• Contribute to Sleep
• Don’t write an app
• (You should) Get off of the main thread
• They DID bone it
• I ripped them off