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
