104: Power Struggles

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