notes/docs/lectures/mdp/15_power_management.md

Power Management

Power Saving

• Knowledge about the power consumption of each component and app
• Screen / Network / CPU off or release other device resources as soon as not needed
• Limit resources for less frequently used apps
• Set device to sleep as soon as there are no user interactions
• Global management of running jobs across all apps
  • Perform syncs, uploads & downloads together in a fixed time window

Battery Consumption Statistics

• Framework tracks the time that devices spend in different states
  • e.g. WiFi chip-set: on/off
  • Display: low/high brightness
• Controlling service pushes state changes to BatteryStats service
• Framework pulls the data at these transition points
• App power consumption is calculated based on CPU run time at specific speeds

Android Power Management Concept

• G0 - working
• G1 - sleeping
  • S1 - CPU stops executing instructions, power to CPU and RAM maintained
  • S2 - CPU powered off, cache is flushed
  • S3 - Standby / sleep / suspend power to RAM
  • S4 - Hibernate, suspend disk, RAM powered off
• G2 (S5) - soft off
• G3 - mechanical off

Power Management Design

• A wrapper to Linux Power Management
• Added to the Kernel
  • Wake lock mechanism
• Apps need to request the CPU & screen to be on with WakeLocks; otherwise, Android will shut down the CPU
• Wake locks and timeouts constantly switch the state of the system’s power
  • Overall system power consumption decreases
  • Better use of battery capacity

WAKE_LOCK

Firstly, request the WAKE_LOCK permission in the manifest

<uses-permission android:name="android.permission.WAKE_LOCK"/>

Use newWakeLock() to create a PowerManager.WakeLock object

PowerManager pm = (PowerManager) getSystemService(Context.POWER_SERVICE);
PowerManager.WakeLock wl = pm.newWakeLock(PowerManager.SCREEN_DIM_WAKE_LOCK, 							"My Tag");
wl.acquire();
..screen will stay on during this section..
wl.release();

Flag Value	CPU	Screen	Keyboard
PARTIAL_WAKE_LOCK	On (Don’t sleep even when power button is pressed)	Off	Off
SCREEN_DIM_WAKE_LOCK	On	Dim	Off
SCREEN_BRIGHT_WAKE_LOCK	On	Bright	Off
FULL_WAKE_LOCK	On	Bright	Bright

Creating and holding wake locks can have a dramatic impact on the host device's battery life. Thus you should use wake locks only when strictly necessary and hold them for as short a time as possible.

• If app is performing long-running HTTP downloads, consider DownloadManager
• If app is synchronising data, consider sync adapter
• If your app relies on background services, consider JobScheduler to trigger services at specific intervals
• One use case for wake lock might be a background service that needs to grab a wake lock to keep the CPU running to do work while the screen is off.

Battery Life Enhancement in Android 6.0+

App Standby

Defer background activity for apps with no recent user interaction

Start Conditions:

An app that is not actively used for a certain time will be placed in an idle mode

• Not doing foreground work (activities, services, pending intents, notifications)
• Hasn’t been explicitly launched for a certain time (can be days)

Actions

• No network access
• No background jobs
• Can set alarms
• Can use wake locks
• Is allowed network access once per day

Exit conditions

• When the device is plugged in
  • Notice when your phone is plugged in, the screen will stay on for longer
• When a foreground task is performed by the user

Doze

A deep sleep if the user has not actively used the device for extended periods of time

Starts Doze

• When device is idle, screen is off, on battery & stationary

In doze

• No network access
• No CPU-intensive work
• Wake locks ignored
• No Wi-Fi scan
• Deferred alarm manager alarms
• Only high-priority notifications received
• No job scheduler
• No sync adaptors

Exits Doze

• User interaction
• Device motion
• Screen on
• Alarm clock alarm
• Notifications do not cause Doze exit

Maintenance windows

Maintenance windows complete pending activities (syncs, jobs, etc.)

• SMS & Telephony services are excluded from Doze.

Exemptions

System apps and cloud messaging services pre-loaded on the phone are exempt from App Standby and Doze

Uses whitelist for apps to be partially exempt from Doze & App standby

• isIgnoringBatterOptimizations() to check if it's in the whitelist
• ACTION_IGNORE_BATTERY_OPTIMIZATION_SETTINGS intent to redirect user to battery optimisation options
• REQUEST_IGNORE_BATTERY_OPTIMIZATIONS allow user to add app to white-list directly
• White-listed apps can use network and hold partial wake locks during Doze and App standby, but jobs and syncs are still deferred
• The app should not be on the whitelist unless it cannot use Firebase Cloud Messaging, high-priority messages or the app’s core functionality is affected.

App Stand-By Buckets

Prioritise apps based on how recently and how frequently the apps are used

• Active: currently or recently used
• Working set: regular use
• Frequent: often used but not every day
• Rare: not frequently
• Never: never

Firebase Cloud Messaging

Replaced Google Cloud Messaging

• A cross-platform reliable battery-efficient messaging solution that allows messages to be delivered between the server and client.
• Provides a single and persistent connection to the server.
• Can notify a client app that a new email or other data is available to sync
• Can deliver to single device, groups of devices or devices subscribed to topics.
• Send messages from client apps to the server (e.g. chats, messages, etc.)

Sustained Performance Mode

• Thermal throttling prevents long-running apps from maintaining their performance (e.g. games, cameras, VR, etc.)
• App can request to enter SPM, and keep a consistent level of performance
  • Set using Window.setSustainedPerformanceMode()
• System automatically disables the mode when no longer in focus

Power 2

Background processes

• Threads
• Services
• AsyncTasks
• AlarmManager
  • Based on time
• JobScheduler
  • Based on conditions
• WorkManager
  • Requires Google Play service
  • Automatic use of alarmManager and jobScheduler

The bottom three will guarantee completion when we are not worried about when it will happen

Alarm Manager

• Schedule a task to run at a specific time point or time interval
• AlarmManager holds a CPU wake lock, when the onReceive() method is executing
• Alarm manager operates outside of your application, so it can be used to trigger events when the app is not running or the device is asleep.
• Alarm delivery is inexact
  • Use setWindow and setExact for exact delivery

Usage

• Specify an intent to be broadcast at some time
  • setRepeating(int type, long triggerAtMillis, long intervalMillis, PendingIntent operation)
  • setExact(int type, long triggerAtMillis, PendingIntent operation)
• Extend Broadcast receiver
  • onReceive method is called when the alarm goes off
  • Start a service to actually do the work

Job Scheduler

• Many apps perform tasks asynchronously outside the main activity (downloading files)
• Job Scheduler collects pending jobs across all apps and schedules them to run at about the same time, so that the phone can sleep for longer and save more power
• Specify requirements for network and timing for each job, then the JS robustly optimises the execution time.
• May defer jobs that comply with Doze and App StandBy
• JobService will run on the main thread; you need to manage any asynchronous tasks yourself (using threads)

Usage

• Construct JobInfo objects using JobInfo.Builder using schedule conditions
  • Network type (metered/unmetered)
  • Charging and Idle
  • Content Provider update
  • Back-off (retry) criteria
  • Minimum latency and override deadline
  • Periodic
  • Persistent
  • Extras (customised information from app)
• Pass it to JobScheduler using schedule (JobInfo)
• Jobs implemented in the app’s JobService
• Jobs will be executed (batch & defer jobs smartly) at a certain time, but not an exact time (minimum period length is 15 minutes for periodic jobs)

Work Manager

• Guaranteed but deferrable
• Supports one-off or periodic tasks
• Queryable - check if work is successful
• Chainable: Job 2 is dependent on the execution of job 1
• Opportunistic: do the background work as soon as it can

When to use work manager

✓ Upload media to a server

✓ Parse data and store it in a database

✓ Periodically sync local data with the network

✗ Extract paint colour and update image views (use thread pools)

✗ Parse data and update contents of a View (use thread pools)

✗ Process payment transactions (use a foreground service)

Usage

• Add dependencies

dependencies {
implementation "androidx.work:work-runtime:$versions.work"
}

• Work Request

mWorkManager = WorkManager.getInstance(application);
mWorkManager.enqueue(OneTimeWorkRequest.from(Worker.class));

• Chain workers

WorkContinuation continuation = mWorkManager.beginWith(workA);
continuation.then(workB).then(workC).enqueue(); 

• Set constraint

Constraints constraints = new Constraints.Builder()
    			.setReuiresCharging(false)
    			.build();