Android is one of the mostly widely used operating systems in the world. It is a feature rich system that can be used in a wide range of application areas like CE devices, industrial HMI’s, display interface for medical devices etc. Generally the boot time of the Android – from power up to showing the first screen is a time consuming process. In this series of posts, we will discuss about some of the tools and techniques for a fast Android boot.
This post is split in three parts:
- Android boot process
- Android boot time analysis
- Android boot optimization techniques
Now in this post, we will explain about the android boot process and its internals.
Android Boot Process
Understanding of the boot process of the target platform is the starting point for the optimizing the boot time. Generally on any platform, following components are loaded and executed step by step
- Boot loader
- U-boot (optional)
The Android process has the following sequence
- System Server
- Service Manager
- Other Daemons and processes
The following diagram depicts the boot process.
Up on power up, the processor boots from a ROM area typically located internally. This code determines the boot media and loads the boot loader from the media. The boot loader can be used to initialize the DRAM and load another level of loader or directly the Linux kernel. IT is generally dependent on the processor architecture and implementation.
U-Boot is used as a first or second level boot loader. It reads the Linux and ramdisk images from boot media and validates them. While it may not be mandatory to use u-boot, it offers some flexibility like passing arguments to the kernel (easily used to enter the recovery mode), fast boot modes, as a backup option to reprogram the OS etc.
Linux kernel is the heart of the Android responsible for the process creation, inter process communication, device drivers, file system management etc. Android applies a custom patch on the main stream kernel to support certain features like Wake locks etc needed for operation of the Android.
The kernel can be either loaded as uncompressed image or as a compressed image. Up on loading, it mounts the root file system (typically passed as kernel command line arguments) and starts the first application in user space.
Android typically operates wholly on the user space. The android applications are executed over a Virtual Machine called the Dalvik. The following section explains the internals in detail.
init and init.rc
The first user space application executed on booting the kernel is the init executable located in the root folder. The process parses a start up script called the “init.rc” script. This is written in a language designed for android used to start all the necessary processes, daemons and services for a proper operation of android. It offers various types of execution timings such as early-init, on-boot, on-post-fs etc. A detailed explanation of the scripting model is available on Android documentation site.
Demons and Services
The init process creates various daemons and processes like rild, vold, mediaserver, adb, etc each responsible for its own functionality. Descriptions of these processes are not in the scope of this post. Rather we will discuss more about “Zygote” process.
The service manager process manages all the services running in the system. Every service created registers itself with this process and this information is used for future references by other processes/applications.
Zygote is one of the first init process created on boot. The term “zygote” is based the biological “initial cell formed that divides to produce offsprings”. Similarly “zygote in android” initializes the Dalivik VM and forks to create multiple instances to support each android process. It facilitates using a shared code across the VM instances resulting in a low memory foot print and short load time, ideal for an embedded system.
Zygote apart from installing a listener on the server socket, also preloads classes and resources to be used later in the Android applications. Once done, the system server is started.
Android System services (SystemServer)
SystemServer process starts all services available in the Android. Some of them are described below.
||Manages activities life cycle and new services
||Manages application package handling (install, uninstall, upgrade, permissions)
||Manages all the window manipulations (like input events, orientation).
||Handles Android widgets
||Manages backup scheduling and transfer
||Shows software/hardware status. It works with other managers like Notification, Network Status, Battery Status
||Handles power management while Android’s different modes (lock mode, sleep mode, Adjust brightness)
||Deals with network related activities
||Manage all notifications (Toasts)
||Manages location providers
||Handles (load & save periodically) kernel randomness
||Manages display properties
||Provides telephony information
||Manages the process scheduling
||Handles the users account credential of different online services
||Handles all the data’s on a device
||Manages battery level and charging states
||Used to schedule the user applications to be run at future.
||Handles input devices and key layouts
||Enforces security policies for the device
||Provides Clipboard based copy/past operations.
||Monitors Network connection Status
||Enforces network security policies.
||Handles WiFi peer to peer connection
||Manages Ethernet connectivity.
||Manage WiFi connectivity
||Monitors and handles network connection state changes
|Network Service Discovery Service
||Used to find local network devices to share app data
Once all the services are started and are executing, the Android broadcasts a “ACTION_BOOT_COMPLETED” message implying end of the boot process.
Android Home Screen
The Android package Manager on start up, parses each package (“.apk” file) available in the “/system/app” and “/system/vendor/app” and validates its AndroidManifest.xml. The application that is configured as the “Home” in its manifest is launched there by showing the android UI. Typically the Launcher application is launched as it is the default home application.
Generally this whole process may take around 25 to 60 seconds depending up on various factors to be discussed in later posts. Now with the boot process understood, we will look in to the tools available for profiling the boot activity in the next post – Android Boot Time Analysis.