Android is a multiuser, multitasking system that can run multiple applications in parallel, where all the applications attempt to acquire CPU time to execute its job.
Each application runs independently on an isolated Linux process cloned from the Zygote process, and by default, all the Android components run within the same process with the same name as the application package specified in Android Application Manifest (AAM).
The Linux kernel will fairly allocate small amounts of CPU time for application execution called CPU time slices. This time-slicing approach means that even a single-processor device can appear to be actively working in more than one application at the same time, when in fact, each application is taking very short turns on the CPU.
The Android operating system tries to maintain the application running for as long as possible, but when the available memory is low, it will try to free resources in the system by killing the processes with lower importance first.
This is when process ranking comes into the picture; the Android processes are ranked in the next five categories from the higher priority to the lower priorities:
Foreground process: This is a process that hosts an activity or service that the user is currently interacting with: a service started in the foreground or service running its life cycle callbacks
Visible process: This is a process that hosts a paused activity or service bounded to a visible activity
Service process: This is a process that hosts a service not bound to a visible activity
Background process: This is a process that hosts a non-visible activity; all background processes are sorted over a Least-Recently-Used (LRU) list, therefore, the most recently used processes are the last killed processes when they have the same rank
Empty process: This is a process used to cache inactive Android components and to improve any component startup time
When the system reaches a point that it needs to release resources, the processes available to be killed will be sorted, taking into account the process rank, last used processes, and components running.
The Android application always runs under a unique Linux user ID (UID) assigned to the application during the application installation so that the process runs on a sandboxed environment, which by default, isolates your data and code execution from other apps.
In some cases, a user could explicitly be required to share the UID with another application to have access to their data:
USER PID PPID VSIZE RSS PC NAME root 319 1 1537236 31324 S zygote …. u0_a221 5993 319 1731636 41504 S com.whatsapp u0_a96 3018 319 1640252 29540 S com.dropbox.android u0_a255 4892 319 1583828 34552 S com.accuweather.android…
The preceding table that results from running the adb shell ps command in the computer with Android SDK Table is a list of Android running processes.
The first column shows the user identifier (UID) assigned at the time of installation, the second column is the process ID (PID), the third column shows the parent process ID (PPID) that for Android applications is the Zygote process, and the last column shows the application package.
From this list, we can assure that the WhatsApp application is running under the user ID u0_a221 with the process ID 5993 and the parent process is the Zygote process with the PID 319.