Android Process Scheduling

The following list presents the different types of processes in order of importance (the first process is most important and is killed last):

  1. Foreground process
  2. Visible process
  3. Service process
  4. Background process
  5. Empty process

Note: Android ranks a process at the highest level it can, based upon the importance of the components currently active in the process. For example, if a process hosts a service and a visible activity, the process is ranked as a visible process, not a service process.

This is referenced from here Processes and Threads


Understanding Application Priority and Process States

The order in which processes are killed to reclaim resources is determined by the priority of the hosted applications. An application’s priority is equal to its highest-priority component.

Where two applications have the same priority, the process that has been at a lower priority longest will be killed first. Process priority is also affected by interprocess dependencies; if an application has a dependency on a Service or Content Provider supplied by a second application, the secondary application will have at least as high a priority as the application it supports.

All Android applications will remain running and in memory until the system needs its resources for other applications.

It’s important to structure your application correctly to ensure that its priority is appropriate for the work it’s doing. If you don’t, your application could be killed while it’s in the middle of something important.
The following list details each of the application states shown in Figure , explaining how the state is determined by the application components comprising it:

Active Processes Active (foreground) processes are those hosting applications with components currently interacting with the user. These are the processes Android is trying to keep responsive by reclaiming resources. There are generally very few of these processes, and they will be killed only as a last resort.

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Active processes include:

1.Activities in an “active” state; that is, they are in the foreground and responding to user events. You will explore Activity states in greater detail later in this chapter.

2.Activities, Services, or Broadcast Receivers that are currently executing an onReceive event handler.

3.Services that are executing an onStart, onCreate, or onDestroy event handler.

Visible Processes Visible, but inactive processes are those hosting “visible” Activities. As the name suggests, visible Activities are visible, but they aren’t in the foreground or responding to user events. This happens when an Activity is only partially obscured (by a non-full-screen or transparent Activity). There are generally very few visible processes, and they’ll only be killed in extreme circumstances to allow active processes to continue.

Started Service Processes Processes hosting Services that have been started. Services support ongoing processing that should continue without a visible interface. Because Services don’t interact directly with the user, they receive a slightly lower priority than visible Activities. They are still considered to be foreground processes and won’t be killed unless resources are needed for active or visible processes.

Background Processes Processes hosting Activities that aren’t visible and that don’t have any Services that have been started are considered background processes. There will generally be a large number of background processes that Android will kill using a last-seen-first-killed pat- tern to obtain resources for foreground processes.

Empty Processes To improve overall system performance, Android often retains applications in memory after they have reached the end of their lifetimes. Android maintains this cache to improve the start-up time of applications when they’re re-launched. These processes are rou- tinely killed as required.

For more info look at here(I found on this blog) Memory Management in Android


I think Android is basic Linux so, whatever scheduler works for Linux is same in Android. 

The difference between Android scheduler and Linux scheduler

Scheduler — 5 files — The Android kernel also contains slight changes to the CPU process scheduler and time-keeping algorithms. We don’t know the history of these changes, and the impact was not evident based on a cursory examination.

Process Preemption:

As mentioned, the Linux operating system is preemptive. When a process enters the TASK_RUNNING state, the kernel checks whether its priority is higher than the priority of the currently executing process. If it is, the scheduler is invoked to pick a new process to run (presumably the process that just became runnable). Additionally, when a process’s timeslice reaches zero, it is preempted, and the scheduler is invoked to select a new process.

The Scheduling Policy in Action

Consider a system with two runnable tasks: a text editor and a video encoder. The text editor is I/O-bound because it spends nearly all its time waiting for user key presses (no matter how fast the user types, it is not that fast). Despite this, when it does receive a key press, the user expects the editor to respond immediately. Conversely, the video encoder is processor-bound. Aside from reading the raw data stream from the disk and later writing the resulting video, the encoder spends all its time applying the video codec to the raw data. It does not have any strong time constraints on when it runs—if it started running now or in half a second, the user could not tell. Of course, the sooner it finishes the better.

In this system, the scheduler gives the text editor a higher priority and larger timeslice than the video encoder, because the text editor is interactive. The text editor has plenty of timeslice available. Furthermore, because the text editor has a higher priority, it is capable of preempting the video encoder when needed. This ensure the text editor is capable of responding to user key presses immediately. This is to the detriment of the video encoder, but because the text editor runs only intermittently, the video encoder can monopolize the remaining time. This optimizes the performance of both applications.

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