Android memory management (In details)

This is now android works. The way I see this, this is very much similar to "pre-fetch" concept in windows 7.
 
 I have a 6 GB RAM laptop. Base OS uses less than 1.5 GB of RAM. But like an hour or so when I see my RAM usage, its to the tune of 3-4 GB. What I have noticed is that my most frequently/recently used apps are loaded to RAM and kept there idle. Some amount of RAM is always kept free instead of using up all RAM. This way apps start faster. When I load a different memory heavy program, it pushes the existing one out and loads this.
 
 More or less the same in android too. When u go to any task manager app n see the running apps, u'll notice that many of the apps loaded are the ones u use frequently.
 
 These apps do NOT use any CPU. They are just loaded to memory and kept there for quick access.
 
 When I boot up my phone I have like 190+ MB free RAM. Though left in standy mode, within an hour, I see my free RAM fall to 80-120 MB range. I never saw it go less than 80 MB. And the apps in memory are the ones I used the last time, and the ones I use all the time.
 
 Even if u use a task killer to kill these "inactive" apps at intervals, they would be loaded again sooner or later. That's the principle of android. So by using task killers, though u feel u r freeing up memory, in fact, u r only draining ur battery. What's the use of memory if u r not using it effectively.
 
 Don't worry abt free RAM amount. Let android manage it. Systems are intelligent enough these days.
 
 Hope this helps. Below is more about the same in detail. [/color]Android Memory Management
 
 Android is a Linux based OS with 2.6.x kernel, stripped down to handle most tasks pretty well. It uses native open source C libraries that have powered Linux machines for years. All the basic OS operations like I/O, memory management, and so on, are handled by the native stripped-down Linux kernel.
 How to use memory for each application
 
 Android’s process and memory management is a little unusual. Like Java and .NET, Android uses its own run time and virtual machine to manage application memory. Unlike either of these frameworks, the Android run time also manages the process lifetimes. Android ensures application responsiveness by stopping and killing processes as necessary to free resources for higher-priority applications.
 
 Each Android application runs in a separate process within its own Dalvik instance, relinquishing all responsibility for memory and process management to the Android run time, which stops and kills processes as necessary to manage resources.
 
 Dalvik and the Android run time sit on top of a Linux kernel that handles low-level hardware interaction including drivers and memory management, while a set of APIs provides access to all of the under- lying services, features, and hardware.
 
 Dalvik Virtual Machine Dalvik is a register-based virtual machine that’s been optimized to ensure that a device can run multiple instances efficiently. It relies on the Linux kernel for threading and low-level memory management.
 
 The Dalvik Virtual Machine
 
 One of the key elements of Android is the Dalvik virtual machine. Rather than use a traditional Java virtual machine (VM) such as Java ME (Java Mobile Edition), Android uses its own custom VM designed to ensure that multiple instances run efficiently on a single device.
 
 The Dalvik VM uses the device’s underlying Linux kernel to handle low-level functionality including security, threading, and process and memory management.
 
 All Android hardware and system service access is managed using Dalvik as a middle tier. By using a VM to host application execution, developers have an abstraction layer that ensures they never have to worry about a particular hardware implementation.
 
 The Dalvik VM executes Dalvik executable files, a format optimized to ensure minimal memory foot- print. The .dex executables are created by transforming Java language compiled classes using the tools supplied within the SDK.
 
 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 (see the attached image) 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.
 
 Active processes include:
 
 * 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.
 * Activities, Services, or Broadcast Receivers that are currently executing an onReceive event handler.
 * 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.
 
 How to use memory efficiently
 
 Android manages opened applications which are running in the background, so officially you shouldn’t care about that. This means that it closes the applications when the system needs more memory. However, most android users are not very satisfied with how it does its things because sometimes it leaves too many processes running which causes sluggishness’ in everyday performance. We can use advanced task killer/task manager and it does its job very well.
 

Source-http://mobworld.wordpress.com/2010/07/05/memory-management-in-android/

yes.

this is the way linux and unix based system manage memory, and i usually dont use task killers.

yes.

this is the way linux and unix based system manage memory, and i usually dont use task killers.

So all custom from has this feature? I have used 2 custom rom till date i.e funseries  and cyanbook. Only cyan book which is based on cynogenMod 9 has this memory management feature.

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it's a function of Linux kernel, so yes.

Custom kernels will have it even more tweaked..

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