[XPERIA X8 General] Chris95X8:
I've seen some people complaining about how low RAM they have left in their phones and others suggesting task killers to manage the RAM and save battery. Well, that is not the case. So, I've gathered some information from the web so you can understand...

How processes and activities work on Android

Activity lifecycle

An activity has essentially three states:

It is active or running when it is in the foreground of the screen (at the top of the activity stack for the current task). This is the activity that is the focus for the user’s actions.

It is paused if it has lost focus but is still visible to the user. That is, another activity lies on top of it and that activity either is transparent or doesn’t cover the full screen, so some of the paused activity can show through. A paused activity is completely alive (it maintains all state and member information and remains attached to the window manager), but can be killed by the system in extreme low memory situations.

It is stopped if it is completely obscured by another activity. It still retains all state and member information. However, it is no longer visible to the user so its window is hidden and it will often be killed by the system when memory is needed elsewhere.

If an activity is paused or stopped, the system can drop it from memory either by asking it to finish (calling its finish() method), or simply killing its process. When it is displayed again to the user, it must be completely restarted and restored to its previous state.

The foreground lifetime of an activity happens between a call to onResume() until a corresponding call to onPause(). During this time, the activity is in front of all other activities on screen and is interacting with the user. An activity can frequently transition between the resumed and paused states — for example, onPause() is called when the device goes to sleep or when a new activity is started, onResume() is called when an activity result or a new intent is delivered. Therefore, the code in these two methods should be fairly lightweight.

Taken from the Android Developers site​

Android's built-in task-manager​

What people don’t seem to realize is that android is designed to have a large number of tasks stored in memory at all times. Why? Well basically we are talking about a mobile device. On a mobile device things tend to be slower. The hardware isn’t as robust as say a desktop or a laptop, so in order to get that same “snappy” feeling, there have to be workarounds.

One of these is how android deals with memory. Android will load up your apps and then keep them running until they absolutely HAVE to kill them. This is because that way, if you want to re-open an app, the system already has it loaded and can then just resume it instead of reloading it. This provides a significant performance increase.

What a lot of people don’t realize as well is that android kernels have their own task manager. This means that:

it will be more efficient than any app-based task manager as it is run at the kernel level, and

it should be left up to that task killer to decide when to free up memory

There is only one case where having a task killer is a good idea, and that is when you want to kill ONE SPECIFIC APP. Killing all apps is never a good idea. You don’t know what operations they are performing or if they are necessary.

Said from a very experienced ROM cooker​

CPU usage matters more​

This set-up implies that the goal of killing these apps is to free up memory. Nowhere on the list does it mention the number of CPU cycles each app is consuming, only the memory you’ll free by killing it. As we’ve learned, full memory is not a bad thing—we want to watch out for the CPU, the resource that actually slows down your phone and drains your battery life.

Thus, killing all but the essential apps (or telling Android to kill apps more aggressively with the “autokill” feature) is generally unnecessary. Furthermore, it’s actually possible that this will worsen your phone’s performance and battery life. Whether you’re manually killing apps all the time or telling the task killer to aggressively remove apps from your memory, you’re actually using CPU cycles when you otherwise wouldn’t—killing apps that aren’t doing anything in the first place.

In fact, some of the processes related to those apps will actually start right back up, further draining your CPU. If they don’t, killing those processes can cause other sorts of problems—alarms don’t go off, you don’t receive text messages, or other related apps may force close without warning. All in all, you’re usually better off letting your phone work as intended—especially if you’re more of a casual user. In these instances, a task killer causes more problems than it solves.

Said from Whitson Gordon of Lifehacker​

The more RAM available, the more Android will find ways to use it up which means your battery will be dead in hours.

From droid-life.com​

So that's it. Let the lovely green robot do the job for you while you're playing games or using the xda app or whatever... :) ;)