Why does droid charge only have 30 - 150 megabytes of free memory?
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The article below was actually written as an explanation of why 3rd party task killers are not needed, but it give a pretty good explanation of how the OS manages ram:
How Android Manages Processes
In Android, processes and Applications are two different things. An app can stay "running" in the background without any processes eating up your phone's resources. Android keeps the app in its memory so it launches more quickly and returns to its prior state. When your phone runs out of memory, Android will automatically start killing tasks on its own, starting with ones that you haven't used in awhile.
The problem is that Android uses RAM differently than, say, Windows. On Android, having your RAM nearly full is a good thing. It means that when you relaunch an app you've previously opened, the app launches quickly and returns to its previous state. So while Android actually uses RAM efficiently, most users see that their RAM is full and assume that's what's slowing down their phone. In reality, your CPU—which is only used by apps that are actually active—is almost always the bottleneck.
Why Task Killers Are (Usually) Bad News
Apps like Advanced Task Killer, the most popular task killer in the Market, act on the incorrect assumption that freeing up memory on an Android device is a good thing. When launched, it presents you with a list of "running" apps and the option to kill as many as you want. You can also hit the Menu button to access a more detailed "Services" view, that lists exactly which parts of each application are "running", how much memory they take up, and how much free memory is available on your phone. 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.
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