COMPUTER RESOURCES Keep your Mac lively by avoiding memory fragmentation.

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Regardless of how much RAM you've installed in your Mac, at times you won't have quite enough to do the job at hand. At one time or another, you've probably tried to open just one more document and been prompted to close some application because you're out of memory. However, you may find that quitting an application still won't free up enough memory for some operations. It can be frustrating to close applications only to discover that you still don't have enough RAM. In fact, you may close an application only to find that you have little or no more memory than you had before you closed the application.

This is caused by a condition called memory fragmentation. Memory fragmentation occurs when quitting one application doesn't free up enough memory for another application to use—leaving a chunk of wasted memory.

What is memory fragmentation: Memory fragmentation results from the way your Mac assigns memory. As we load an application, the system assigns that application to the next available block of memory large enough to hold it. Therefore, if you open a large application, then a small application followed by another large one, quitting the small application won't provide usable memory for any application that exceeds the amount of freed memory. Instead, your Mac assigns that new application to the memory remaining after the last application you loaded.

In fact, your Mac's RAM now contains a blank spot—the spot once occupied by the small application that you launched in between the two larger applications. The holes in memory that occur when you launch and close applications constitute memory fragmentation.

Another condition that can cause memory fragmentation is memory leak, which occurs when you quit an application and it doesn't release the memory it was using. Not many programs do this, but it does happen occasionally.

Not only is memory fragmentation an inefficient use of memory, but it can also create problems, including unexplained application crashes and the inability to launch an application even though the About This Computer dialog box seems to show that there's enough memory.

What can you do about memory fragmentation?
You should pay particular attention to memory fragmentation if you leave your computer on for long periods of time. If you do this, leave your most-used applications running too. Quitting and relaunching them creates memory fragmentation. When the memory becomes too fragmented for you to continue opening the applications you need, the best cure is to simply restart your Mac.

Fortunately, with a little planning, you can avoid most of the problems associated with memory fragmentation. When you begin a long Mac sessions, think about the applications you needs to run during the session. For example, if you know you'll need AppleWorks during the entire session, then launch it first. Leave for last those applications that you expect to launch and use for a brief time. If you follow this plan-ahead approach, all your short-term programs will load into memory after the ones you intend to keep loaded.

Fortunately, Mac OS X automatically handles memory fragmentation before it ever occurs. Apple says you "really never have to restart". Ya, sure.  Don't believe it.  However, you can open Applications / Utilities / Activity Monitor. This application lists all programs that are open and the resources they are eating up. You can close applications and Apple utilities and get memory back to avoid restarting.