Reader Elsa Pederson finds her Mac mysteriously out of memory. She writes:
Today I saw a message I'd never seen before. It indicated that my Mac had run out of application memory. In order to keep working I had to force quit some of my open applications. Is there a way I could have known which application was causing the problem?
Yes. In some cases, the misbehaving application will appear in red, followed by the words "Not Responding." This strongly hints that this application has gone on a RAM binge and has consumed so much of the stuff that it can no longer move. One way to tell if that's the problem application is to force quit a different application. If the once-red application starts responding again, it tells you it's now happy because it has more RAM to chew on.
The list of applications may not contain a single red one, however. In either case, you should check Activity Monitor (/Applications/Utilities), as it will detail exactly what's going on.
Once you've launched Activity Monitor click on the Real Mem column so that applications are organized by the amount of RAM they're using. If you see one that's gobbling up enormous amounts of memory, you've nailed the culprit. Quit and relaunch it and see how much memory it now uses. It should be quite a bit less than before. If the error pops up again in the next day or so, see if there's an update for the application as the version you're currently using could have a flaw that's causing it to over-eat.
If you wish to learn more about the amount of memory your Mac's applications consume, click Activity Monitor's System Memory button at the bottom of the window. When you do you'll see a pie chart that tells you how your computer's memory is allocated--free, wired, active, inactive, and used memory appear here. If your free memory is down to a couple of dozen megabytes and the pie chart shows almost no green (the default color assigned to free memory) you're riding on the edge or another error. Again, check that Real Mem column and kill any application that's hogging RAM.
If you're obsessive about this kind of thing, there's an easier way to quickly eyeball your Mac's memory usage. With Activity Monitor running, click and hold on its Dock icon and from the menu that appears choose Dock Icon > Show Memory Usage. The memory chart that appears at the bottom of Activity Monitor now shows in the Dock, and is updated in real time.
More obsessive still (and yes, this includes me)? Check out Bjango's $16 iStat Menus 4. With it you can track CPU usage, memory usage, network activity, disk usage and activity, internal temperatures, and the current state of your Mac's battery. A snapshot of all this information (or less, if you configure the application that way) appears in your Mac's menu bar. When my Mac acts up, the first things I do is glance at iStat's stats. Completely worthwhile.