Activity Monitor in OS X Mountain Lion
In Unix, the underlying operating system that powers OS X, applications and other things going on behind the scenes are called processes. Each application and the operating system itself can run a number of processes at the same time.
In the following figure, you see 78 different processes running, most of them behind the scenes. Note that when this picture was taken, half a dozen or more programs were running, including the Finder, FaceTime, the Mac App Store, and Activity Monitor itself.
To display the three CPU Monitor windows on the right side of the Activity Monitor window as shown, choose Window→CPU Usage (keyboard shortcut Command+2), CPU History (keyboard shortcut Command+3), and/or Floating CPU Window (no keyboard shortcut).
You also select what appears in the Activity Monitor’s Dock icon — CPU Usage, CPU History, Network Usage, Disk Activity, Memory Usage, or the Activity Monitor icon — by choosing View→Dock Icon. All but the Activity Monitor icon appear live, meaning that they update every few seconds to reflect the current state of affairs.
To choose how often these updates occur, choose View→Update Frequency.
But be careful — setting Activity Monitor to update more frequently causes it to use more CPU cycles, which can decrease overall performance slightly.
Finally, the bottom portion of the Activity Monitor window can display one of five monitors. Just click the appropriate tab — CPU, System Memory, Disk Activity, Disk Usage, or Network — to see that particular monitor.
Because all Macs that can run Mountain Lion have at least a dual-core processor, you’ll see at least two, and possibly four or more, CPUs displayed in Activity Monitor, one for each core.
Geeks and troubleshooters (and even you) can use Activity Monitor to identify what processes are running, which user owns the process, and how much CPU capacity and memory the process is using. You can even use this feature to quit or force-quit a process that you think might be causing problems for you.
Messing around in Activity Monitor isn’t a good idea for most users. If you’re having problems with an application or with OS X, try quitting open applications; force-quitting applications (press Command+Option+Esc — the Mac “three-finger salute”); or logging out and then logging back in again before you start mucking around with processes.