No matter how well an application is designed and tested, there are always cases when due to input error or due to a dearth of resources, applications freeze, or slow down to the extent where user interaction isn’t possible. At other times, apps start taking so many resources that they slow the whole system down, necessitating the system or the user to quit them, and free resources that other applications might need to function.
On Windows, to do this, there is a familiar mechanism of opening up the Task Manager, by using the popular keyboard shortcut Ctrl + Alt + Delete. A similar mechanism is available on Mac OS X, but is divided into 2 entities, to separate important system apps and background tasks from the user applications which are commonly visible.
Command + Option + Escape
Using this keyboard shortcut on a Mac brings up a ‘Force Quit Applications’ dialog, which is probably the easiest way to kill an unresponsive application. Usually, if an app is not responding, a (Not Responding) label is appended to its name in the list of applications. This enables you to quickly find and terminate apps that are unresponsive.
In case if Finder becomes unresponsive (though it very rarely does that), you will notice that there is an option to ‘Relaunch’ Finder, but not to force quit it. This is because Finder is the base application on Mac OS X that manages how you interact with all other applications and therefore it must be running at all times.
Though the previous mentioned method will work for you 99% of the times, but it has a limitation that it lists only the applications that are running in the front system, and not the background services. For example, a process called SystemUIServer is responsible on Mac OS X for displaying the menu bar and the status bar on the top of all applications. If something goes wrong with SystemUIServer, there is no way to force quit it from the Cmd + Opt + Esc shortcut.
To deal with such situations, you can launch the Activity Monitor (from /Applications/Utilities).
Activity Monitor shows you a list of all running processes and the amount of system resources reserved/in use by them. You can sort processes by name, CPU usage or memory usage. This sorting feature is especially useful in cases such as killing a Google Chrome tab running a Flash video, and clogging up CPU resources. If you sort by CPU usage, you will see one instance of Google Chrome taking up a lot of CPU, and you can immediately identify it as the culprit for your system’s slowdown.
To force quit an application/process from Activity Monitor, simply select the process, and click on the red ‘Quit Process’ button on the top left of the window.