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What Is Activity Monitor (or How to Take Your Mac's Pulse)

by Giles Turnbull
Activity Monitor

Activity Monitor is a GUI application included with Mac OS X. It provides graphical representations of your computer's CPU, system memory, disk activity, disk usage, and network processes. This little utility is easy to overlook, but it can help you manage your memory and processing power when running lots of applications. If you prefer to work in the text-only Terminal application instead, you can view much of this information by using the top command.

In this article

  1. What to Look For
  2. What's Keeping My Computer Busy?
  3. What's Happening with That Process?
  4. What's My Computer Actually Doing?
  5. What Can I Use All of This For?

Your computer is always, always, doing things. Even when you've just booted up, and it seems no applications are running, stuff is happening in the background.

There's the Finder, for one thing. The Mac OS X file manager runs automatically and starts consuming system resources straight away. And don't forget about the daemons and utilities running where you can't see them. Some simply don't have any graphical interface for you to see, others do, and have been told to keep it out of sight until needed.

Everything that's running on your computer consumes resources. But what are these resources and why do they matter?

Computers are very good at doing tasks that humans don't enjoy or tend to get wrong. But even the smartest computer's resources are finite. There's a limited amount of memory available for the OS and its applications to run in; a limited number of instructions that the CPU can handle at any one time; and a limited amount of disk space for them all to write files to.

Sometimes, a computer starts to run short of one or more of these resources, and that's when problems start. If there's not enough memory, applications start to run very slowly, or freeze up altogether. That's when Activity Monitor comes in handy. You can find this utility in your /Applications/Utilities folder. If you're running OS X 10.4, a Spotlight search for "activity" will find it for you.

What to Look For

When you open Activity Monitor, you will probably see something like this:

Activity Monitor's default view

Let's dissect the window. At the top there's a toolbar. You can customize this to your wishes as with any other Cocoa app. The main information area, the processes list, displays statistics about your computer's performance in real time. Each process represents an application, utility, or other program or daemon that is currently in use.

Underneath are a series of tabs for switching between further detailed views of different sources of information--different system resources. They are: CPU, System Memory, Disk Activity, Disk Usage, and Network. First, let's look at the processes list.

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