Numerous utilities (most of them free) can provide up-to-the-minute vital statistics about your Mac. In most cases, these programs run in the background all the time, but if you prefer, you can run them manually when you get curious about your Mac's current state. I provide a list of several such utilities just ahead. But first, you should understand what information you might want to monitor and why.
Mac OS X manages your computer's RAM efficiently for the most part. Applications can dynamically adjust the amount of memory they use, and even if all your RAM is actively in use, a virtual memory system lets Mac OS X use a portion of your hard disk to extend your RAM, automatically swapping (or "paging") data between the disk and the physical RAM as needed.
Even so, if you have enough applications open at once, and if they require enough memory to perform their respective tasks, you can get to a point where the data swapping occurs constantly. This slows everything on your Mac way down, and it also uses up disk space.
You should also be aware of a type of bug known as a memory leak. Applications usually ask the system for a certain amount of memory for any given task and then give it back when they're done with it. But sometimes, due to a programming error, an application keeps taking memory and not returning any, so that by doing nothing more than staying open, it constantly chews up more and more RAM. You can recover the used memory simply by quitting the applicationbut you might never know you have this problem in the first place without monitoring your RAM usage.
For all these reasons, I recommend keeping an eye on how much RAM is currently in use. If the free RAM drops near zero, consider closing windows, quitting applications, or even restarting your machine to reduce your Mac's dependence on virtual memory. Better yet, add more RAM (if possible).
In Mac OS X, RAM is not simply "used" or "free" but can be used in any of three different ways: wired (in use and crucial to keep your Mac running); active (in use now, but may be paged out to disk later); or inactive (not currently in use, and possibly paged out to disk, but also stored in RAM for fast access when needed). Most RAM-monitoring utilities break down RAM into these three categories plus "free," and generally include documentation that explains RAM usage in greater detail.
With hard disk capacity constantly on the rise, you're now less likely to run out of space than you were a few years ago. Nevertheless, the consequences of running out of space can be severe. For one thing, as your hard disk approaches its maximum capacity, your Mac may run more slowly as files become increasingly fragmented. Worse, you could lose data, because your Mac has no space to save a file. And even more seriously, your computer may hang, crash, or fail to start up if it runs out of physical RAM and runs out of disk space to use for virtual memory.
In general, I recommend leaving at least 10 to 15 percent of your hard disk space empty to provide breathing room for file storage, virtual memory, disk image creation, and other tasks. When your disk gets close to that level, delete any unneeded files (see the sidebar below for advice about what files to delete), and archive seldom-used files to CD, DVD, or an external hard drive.
Although you can tell how much free space is on a disk by selecting it in the Finder and choosing File > Get Info, you may not notice if it gets dangerously full while you're busy working. (Mac OS X does display a warning message when space gets critically low, but it appears much too late for my taste.) Several utilities display a live status indicator (in your menu bar, a Dock icon, or a floating window) showing your disks' current free space.
Your Mac contains one or more CPUschips that do the bulk of the computer's information processing. Depending on what software is running and what that software is doing, the CPU load goes up and down. Because all your applications share the available CPU power, it's generally true that the higher the overall load, the slower your software will run. In addition, greater CPU load means a higher internal temperature, forcing your computer's fans to work harder.
Having your CPU(s) run at 100 percent capacity from time to time is normal. However, if the load is always at or near maximumor if it's high even when your computer is relatively inactiveyou may have a problem. For example, a background application could have a bug that causes it to use too much processor capacity, slowing down your foreground tasks. Or you may be running more applications than your hardware can handle gracefully. In any case, keeping an eye on CPU usage can help you spot potential problems before they get out of hand. Some CPU monitoring tools display a breakdown of usage by application, so that if one program is hogging too much of the CPU capacity, you can force it to quit.
Extreme heat can damage delicate components inside your Mac. This is why all Macs have carefully designed cooling systems, which usually rely on two or more fans to vent heat away from the processor, hard drive, and other vital components. These fans, in turn, rely on one or more internal temperature sensors that tell them when to turn on or off or to increase or decrease speed.
If a fan malfunctions, if dust blocks the flow of air through your computer, or if a defect in your computer causes it to overheat for some reason, bad things can happen. Your Mac may hang, shut down unexpectedly, or display other improper behavior. Depending on the nature and severity of the problem, you might be looking at an intermittent inconvenience or an expensive trip to the repair shop. In any case, it behooves you to be alert to excessive temperatures.
Several utilities monitor each of your computer's internal temperature sensors, so that you can easily see when heat exceeds safe limits and take action before damage occurs.
The types, positions, and design of temperature sensors vary from one Mac model to the next. Not all Macs' sensors work with monitoring utilities or provide live updates of their readings.
Some utilities monitor other statistics that may be interesting (though not necessarily relevant to your Mac's health). These include:
Although this is by no means an exhaustive list, the following utilities all provide one or more monitoring services: