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Activity Monitor 101: System memory

Activity Monitor (located in Applications/Utilities) is a great tool for monitoring system resource usage, and is a great way of locating rendering bottlenecks in your system.

In particular, the Memory tab shows you a great deal of information (note: this is from Leopard; Tiger's Activity Monitor looks a little different).



But what does this information mean? What exactly is wired memory? Here is a breakdown of what each item in the Memory tab refers to.

(For clarification, a swap file is a file on disk used to store currently inactive memory in order to make room in the system memory for active applications. For example, if you have Firefox running and you minimize it and then open iTunes, there is a good chance that a lot of Firefox's memory will be written to disk and the space freed will be assigned to iTunes. When you restore the Firefox window, it will copy the data from the disk back into memory again. Your hard disk is much slower than your memory chips so it is best to keep swap file usage to a minimum. This can be achieved by adding more memory to your system.)

Free: Unused memory that is available for any application to use.

Wired: Memory used by the operating system that cannot be touched by other applications. This memory cannot be written to a swap file.

Active: Memory currently used by applications. Note that this might not represent all of the memory used, as some may be stored in swap files.

Inactive: Memory stored from an application that is no longer running. OS X stores application data in a cache so that you can quickly relaunch applications a second time. If you start running out of free memory, the operating system will automatically start using inactive memory.

To clear inactive memory manually, go to Applications/Utilities/Terminal. When it launches, type purge and press enter. Note: it takes several seconds to process.

Used: Memory currently in use that cannot be immediately written to a swap file or allocated elsewhere.

VM size: The size of the swap file.

Page Ins / Page Outs: How often OS X is reading data from and writing to the swap file.

When deciding if you need an upgrade, consider the following:

• Are your page ins/outs approximately equal to one another? Are they increasing rapidly?
• Is the majority of your memory wired?
• Do you have 10% or less of your memory free?

If the answer to any of the above questions is yes, consider more memory.