Mac OS X: Reading system memory usage in Activity Monitor

Four types of memory appear in Activity Monitor's System Memory pie chart: Free, Wired, Active, and Inactive.

The total of the four types equals the amount of random-access memory (RAM) in your computer. RAM is the high-speed memory used to store information that is in use or used most recently. Information in RAM is loaded from your hard disk.


Free memory

This memory is not being used currently

Wired memory

This information can't be moved to disk, so it must stay in RAM. The amount depends on the applications you are using.

Active memory

This information is currently in RAM and has recently been used.

Inactive memory

This information has not recently been used but will remain in RAM until another application needs more memory but no free memory is available. If called upon by a process, this is quickly changed to Active memory; if it has been swapped to the hard disk, it will be moved back to RAM and marked as Active.

Other information

Used

Used memory is total of Wired, Active, and Inactive memory.

VM size

"VM size" is the amount hard disk space being used as virtual memory (this number also includes the amount of installed RAM).

Virtual memory allows Mac OS X to "virtually" use more memory than the amount of RAM you have by using hard disk space to supplement RAM. However, hard disks are much slower than RAM, so Mac OS X automatically distributes information between disk space and RAM for efficient performance.

Page ins / Page outs

"Page ins/outs" refers to the amount of information moved between RAM and the hard disk. This number is cumulative amount of data that Mac OS X has moved between RAM and disk space.

Tip: Page outs occur when your Mac has to write information from RAM to the hard drive (because RAM is full).  Adding more RAM may reduce page outs.