Mac OS X: Reading system memory usage in Activity Monitor
Summary
Should you be worried if your computer only has 10 MB of "Free" memory? Probably not. Learn what the four types of memory in Activity Monitor really mean.
This document applies to Mac OS X and Mac OS X Server versions 10.3 and later.
Products Affected
Mac OS X 10.3, Mac OS X 10.4, Mac OS X 10.5
Four types of memory appear in the System Memory pie chart: Wired, Active, Inactive, and Free. "Used" is simply the total of the first three.
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, and the RAM is emptied when you turn off your computer.
The "VM size" refers to virtual memory, a system of putting information in RAM or caching it to your hard disk as needed. Thus Mac OS X can "virtually" use more memory than the amount of RAM you have. The hard disk is much slower than RAM, so the virtual memory system automatically distributes information between disk space and RAM for efficient performance. "Page ins/outs" refers to the number of times Mac OS X has moved information between RAM and disk space.
Wired memory
This information can't be cached to disk, so it must stay in RAM. The amount depends on what 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 the space in RAM. Then, Inactive memory will be cached to disk. Leaving Inactive memory in RAM for as long as possible is to your advantage. If called upon by a process, it is quickly changed to Active memory.
If the inactive memory is cached to disk and is called upon by a process, it will be returned to RAM and marked as Active memory.
Free memory
This memory is not being used.
Additional Information
What does all this mean?
This means you shouldn't worry when the Free memory is low. The only time Free memory should be high is right after the computer starts up. As you use applications or services, memory is used and transitions to Inactive. Applications that need more memory will take from the Inactive, but the Inactive is there just in case you need it again. If the combination of Free and Inactive is very low, then you might need more memory.