OS X Memory Usage Doesn't Add Up

Question

It looks like Active + Wired memory displayed in Activity Monitor should add up to the sum of the real memory used by all processes, but 3.27 GB != ~1GB:

As you can see there aren't really any user applications running, and yet theres more than 2GB of active memory being used. Does anyone know what's going on? Shouldn't the active memory become either free or inactive after an application is closed? Also is the kernel_task process supposed to take up that much memory?

In case anyone is wondering, this a new (less than a year old) Macbook Pro with plenty of hard disk space running Snow Leopard.

Answer 1

From the Help Viewer:

Here is an explanation of some of the information displayed at the bottom of the memory pane:

• Wired:
  Wired memory contains information that must always stay in RAM

• Active:
  Active memory that contains information that is actively being used.

• Inactive:
  Inactive memory contains information that is not actively being used. Leaving this information in RAM is to your advantage if you (or a client of your computer) come back to it later.

• Used:
  Used memory is being used by a process or by the system.

  (Used memory is the sum of wired, active, and inactive memory. If the system requires memory it takes free memory before used memory.)

• Free:
  Free memory is not being used and is immediately available.

• VM size:
  Virtual memory, or VM, is hard disk space that can be used as memory. VM size is the amount of disk space being used as memory. Mac OS X can use more memory than the amount of physical RAM you have. A 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:
  The number of gigabytes of information Mac OS X has moved between RAM and disk space