MacOS – what do App Memory, Wired Memory, and Compressed mean

OS X, like most modern operating systems, uses a virtual memory system for managing memory. Among other purposes, this allows the operating system to treat the computer as having an unlimited pool of memory. To achieve this, the OS will page unused parts of RAM out to a disk store known as the swapfile.

Of course, RAM is not unlimited, so OS X groups RAM into four categories: wired, active, inactive, and free. Wired memory is required by the operating system, and can never be paged out of memory. Active memory is memory used by currently-running programs. Inactive memory was used recently by programs which have now been terminated (or haven't been unused in a long time). Free memory is, as the name suggests, RAM that is not being used.

When you launch a program, it gets loaded into active memory. When you quit a program, however, it doesn't get removed from RAM; rather, it gets bumped into inactive memory. This is why it is often faster to re-launch a program -- it is still in RAM (try this with a big program like Firefox).

Once all your memory is used (free memory is 0), the OS will write out inactive memory to the swapfile to make more room in active memory.

If a program gets paged out to the swapfile, and you re-launch it, it'll get pulled from the swapfile into active memory.

So in short, you actually shouldn't care if your free memory is low. In fact, you want it to be low -- free memory is wasted memory (as the OS isn't using it for anything).

When examining how much memory your computer is using, you actually want to pay attention mostly to Swap used, which tells you the size of the virtual memory swapfile, and Page ins, which tells you how often the OS has to pull memory from the swapfile into active memory.

From what I can tell, wired memory belongs to the kernel, the innermost core of Mac OS X. It's many layers removed from the icons in the menubar, which are just ordinary apps showing themselves in an odd way.

Wired memory is used for some of the core functions of the operating system—things like keeping track of all the applications on your system, or open files and network connections, or chunks of memory used by various drivers. The "page tables" that form a map of your system's memory are also stored in wired memory, and a system with more memory needs larger page tables. I suspect that the memory used by the integrated video chips in most Macs is wired as well, but I can't find anything that says that outright. In any case, much of this information is needed to manage and access memory, and so it can't itself be swapped out to disk!

To understand why, imagine an enormous library. Think of, for example, the Bodleian Library at the University of Oxford, which has eleven million printed items. There's no way you could possibly fit all that stuff into a single building—certainly not one in the middle of a university campus.

So instead, imagine that the librarians construct a vast warehouse. (The real Bodleian Library has about forty sub-libraries of various descriptions, plus storage for really rare books, but this is a thought experiment.) Most of the books are kept in the warehouse, but anything that's been used recently is kept in the library. If you show up at the library looking for a book, and it's in the stacks there, you can read it immediately. If not, ask a librarian and the book you want will be transported from the warehouse and given to you the next day.

The information in wired memory, then, would be things like the card catalog, keys to the book delivery trucks, and maps of the route to the warehouse. If you stored these things in the warehouse, you could never retrieve books to bring them back to the library—so they must be kept in the library at all times, lest the whole system break down.

Anyway, getting back to practical considerations: wired memory is basically used by your computer for internal bookkeeping of various sorts. You shouldn't worry about it.

And don't worry if you have lots of "inactive" memory and little "free" memory, either. Inactive memory is basically memory that Mac OS is keeping something in on the off chance it's needed again; if your system needs that memory for something else, it'll be converted to free memory without any performance hit.

To extend the library metaphor, imagine that the library keeps books that have been recently used in the stacks. The space is there in the stacks; there's no use leaving it empty, so you might as well keep the books you already have there. There's no harm in it, and sometimes someone will be able to pick up a book immediately that he would otherwise have had to wait for.

Similarly, inactive memory can only speed things up; it can't hurt your computer, so don't worry about it being too high.

Really, you shouldn't worry about your memory use unless wired + active starts getting close to the 8 GB in your computer. That's when you could get into trouble, because that's when Mac OS will start consuming disk space to add "extra" memory, damaging performance considerably.

Otherwise, you might as well have something in that memory. Ultimately, unused memory is just a waste of power—if you have it, you might as well put it to use.