https://developer.apple.com/library/content/documentation/Performance/Conceptual/ManagingMemory/Articles/AboutMemory.html#//apple_ref/doc/uid/20001880-99714-TPXREF106

Wired memory (also called resident memory) stores kernel code and data structures that must never be paged out to disk. Applications, frameworks, and other user-level software cannot allocate wired memory. However, they can affect how much wired memory exists at any time. For example, an application that creates threads and ports implicitly allocates wired memory for the required kernel resources that are associated with them. 

Table 2 lists some of the wired-memory costs for application-generated entities.

As you can see, every thread, process, and library contributes to the resident footprint of the system. In addition to your application using wired memory, however, the kernel itself requires wired memory for the following entities:

• VM objects

• the virtual memory buffer cache

• I/O buffer caches

• drivers

Wired data structures are also associated with the physical page and map tables used to store virtual-memory mapping information, Both of these entities scale with the amount of available physical memory. Consequently, when you add memory to a system, the amount of wired memory increases even if nothing else changes. When a computer is first booted into the Finder, with no other applications running, wired memory can consume approximately 14 megabytes of a 64 megabyte system and 17 megabytes of a 128 megabyte system. 

Wired memory pages are not immediately moved back to the free list when they become invalid. Instead they are “garbage collected” when the free-page count falls below the threshold that triggers page out events. 

Page Lists in the Kernel

The kernel maintains and queries three system-wide lists of physical memory pages:

• The active list contains pages that are currently mapped into memory and have been recently accessed. 

• The inactive list contains pages that are currently resident in physical memory but have not been accessed recently. These pages contain valid data but may be removed from memory at any time. 

• The free list contains pages of physical memory that are not associated with any address space of VM object. These pages are available for immediate use by any process that needs them. 

When the number of pages on the free list falls below a threshold (determined by the size of physical memory), the pager attempts to balance the queues. It does this by pulling pages from the inactive list. If a page has been accessed recently, it is reactivated and placed on the end of the active list. In OS X, if an inactive page contains data that has not been written to the backing store recently, its contents must be paged out to disk before it can be placed on the free list. (In iOS, modified but inactive pages must remain in memory and be cleaned up by the application that owns them.) If an inactive page has not been modified and is not permanently resident (wired), it is stolen (any current virtual mappings to it are destroyed) and added to the free list. Once the free list size exceeds the target threshold, the pager rests.

The kernel moves pages from the active list to the inactive list if they are not accessed; it moves pages from the inactive list to the active list on a soft fault (see Paging In Process). When virtual pages are swapped out, the associated physical pages are placed in the free list. Also, when processes explicitly free memory, the kernel moves the affected pages to the free list.

Paging Out Process

In OS X, when the number of pages in the free list dips below a computed threshold, the kernel reclaims physical pages for the free list by swapping inactive pages out of memory. To do this, the kernel iterates all resident pages in the active and inactive lists, performing the following steps:

1. If a page in the active list is not recently touched, it is moved to the inactive list.

2. If a page in the inactive list is not recently touched, the kernel finds the page’s VM object.

3. If the VM object has never been paged before, the kernel calls an initialization routine that creates and assigns a default pager object.

4. The VM object’s default pager attempts to write the page out to the backing store.

5. If the pager succeeds, the kernel frees the physical memory occupied by the page and moves the page from the inactive to the free list.

Wired Memory