Physical memory is managed on a page-by-page basis through the vm_page_t structure. Pages of physical memory are categorized
through the placement of their respective vm_page_t structures
on one of several paging queues.
A page can be in a wired, active, inactive, cache, or free state. Except for the wired
state, the page is typically placed in a doubly link list queue representing the state
that it is in. Wired pages are not placed on any queue.
FreeBSD implements a more involved paging queue for cached and free pages in order to
implement page coloring. Each of these states involves multiple queues arranged according
to the size of the processor's L1 and L2 caches. When a new page needs to be allocated,
FreeBSD attempts to obtain one that is reasonably well aligned from the point of view of
the L1 and L2 caches relative to the VM object the page is being allocated for.
Additionally, a page may be held with a reference count or locked with a busy count.
The VM system also implements an ``ultimate locked'' state for a page using the PG_BUSY
bit in the page's flags.
In general terms, each of the paging queues operates in a LRU fashion. A page is
typically placed in a wired or active state initially. When wired, the page is usually
associated with a page table somewhere. The VM system ages the page by scanning pages in
a more active paging queue (LRU) in order to move them to a less-active paging queue.
Pages that get moved into the cache are still associated with a VM object but are
candidates for immediate reuse. Pages in the free queue are truly free. FreeBSD attempts
to minimize the number of pages in the free queue, but a certain minimum number of truly
free pages must be maintained in order to accommodate page allocation at interrupt
time.
If a process attempts to access a page that does not exist in its page table but does
exist in one of the paging queues (such as the inactive or cache queues), a relatively
inexpensive page reactivation fault occurs which causes the page to be reactivated. If
the page does not exist in system memory at all, the process must block while the page is
brought in from disk.
FreeBSD dynamically tunes its paging queues and attempts to maintain reasonable ratios
of pages in the various queues as well as attempts to maintain a reasonable breakdown of
clean vs. dirty pages. The amount of rebalancing that occurs depends on the system's
memory load. This rebalancing is implemented by the pageout daemon and involves
laundering dirty pages (syncing them with their backing store), noticing when pages are
activity referenced (resetting their position in the LRU queues or moving them between
queues), migrating pages between queues when the queues are out of balance, and so forth.
FreeBSD's VM system is willing to take a reasonable number of reactivation page faults to
determine how active or how idle a page actually is. This leads to better decisions being
made as to when to launder or swap-out a page.
