内存各部分占用的含义

The kernel divides the main memory into five lists:

Wired: Wired pages are locked in memory and cannot be paged out. Typically, these pages are being used by the kernel or the physical-memory pager, or they have been locked down with mlock. In addition, all the pages being used to hold the user structure and thread stacks of loaded (i.e., not swapped-out) processes are also wired. Wired pages cannot be paged out.

Active: Active pages are being used by one or more regions of virtual memory. Although the kernel can page them out, doing so is likely to cause an active process to fault them back again.

Inactive: Inactive pages have contents that are still known, but they are not usually part of any active region. If the contents of the page are dirty, the contents must be written to backing store before the page can be reused. Once the page has been cleaned, it is moved to the cache list. If the system becomes short of memory, the pageout daemon may try to move active pages to the inactive list in the hopes of finding pages that are not really in use. The selection criteria that are used by the pageout daemon to select pages to move from the active list to the inactive list are described later in this section. When the free-memory and cache lists drop too low, the pageout daemon traverses the inactive list to create more cache and free pages.

Cache: Cache pages have contents that are still known, but they are not usually part of any active region. If they are mapped into an active region, they must be marked read-only so that any write attempt will cause them to be moved off the cache list. They are similar to inactive pages except that they are not dirty, either because they are unmodified, since they were paged in, or because they have been written to their backing store. They can be moved to the free list when needed.

Free: Free pages have no useful contents and will be used to fulfill new page-fault requests. The idle process attempts to keep about 75 percent of the pages on the free list zeroed so that they do not have to be zeroed while servicing an anonymous-region page fault. Pages with unknown contents are placed at the front of the free list. Zeroed pages are placed at the end of the free list. The idle process takes pages from the front of the free list, zeros them, marks them as having been zeroed, and puts them on the end of the free list. Page faults that will be filling a page take one from the front of the free list. Page faults that need a zero-filled page take one from the end of the free list. If marked as zeroed, it does not have to be zeroed during the fault service.