The pages are used mainly for two purposes: page cache and process address space. The page cache is pages mapped to a file on disk. The pages that belong to a process address space (called anonymous memory because it is not mapped to any files, and it has no name) are used for heap and stack. When kswapd reclaims pages, it would rather shrink the page cache than page out (or swap out) the pages owned by processes. A large proportion of page cache that is reclaimed and process address space that is reclaimed might depend on the usage scenario and will affect performance. You can take some control of this behavior by using /proc/sys/vm/swappiness.
Page out and swap out: The phrases “page out” and “swap out” are sometimes confusing. The phrase “page out” means take some pages (a part of entire address space) into swap space while “swap out” means taking entire address space into swap space. They are sometimes used interchangeably.