Editing Page replacement algorithm (section)

== Precleaning ==
Most replacement algorithms simply return the target page as their result. This means that if target page is ''dirty'' (that is, contains data that have to be written to the stable storage before page can be reclaimed), I/O has to be initiated to send that page to the stable storage (to ''clean'' the page). In the early days of virtual memory, time spent on cleaning was not of much concern, because virtual memory was first implemented on systems with [[full duplex]] channels to the stable storage, and cleaning was customarily overlapped with paging. Contemporary commodity hardware, on the other hand, does not support full duplex transfers, and cleaning of target pages becomes an issue.

To deal with this situation, various ''precleaning'' policies are implemented. Precleaning is the mechanism that starts I/O on dirty pages that are (likely) to be replaced soon. The idea is that by the time the precleaned page is actually selected for the replacement, the I/O will complete and the page will be clean. Precleaning assumes that it is possible to identify pages that will be replaced ''next''. Precleaning that is too eager can waste I/O bandwidth by writing pages that manage to get re-dirtied before being selected for replacement.