A phase change occurs when the set of pages in active use changes. In practice, it is difficult to implement exactly. The page-fault frequency policy approximates the working set policy. P is a tunable parameter if last fault occurred less than p units ago, add new page to working set but do nothing else after making decision reset all referenced bits. It is added to the pool of good candidates to swap out.
Small pool implies too much multiprogramming.
Large pool implies that another process could be added. Working Size Rather than keep track of the actual working set, just keep track of its size. The size can be estimated by looking at the referenced bits. A process is not placed on the ready list until the number of free frames exceeds the process's working set size. Missing Used Field DEC VAX lacks a Used field.