置换策略：也称淘汰策略，涉及的是当内存可用空间不能装下需要调入的信息时，决定调出已占用内存某个区域的那一块信息以便腾出空间。

OPT该算法的基本思想是：发生缺页时，有些页面在内存中，其中有一页将很快被访问(包含紧接着的下一条指令的那页)，而其他页则可能要到10、100、或1000条指令之后才被访问，每个页都可以用在该页面首次被访问前所要执行的指令数进行标记

-Replacement Policy: also known as the phase-out strategy, involving the memory space available when required under can not be installed transferred the information, decided to transfer out of an area has been occupied by the memory of that piece of information in order to make room. The basic idea of the algorithm OPT are: page fault occurs, some pages in memory, of which one will soon be visited (including followed by an instruction that the next page), while other pages have to move to 10,100 , or 1000 instructions only after visit, every page can be used for the first time to visit the page number of instructions to be executed before tagging