四川大学操作系统Chapter08讲述.ppt

* Page Buffering(页缓冲) Replaced page is added to one of two lists Free page list if page has not been modified (空闲页表) Modified page list (修改页表) Modified pages are written out in clusters rather than one at a time * 8.2 Operating System Software 8.2.1 Fetch Policy 8.2.2 Placement Policy 8.2.3 Replacement Policy 8.2.4 Resident Set Management 8.2.5 Clearing Policy 8.2.6 Load Control * Resident Set Size(驻留集大小) Fixed-allocation(固定分配) Gives a process a fixed number of pages within which to execute When a page fault occurs, one of the pages of that process must be replaced Variable-allocation(可变分配) Number of pages allocated to a process varies over the lifetime of the process * Replace Scope(替换范围) Local Replace Policy(局部置换) Chooses only among the resident pages of the process that generated the page fault in selecting a page to replace Global Replace Policy(全局置换) Considers all unlocked pages in main memory as candidates for replacement, regardless of which process owns a particular page * Fixed Allocation, Local Scope(固定分配，局部置换) Decide ahead of time the amount of allocation to give a process (事先确定给进程分配多少页) If allocation is too small, there will be a high page fault rate (如果太少，导致高缺页率) If allocation is too large there will be too few programs in main memory (如果分配太多，内存中驻留的程序会过少) * Variable Allocation, Global Scope(可变分配，全局置换) Easiest to implement Adopted by many operating systems Operating system keeps list of free frames Free frame is added to resident set of process when a page fault occurs If no free frame, replaces one from any of the resident processes * Variable Allocation, Local Scope(可变分配，局部置换) When new process added, allocate number of page frames based on application type, program request, or other criteria When page fault occurs, select page from among the resident set of the process that suffers the fault Reevaluate(重新评估) allocation of processes from time to time and increase or decrease allocation to improve overall performance * 8.2 Operating System Software 8