第十讲 虚拟存储系统.ppt

Chapter 10: Virtual Memory Background Demand Paging Process Creation Page Replacement Allocation of Frames Thrashing Operating System Examples Background Virtual memory – separation of user logical memory from physical memory. Only part of the program needs to be in memory for execution. Logical address space can therefore be much larger than physical address space. Allows address spaces to be shared by several processes. Allows for more efficient process creation. Virtual memory can be implemented via: Demand paging Demand segmentation Virtual Memory That is Larger Than Physical Memory Demand Paging Bring a page into memory only when it is needed. Less I/O needed Less memory needed Faster response More users Page is needed ? reference to it invalid reference ? abort not-in-memory ? bring to memory Transfer of a Paged Memory to Contiguous Disk Space Valid-Invalid Bit With each page table entry a valid–invalid bit is associated(1 ? in-memory, 0 ? not-in-memory) Initially valid–invalid but is set to 0 on all entries. Example of a page table snapshot. During address translation, if valid–invalid bit in page table entry is 0 ? page fault. Page Table When Some Pages Are Not in Main Memory Page Fault If there is ever a reference to a page, first reference will trap to OS ? page fault OS looks at another table to decide: Invalid reference ? abort. Just not in memory. Get empty frame. Swap page into frame. Reset tables, validation bit = 1. Restart instruction: Least Recently Used block move auto increment/decrement location Steps in Handling a Page Fault What happens if there is no free frame? Page replacement – find some page in memory, but not really in use, swap it out. algorithm performance – want an algorithm which will result in minimum number of page faults. Same page may be brought into memory several times. Performance of Demand Paging Page Fault Rate 0 ? p ? 1.0 if p = 0 no page faults if p = 1, every reference is a fault Effective Acce