上海交通大学：《操作系统 Operating System》课程教学资源（课件讲稿）OS-Lec19_virtual memory

CHAPTER 9:VIRTUAL MEMORY

CHAPTER 9: VIRTUAL MEMORY

CHAPTER 9:VIRTUAL MEMORY o Background o Demand Paging o Copy-on-Write o Page Replacement o Allocation of Frames o Thrashing o Memory-Mapped Files o Allocating Kernel Memory o Other Considerations o Operating-System Examples

CHAPTER 9: VIRTUAL MEMORY  Background  Demand Paging  Copy-on-Write  Page Replacement  Allocation of Frames  Thrashing  Memory-Mapped Files  Allocating Kernel Memory  Other Considerations  Operating-System Examples

OBJECTIVES o To describe the benefits of a virtual memory system o To explain the concepts of demand paging, page-replacement algorithms,and allocation of page frames o To discuss the principle of the working-set model

OBJECTIVES  To describe the benefits of a virtual memory system  To explain the concepts of demand paging, page-replacement algorithms, and allocation of page frames  To discuss the principle of the working-set model

BACKGROUND o 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 o Virtual memory can be implemented via: 。Demand paging ·Demand segmentation

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 page 0 page 1 page 2 memory map page v physical memory virtual memory

VIRTUAL MEMORY THAT IS LARGER THAN PHYSICAL MEMORY 

VIRTUAL-ADDRESS SPACE Max stack heap data code 0

VIRTUAL-ADDRESS SPACE

SHARED LIBRARY USING VIRTUAL MEMORY stack stack shared shared library pages shared library heap heap data data code code

SHARED LIBRARY USING VIRTUAL MEMORY

DEMAND PAGING o Bring a page into memory only when it is needed ·Less I/O needed ·Less memory needed ·Faster response ·ore users o Page is needed =reference to it 。invalid reference→abort not-in-memory>bring to memory o Lazy swapper-never swaps a page into memory unless page will be needed Swapper that deals with pages is a pager

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  Lazy swapper – never swaps a page into memory unless page will be needed  Swapper that deals with pages is a pager

TRANSFER OF A PAGED MEMORY TO CONTIGUOUS DISK SPACE swap out 0☐1☐2☐3☐ program A 4古5d7 8☐9☐10☐11☐ 12☐13☐14☐15☐ program B swap in 16☐17☐18☐19☐ 20☐21☐22☐23☐ main memory

TRANSFER OF A PAGED MEMORY TO CONTIGUOUS DISK SPACE

VALID-INVALID BIT 0 With each page table entry a valid-invalid bit is associated (w→in-memory,i→not-in-memory） o Initially valid-invalid bit is set to i on all entries o Example of a page table snapshot: Frame valid-invalid bit V i i page table o During address translation,if valid-invalid bit in page table entry isI→page fault

VALID-INVALID BIT  With each page table entry a valid–invalid bit is associated (v  in-memory, i  not-in-memory)  Initially valid–invalid bit is set to i on all entries  Example of a page table snapshot:  During address translation, if valid–invalid bit in page table entry is I  page fault v v v v i i i …. Frame # valid-invalid bit page table