Virtual Memory Silberschatz: 9. Operating Systems. Autumn CS4023

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Chester Boyd
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1 Operating Systems Autumn

2 Outline 1 Virtual Memory Silberschatz: 9

3 Outline Virtual Memory Silberschatz: 9 1 Virtual Memory Silberschatz: 9

4 Dynamic Loading Virtual Memory Silberschatz: 9 Routine (i.e. function) is not loaded until it is called Better memory-space utilization; unused routine is never loaded Useful when large amounts of code are needed to handle infrequently occurring cases No special support from the operating system is required implemented through program design

5 Dynamic Linking Virtual Memory Silberschatz: 9 Linking postponed until execution time Small piece of code, stub, used to locate the appropriate memory-resident library routine Stub replaces itself with the address of the routine, and executes the routine Operating system needed to check if routine is in processes memory address Dynamic linking is particularly useful for libraries System also known as shared libraries

6 Swapping Virtual Memory Silberschatz: 9 A process can be swapped temporarily out of memory to a backing store, and then brought back into memory for continued execution Backing store fast disk large enough to accommodate copies of all memory images for all users; must provide direct access to these memory images Roll out, roll in swapping variant used for priority-based scheduling algorithms; lower-priority process is swapped out so higher-priority process can be loaded and executed Major part of swap time is transfer time; total transfer time is directly proportional to the amount of memory swapped Modified versions of swapping are found on many systems (i.e., UNIX, Linux, and Windows) System maintains a ready queue of ready-to-run processes which have memory images on disk

7 Schematic View of Swapping

8 Virtual Memory Virtual Memory Silberschatz: 9 Virtual memory: separation of user logical memory from physical memory Only part of the program / program s data structures 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

9 Virtual Memory That is Larger Than Physical Memory

10 Virtual Address Space

11 Shared Library Using Virtual Memory

12 Outline Virtual Memory Silberschatz: 9 1 Virtual Memory Silberschatz: 9

13 Basics Bring a page into memory only when it is needed Less I/O needed Less memory needed Faster response More users Page is needed because a reference was made to it if invalid reference then abort if not-in-memory then bring in to memory

14 Valid-Invalid Bit Virtual Memory Silberschatz: 9 With each page table entry a valid-invalid bit is associated (v means in-memory, i not-in-memory) Initially valid-invalid bit is set to i on all entries Example of a page table snapshot: Frame # V/I v v i. i i Page Table During address translation, if valid-invalid bit in page table entry is i then this triggers a page fault

15 Page Table When Some Pages Are Not in Main Memory

16 Page Faulting Virtual Memory Silberschatz: 9 If there is a reference to a page, first reference to that page will trap to operating system: page fault Operating system looks at another table to decide whether it is: an invalid reference; if so then abort just not in memory Get empty frame Move page into frame (performed by the pager) Update tables Set valid-invalid bit = v Restart the instruction that caused the page fault

17 Steps in Handling a Page Fault

18 Performance of Page Fault Rate 0 ρ 1.0 if ρ = 0, no page faults if ρ = 1, every reference is a fault Effective Access Time (EAT): EAT = (1 ρ) memory access +ρ (page fault overhead +paging out +paging in +restart overhead)

19 Example of Memory access time = 200 nanoseconds (= s) Average page-fault service time = 8 milliseconds Effective Access Time EAT = (1 ρ) 200ns + ρ(8ms) = (1 ρ) 200ns + ρ(8, 000, 000) = 200ns + ρ7, 999, 800 If one access out of 1,000 causes a page fault, then EAT = ms = s This is a slowdown by a factor of 40!!

현재이이미지를표시할수없습니다. Chapter 9: Virtual Memory

현재이이미지를표시할수없습니다. Chapter 9: Virtual Memory 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