Chapter08
- 1. Virtual Memory Chapter 8 1
- 2. Hardware and Control Structures • Memory references are dynamically translated into physical addresses at run time – A process may be swapped in and out of main memory such that it occupies different regions • A process may be broken up into pieces that do not need to located contiguously in main memory • All pieces of a process do not need to be loaded in main memory during execution 2
- 3. Execution of a Program • Operating system brings into main memory a few pieces of the program • Resident set - portion of process that is in main memory • An interrupt is generated when an address is needed that is not in main memory • Operating system places the process in a blocking state 3
- 4. Execution of a Program • Piece of process that contains the logical address is brought into main memory – Operating system issues a disk I/O Read request – Another process is dispatched to run while the disk I/O takes place – An interrupt is issued when disk I/O complete which causes the operating system to place the affected process in the Ready state 4
- 5. Advantages of Breaking up a Process • More processes may be maintained in main memory – Only load in some of the pieces of each process – With so many processes in main memory, it is very likely a process will be in the Ready state at any particular time • A process may be larger than all of main memory 5
- 6. Types of Memory • Real memory – Main memory • Virtual memory – Memory on disk – Allows for effective multiprogramming and relieves the user of tight constraints of main memory 6
- 7. Thrashing • Swapping out a piece of a process just before that piece is needed • The processor spends most of its time swapping pieces rather than executing user instructions 7
- 8. Principle of Locality • Program and data references within a process tend to cluster • Only a few pieces of a process will be needed over a short period of time • Possible to make intelligent guesses about which pieces will be needed in the future • This suggests that virtual memory may work efficiently 8
- 9. Support Needed for Virtual Memory • Hardware must support paging and segmentation • Operating system must be able to management the movement of pages and/or segments between secondary memory and main memory 9
- 10. Paging • Each process has its own page table • Each page table entry contains the frame number of the corresponding page in main memory • A bit is needed to indicate whether the page is in main memory or not 10
- 11. Paging 11
- 12. Modify Bit in Page Table • Modify bit is needed to indicate if the page has been altered since it was last loaded into main memory • If no change has been made, the page does not have to be written to the disk when it needs to be swapped out 12
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- 14. Two-Level Scheme for 32-bit Address 14
- 15. Page Tables • The entire page table may take up too much main memory • Page tables are also stored in virtual memory • When a process is running, part of its page table is in main memory 15
- 16. Inverted Page Table • Used on PowerPC, UltraSPARC, and IA-64 architecture • Page number portion of a virtual address is mapped into a hash value • Hash value points to inverted page table • Fixed proportion of real memory is required for the tables regardless of the number of processes 16
- 17. Inverted Page Table • Page number • Process identifier • Control bits • Chain pointer 17
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- 19. Translation Lookaside Buffer • Each virtual memory reference can cause two physical memory accesses – One to fetch the page table – One to fetch the data • To overcome this problem a high-speed cache is set up for page table entries – Called a Translation Lookaside Buffer (TLB) 19
- 20. Translation Lookaside Buffer • Contains page table entries that have been most recently used 20
- 21. Translation Lookaside Buffer • Given a virtual address, processor examines the TLB • If page table entry is present (TLB hit), the frame number is retrieved and the real address is formed • If page table entry is not found in the TLB (TLB miss), the page number is used to index the process page table 21
- 22. Translation Lookaside Buffer • First checks if page is already in main memory – If not in main memory a page fault is issued • The TLB is updated to include the new page entry 22
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- 27. Page Size • Smaller page size, less amount of internal fragmentation • Smaller page size, more pages required per process • More pages per process means larger page tables • Larger page tables means large portion of page tables in virtual memory • Secondary memory is designed to efficiently transfer large blocks of data so a large page size is better 27
- 28. Page Size • Small page size, large number of pages will be found in main memory • As time goes on during execution, the pages in memory will all contain portions of the process near recent references. Page faults low. • Increased page size causes pages to contain locations further from any recent reference. Page faults rise. 28
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- 30. Example Page Sizes 30
- 31. Segmentation • May be unequal, dynamic size • Simplifies handling of growing data structures • Allows programs to be altered and recompiled independently • Lends itself to sharing data among processes • Lends itself to protection 31
- 32. Segment Tables • Corresponding segment in main memory • Each entry contains the length of the segment • A bit is needed to determine if segment is already in main memory • Another bit is needed to determine if the segment has been modified since it was loaded in main memory 32
- 33. Segment Table Entries 33
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- 35. Combined Paging and Segmentation • Paging is transparent to the programmer • Segmentation is visible to the programmer • Each segment is broken into fixed-size pages 35
- 36. Combined Segmentation and Paging 36
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- 39. Fetch Policy • Fetch Policy – Determines when a page should be brought into memory – Demand paging only brings pages into main memory when a reference is made to a location on the page • Many page faults when process first started – Prepaging brings in more pages than needed • More efficient to bring in pages that reside contiguously on the disk 39
- 40. Placement Policy • Determines where in real memory a process piece is to reside • Important in a segmentation system • Paging or combined paging with segmentation hardware performs address translation 40
- 41. Replacement Policy • Placement Policy – Which page is replaced? – Page removed should be the page least likely to be referenced in the near future – Most policies predict the future behavior on the basis of past behavior 41
- 42. Replacement Policy • Frame Locking – If frame is locked, it may not be replaced – Kernel of the operating system – Control structures – I/O buffers – Associate a lock bit with each frame 42
- 43. Basic Replacement Algorithms • Optimal policy – Selects for replacement that page for which the time to the next reference is the longest – Impossible to have perfect knowledge of future events 43
- 44. Basic Replacement Algorithms • Least Recently Used (LRU) – Replaces the page that has not been referenced for the longest time – By the principle of locality, this should be the page least likely to be referenced in the near future – Each page could be tagged with the time of last reference. This would require a great deal of overhead. 44
- 45. Basic Replacement Algorithms • First-in, first-out (FIFO) – Treats page frames allocated to a process as a circular buffer – Pages are removed in round-robin style – Simplest replacement policy to implement – Page that has been in memory the longest is replaced – These pages may be needed again very soon 45
- 46. Basic Replacement Algorithms • Clock Policy – Additional bit called a use bit – When a page is first loaded in memory, the use bit is set to 1 – When the page is referenced, the use bit is set to 1 – When it is time to replace a page, the first frame encountered with the use bit set to 0 is replaced. – During the search for replacement, each use bit set to 1 is changed to 0 46
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- 50. Comparison of Placement Algorithms 50
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- 52. Basic Replacement Algorithms • Page Buffering – Replaced page is added to one of two lists • Free page list if page has not been modified • Modified page list 52
- 53. Resident Set Size • Fixed-allocation – 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 – Number of pages allocated to a process varies over the lifetime of the process 53
- 54. Fixed Allocation, Local Scope • Decide ahead of time the amount of allocation to give a process • If allocation is too small, there will be a high page fault rate • If allocation is too large there will be too few programs in main memory 54
- 55. Variable Allocation, Global Scope • 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 another process 55
- 56. Variable Allocation, Local Scope • 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 allocation from time to time 56
- 57. Cleaning Policy • Demand cleaning – A page is written out only when it has been selected for replacement • Precleaning – Pages are written out in batches 57
- 58. Cleaning Policy • Best approach uses page buffering – Replaced pages are placed in two lists • Modified and unmodified – Pages in the modified list are periodically written out in batches – Pages in the unmodified list are either reclaimed if referenced again or lost when its frame is assigned to another page 58
- 59. Load Control • Determines the number of processes that will be resident in main memory • Too few processes, many occasions when all processes will be blocked and much time will be spent in swapping • Too many processes will lead to thrashing 59
- 60. Multiprogramming 60
- 61. Process Suspension • Lowest priority process • Faulting process – This process does not have its working set in main memory so it will be blocked anyway • Last process activated – This process is least likely to have its working set resident 61
- 62. Process Suspension • Process with smallest resident set – This process requires the least future effort to reload • Largest process – Obtains the most free frames • Process with the largest remaining execution window 62
- 63. UNIX and Solaris Memory Management • Paging System – Page table – Disk block descriptor – Page frame data table – Swap-use table 63
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- 67. UNIX and Solaris Memory Management • Page Replacement – Refinement of the clock policy 67
- 68. Kernel Memory Allocator • Lazy buddy system 68
- 69. Linux Memory Management • Page directory • Page middle directory • Page table 69
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- 72. Windows Memory Management • Paging – Available – Reserved – Committed 72