virtual memory
- 1. Operating Systems: Internals and Design Principles, 6/E William Stallings Chapter 8 Virtual Memory Patricia Roy Manatee Community College, Venice, FL ©2008, Prentice Hall
- 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
- 3. Hardware and Control Structures • A process may be broken up into pieces, which do not need to be located contiguously in main memory • It is not necessary for all pieces of a process to be loaded in main memory during execution of the process
- 4. 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
- 5. 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
- 6. Improved System Utilization • 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
- 7. 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
- 8. 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
- 9. 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
- 10. Support Needed for Virtual Memory • Hardware must support paging and segmentation • Operating system must be able to do the management the movement of pages and/or segments between secondary memory and main memory
- 11. 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
- 12. Paging
- 13. 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 replaced
- 15. Two-Level Hierarchical Page Table
- 16. Page Tables • Page tables are also stored in virtual memory • When a process is running, part of its page table is in main memory
- 18. 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
- 19. Inverted Page Table • Page number • Process identifier • Control bits • Chain pointer
- 21. 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)
- 22. Translation Lookaside Buffer • Contains page table entries that have been most recently used
- 23. 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
- 24. 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
- 29. 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
- 30. Page Size • Secondary memory is designed to efficiently transfer large blocks of data so a large page size is better
- 31. 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.
- 32. Page Size
- 34. 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
- 35. Segment Tables • Starting address 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
- 37. Segmentation
- 38. Combined Paging and Segmentation • Paging is transparent to the programmer • Segmentation is visible to the programmer • Each segment is broken into fixed-size pages
- 39. Combined Paging and Segmentation
- 42. 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
- 43. 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
- 44. Replacement 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
- 45. Replacement Policy • Frame Locking – If frame is locked, it may not be replaced – Kernel of the operating system – Key control structures – I/O buffers – Associate a lock bit with each frame
- 46. 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
- 47. 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.
- 48. 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
- 49. 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
- 50. Clock Policy
- 51. Clock Policy
- 52. Clock Policy
- 53. Comparison
- 54. Behavior of Page Replacement Algorithms
- 55. 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
- 56. 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
- 57. 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 – Processor idle time – Swapping
- 58. 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
- 59. 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
- 60. Cleaning Policy • Demand cleaning – A page is written out only when it has been selected for replacement • Precleaning – Pages are written out in batches
- 61. 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
- 62. 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
- 63. Multiprogramming
- 64. 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
- 65. 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