Chap8 Virtual Memory. 1997-2003.ppt
- 2. Process (broken: Some portion in memory, Non- contagious location) Pages (invisible to programmer) Segmentation Like Dynamic Partitioning Suffers External Fragmentation Visible to programmer Growing Data Structure Handled Modularity Sharing and Protection Support etc.
- 3. Characteristics of Paging and Segmentation 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
- 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. Advantages of Breaking up 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 It is possible for a process to be larger than all the main memory
- 7. Advantages of Breaking up Processes Programmer is dealing with memory the size of the hard disk It would be wasteful to load in many pieces of the process when only a few pieces will be used Time can be saved because unused pieces are not swapped in and out of memory
- 8. Types of Memory Real memory main memory Virtual memory memory on disk
- 9. 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
- 10. 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
- 11. 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
- 12. 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
- 13. Modify Bit in Page Table A 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
- 14. Paging Virtual Address Page Table Entry Page Number Offset P M Frame Number Other Control Bits
- 15. Address Translation in a Paging System Program Paging Main Memory Virtual Address Register Page Table Page Frame Offset P# Frame # Page Table Ptr Page # Offset Frame # Offset +
- 16. 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
- 17. 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 special cache is set up for page table entries called the TLB - Translation Lookaside Buffer
- 18. Translation Lookaside Buffer Contains page table entries that have been most recently used Works similar to main memory cache
- 19. Translation Lookaside Buffer Given a virtual address, processor examines the TLB If page table entry is present (a hit), the frame number is retrieved and the real address is formed If page table entry is not found in the TLB (a miss), the page number is used to index the process page table
- 20. 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
- 21. Use of a Translation Lookaside Buffer Virtual Address Translation Lookaside Buffer Page Table TLB miss Page fault Real Address TLB hit Offset Main Memory Secondary Memory Load page Page # Offset Frame # Offset
- 22. START CPU checks the TLB Page table entry in TLB? Access page table Page in main memory? Update TLB CPU generates Physical Address Yes No Yes Page fault handling routing No Operation of TLB
- 23. Page Fault Handling Routine OS instructs CPU to read the page from disk CPU activates I/O Hardware Memory full? Perform Page Replacement Page transferred from disk to main memory Page tables updated Yes No
- 24. 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
- 25. 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.
- 26. Page Size (Multiple) Multiple page sizes provide the flexibility needed to effectively use a TLB Large pages can be used for program instructions Small pages can be used for threads
- 27. Segmentation May be dynamic Simplifies handling of growing data structures Allows programs to be altered and recompiled independently Used for sharing data among processes Lends itself to protection
- 28. Segment Tables Each entry contains the starting address of the 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 A bit is needed to determine if the segment has been modified since it was loaded in main memory
- 29. Segmentation Virtual Address Segment Table Entry Segment Number Offset P M Other Control Bits Length Segment Base
- 30. Address Translation in a Segmentation System Base + d Program Segmentation Main Memory Virtual Address Register Segment Table Segment d S# Length Base Seg Table Ptr Seg # Offset = d Segment Table + +
- 31. Segmentation Ex. Page Size: 100 B, Total 3 Segments. Segment:0, 257 Segment:1, 225 Segment:2, 350
- 32. Combined Paging and Segmentation Paging is transparent to the programmer Paging eliminates external fragmentation Segmentation is visible to the programmer Segmentation allows for growing data structures, modularity, and support for sharing and protection Each segment is broken into fixed-size pages
- 33. Combined Segmentation and Paging Virtual Address/Logical Address Segment Table Entry Page Entry Table Segment Number Page Number Offset Other Control Bits Length Segment Base P M Other Control Bits Frame Number Segment Number Offset Relative Address
- 34. Address Translation in Segmentation/Paging System Main Memory Page Frame Offset Paging Page Table P# + Frame # Offset Seg Table Ptr + S # Segmentation Program Segment Table Seg # Page # Offset
- 36. Covert Relative to Virtual Add & Validate Ex.
- 37. Protection Ring 0 contains kernel functions of the operating system A program may access only data that resides on the same ring, or a less privileged ring A program may call services residing on the same, or a more privileged ring
- 38. Ring Protection Control Transfer Between Programs Ring 0 Ring 1 Ring 2 Call Return Call Return Jump
- 39. Ring Protection Data Access Ring 0 Ring 1 Ring 2 Data access Data access
- 40. Operating System Policies for Virtual Memory 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. Operating System Policies for Virtual Memory Placement Policy determines where in real memory a process piece resides usually irrelevant because hardware determines the real address
- 44. Operating System Policies for Virtual Memory Replacement Policy deals with the selection of a page in memory to be replaced when a new page is brought in frame locking used for frames that cannot be replaced used for the kernel and key control structures of the operating system key control structures I/O buffers
- 45. Operating System Policies for Virtual Memory Replacement Policy 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
- 46. Operating System Policies for Virtual Memory Replacement Policy - 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.
- 47. Operating System Policies for Virtual Memory Replacement Policy 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
- 48. Operating System Policies for Virtual Memory Replacement Policy - Clock Policy Additional bit called a use bit When a page is first loaded in memory, the use bit is set to 0 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
- 49. Clock Policy State of buffer just prior to a page replacement 0 1 2 3 4 5 6 7 8 n . . . Page 9 use = 1 Page 19 use = 1 Page 1 use = 0 Page 45 use = 1 Page 191 use = 1 Page 556 use = 0 Page 13 use = 0 Page 67 use = 1 Page 33 use = 1 Page 222 use = 0 next frame pointer
- 50. Clock Policy State of buffer just after the next page replacement 0 1 2 3 4 5 6 7 8 n . . . Page 9 use = 1 Page 19 use = 1 Page 1 use = 0 Page 45 use = 0 Page 191 use = 0 Page 727 use = 1 Page 13 use = 0 Page 67 use = 1 Page 33 use = 1 Page 222 use = 0
- 51. Page Buffering Replaced page is added to one of two lists free page list if page has not been modified modified page list
- 52. 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. Cleaning Policy Demand cleaning a page is written out only when it has been selected for replacement Precleaning pages are written out in batches
- 54. 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
- 55. 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 processor will be idle Too many processes will lead to thrashing
- 56. 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
- 57. 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
- 58. UNIX and Solaris Memory Management Paging system for processes Kernel memory allocator for memory allocation for the kernel
- 59. UNIX and Solaris Memory Management Data Structures Page table - one per process Disk block descriptor - describes the disk copy of the virtual page Page frame data table - describes each frame of real memory Swap-use table - one for each swap device
- 60. UNIX and Solaris Memory Management Page Replacement refinement of the clock policy known as the two-handed clock algorithm Kernel Memory Allocator most blocks are smaller than a typical page size
- 61. Windows NT Memory Management All process share the same 2 Gbyte of system space Pages available reserved for a process but does not count against the process’s memory quota committed