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Chapter 9: Virtual Memory

Chapter 9: Virtual Memory. 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. Background.

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Chapter 9: Virtual Memory

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  1. Chapter 9: Virtual Memory

  2. 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

  3. 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

  4. Page Fault • 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: • Invalid reference  abort • Just not in memory • Get empty frame • Swap page into frame • Reset tables • Set validation bit = v • Restart the instruction that caused the page fault

  5. Steps in Handling a Page Fault

  6. What happens if there is no free frame? • Page replacement – find some page in memory, but not really in use, swap it out • algorithm should be efficient; want algorithm to minimize number of page faults • Dilemma: same page may be brought into memory several times

  7. Need For Page Replacement

  8. Basic Page Replacement • Find the location of the desired page on disk • Find a free frame: - If there is a free frame, use it - If there is no free frame, use a page replacement algorithm to select a victimframe • Bring the desired page into the (newly) free frame; update the page and frame tables • Restart the process

  9. Page Replacement

  10. Page Replacement Algorithms • Want lowest page-fault rate to keep memory overhead down • Page replacement algorithms are evaluated by running it on a particular string of memory references (reference string) and computing the number of page faults on that string • Assumptions: all pages are 100 Bytes in size (frames 100B) if only page 1 loaded (in a frame) then any memory request for addresses 100 – 199 will not cause page fault; however, a request for address 234 will cause page fault s.t. page 2 will be loaded into a frame. • In all our examples, the reference string is 0100, 0210, 0302, 0467, 0189, 0234, … , 0521 1, 2, 3, 4, 1, 2, 5, 1, 2, 3, 4, 5

  11. Graph of Page Faults Versus The Number of Frames

  12. FIFO Page Replacement

  13. First-In-First-Out (FIFO) Algorithm • Reference string: 1, 2, 3, 4, 1, 2, 5, 1, 2, 3, 4, 5 • 3 frames (3 pages can be in memory at a time per process) • 4 frames • Belady’s Anomaly: more frames  more page faults 1 1 4 5 2 2 1 3 9 page faults 3 3 2 4 1 1 5 4 2 2 1 10 page faults 5 3 3 2 4 4 3

  14. Optimal Algorithm • Replace page that will not be used for longest period of time • 4 frames example 1, 2, 3, 4, 1, 2, 5, 1, 2, 3, 4, 5 • How do you know which page to replace? i.e. which will not be used for a long period of time. You cannot. There fore this algorithm is used for as the theoretical best solution. Not implementable. • Optimal is used for comparison against implementable algorithms. 1 4 2 6 page faults 3 4 5

  15. Optimal Page Replacement

  16. Least Recently Used (LRU) Algorithm • Reference string: 1, 2, 3, 4, 1, 2, 5, 1, 2, 3, 4, 5 • Counter implementation • Every page entry has a counter; every time page is referenced through this entry, copy the clock into the counter • When a page needs to be changed, look at the counters to determine which counter has the oldest time stamp 1 1 1 5 1 2 2 2 2 2 5 4 3 4 5 3 3 4 3 4

  17. LRU Page Replacement - Least used page

  18. LRU Algorithm (Cont.) • Stack implementation – keep a stack of page numbers in a double link form: • Page referenced: • move it to the top (stack) • costs: requires 6 pointers to be changed • No search for replacement b/c the bottom of the stack will always contain the least recently used page.

  19. Use Of A Stack to Record The Most Recent Page References top of stack top of stack

  20. LRU Approximation Algorithms • Reference bit • With each page associate a bit, initially = 0 • When page is referenced set reference bit = 1 • Replace the one which is 0 (if one exists) • We do not know the order, however • Second chance • Need reference bit • Clock algorithm b/c it simulates a clock by using a circular queue • Select victim by cycling through pages seeking a reference bit = 0: • if reference bit = 1, set bit to 0, move to next page • replace next page with bit = 0 • NOTE: if all bits originally set, then algorithm becomes FIFO.

  21. Second-Chance (clock) Page-Replacement Algorithm

  22. Counting Algorithms • Keep a counter of the number of references that have been made to each page • LFU Algorithm: replaces page with smallest count • MFU Algorithm: based on the argument that the page with the smallest count was probably just brought in and has yet to be used

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