Lecture 15: Page Replacement Algorithms Tanenbaum Ch' 4'4

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Lecture 15Page Replacement Algorithms
(Tanenbaum Ch. 4.4)
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Where We Are Memory Management

• Covered so far (and not on slides)
• Paging
• Page tables
• Logical -gt physical address translation
• Virtual memory with paging
• Motivation, consequences
• Structure of page entry
• Next
• Page replacement algorithms (today)
• Multi-level page tables
• Inverted page tables
• Design issues for paging systems
• Segmentation

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Page Replacement Algorithms

• Page fault forces choice
• which page must be removed
• make room for incoming page
• Modified page must first be saved
• unmodified just overwritten
• Better not to choose an often used page
• will probably need to be brought back in soon
• Want lowest page-fault rate
• Evaluate algorithm by running it on a particular
  string of memory references (reference string)
  and computing the number of page faults on that
  string

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Optimal Page Replacement
Replace page for which the time to the next
reference is the longest
Problem to know the future.
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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)
• Beladys Anomaly more frames ? more page faults