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Page Replacement (cont)

Factors to consider in deciding which page to replace
  • best page is one that won't be used again by its process
  • prefer pages that are read-only  (no need to write to disk)
  • prefer pages that are unmodified  (no need to write to disk)
  • prefer pages that are used by only one process  (see later)
OS can't predict whether a page will be required again by its process

But we do know whether it has been used recently (if we record this)

Useful heuristic: LRU replacement

  • a page not used recently may not be needed again soon