NRU (Not Recently Used) Page Replacement Algorithm

Lists of Descriptive Questions Answers and Short Study Notes On Operating System – NRU (Not Recently Used) Page Replacement Algorithm

  • (1) Explain NRU (Not Recently Used) Page Replacement Algorithm

 

Question-1 Explain NRU (Not Recently Used) Page Replacement Algorithm

NRU (Not Recently Used)

  • NRU makes approximation to replace the page based on R and M bits.
  • When a process is started up, both page bits for all pages are set to 0 by operating system.
  • Periodically, the R bit is cleared, to distinguish pages that have not been referenced recently from those that have been.
  • When page fault occurs, the operating system inspects all the pages and divide them into 4 categories based on current values of their R and M bits
    • Case 0 : not referenced, not modified
    • Case 1 : not referenced, modified
    • Case 2 : referenced, not modified
    • Case 3 : referenced, modified
  • The NRU (Not Recently Used) algorithm removes a page at random from the lowest numbered nonempty class.
  • In given example,
    • Page-0 is of class-2 (referenced, not modified)
    • Page-1 is of class-1 (not referenced, modified)
    • Page-2 is of class-0 ( not referenced, not modified)
    • Page-3 is of class-3 (referenced, modified)

So lowest class page-2 needs to be replaced by NRU.