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CGS 3763 Operating Systems Concepts Spring 2013

CGS 3763 Operating Systems Concepts Spring 2013. Dan C. Marinescu Office: HEC 304 Office hours: M- Wd 11:30 - 12:30 A M. Last time: Problem solving – Dynamic Address Translation Today Page replacement algorithms Next time Virtua l memory Reading assignments

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CGS 3763 Operating Systems Concepts Spring 2013

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  1. CGS 3763 Operating Systems Concepts Spring 2013 Dan C. Marinescu Office: HEC 304 Office hours: M-Wd 11:30 - 12:30 AM

  2. Last time: Problem solving – Dynamic Address Translation Today Page replacement algorithms Next time Virtual memory Reading assignments Chapters 8 and 9 of the textbook Lecture 36 – Friday, April 12, 2013 Lecture 36

  3. Page replacement Lecture 36

  4. Page Replacement Algorithms • 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 • In all our examples, the reference string is 1, 2, 3, 4, 1, 2, 5, 1, 2, 3, 4, 5 Lecture 36

  5. Page replacement policies; Belady’s anomaly In the following examples we use a given string of references to illustrate several page replacement policies. We consider a primary device (main memory) with a capacity of three or four blocks and a secondary device (the disk) where a replica of all pages reside. Once a block has the “dirty bit” on it means that the page residing in that block was modifies and must be written back to the secondary device before being replaced. The capacity of the primary device is important. One expects that increasing the capacity, in our case the number of blocs in RAM leads to a higher hit ratio. That is not always the case as our examples will show. This is the Belady’s anomaly. Note: different results are obtained with a different string of references!! Lecture 36

  6. FIFO Page replacement algorithm PS: Primary storage Lecture 36

  7. OPTIMAL page replacement algorithm Lecture 36

  8. LRU page replacement algorithm Lecture 36

  9. LRU, OPTIMAL, MRU LRU looks only at history OPTIMAL “knows” not only the history but also the future. In some particular cases Most Recently Used Algorithm performs better than LRU. Example: primary device with 4 cells. Reference string 0 1 2 3 4 0 1 2 3 4 0 1 2 3 4 LRU F FFFFFFFFFFFFFF MRU F FFF - - - - F - - - F - - Lecture 36

  10. The OPTIMAL replacement policy keeps in the 3-blocks primary memory the same pages as it does in case of the 4-block primary memory. Lecture 36

  11. The FIFO replacement policy does not keep in the 3-blocks primary memory the same pages as it does in case of the 4-block primary memory. Lecture 36

  12. The LRU replacement policy keeps in the 3-blocks primary memory the same pages as it does in case of the 4-block primary memory. Lecture 36

  13. The FIFO replacement policy does not keep in the 3-blocks primary memory the same pages as it does in case of the 4-block primary memory Lecture 36

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