1. Virtual Memory & Paging Algorithms Chapter 4 Tuesday, March 26, 2007

2. Today’s Schedule • Memory Management - Chapter 4 • Managing Page Tables • Multi-Level Page Tables • Inverted Page Tables • Page Replacement Algorithms • NRU • FIFO • Second Chance • Clock

3. Objectives You will be able to describe: • Trade-offs in page table implementation for speed and space • TLB • Multi-level page tables • Inverted page tables • The difference between page replacement policies • first-in first-out • not-recently-used page • clock and second chance

4. Paging Overview

5. Paging Hardware With TLB A TLB to speed up paging

6. TLBs – Translation Lookaside Buffers An Item is missing in the TLB entry? Why?

7. Page Table Entry Typical page table entry

8. page number page offset p d m - n n Address Translation Scheme • Address generated by CPU is divided into: • Page number (p) – used as an index into a pagetable which contains base address of each page in physical memory • Page offset (d) – combined with base address to define the physical memory address that is sent to the memory unit • For given logical address space 2m and page size2n

9. How Big is a Page Table? • For 32-bit Address Space?? • For 64-bit Address Space??

10. Multi-Level Page Tables Second-level page tables • 32 bit address with 2 page table fields • Two-level page tables – save space! Top-level page table

11. Inverted Page Table • One entry for each real page of memory • Entry consists of the virtual address of the page stored in that real memory location, with information about the process that owns that page • Decreases memory needed to store each page table, but increases time needed to search the table when a page reference occurs • Use hash table to limit the search to one — or at most a few — page-table entries

12. Inverted Page Tables Comparison of a traditional page table with an inverted page table

13. Allocating Free Frames After allocation Before allocation

14. 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

15. Optimal Page Replacement Algorithm • Replace page needed at the farthest point in future • Optimal but unrealizable • Estimate by … • logging page use on previous runs of process • although this is impractical

16. Not Recently Used Page Replacement • Each page has Reference bit, Modified bit • bits are set when page is referenced, modified • Pages are classified • Class 0 - not referenced, not modified • Class 1 - not referenced, modified • Class 2 - referenced, not modified • Class 3 - referenced, modified • NRU removes page at random • from lowest numbered non empty class

17. FIFO Page Replacement Algorithm • Maintain a linked list of all pages • in order they came into memory • Page at beginning of list replaced • Disadvantage • page in memory the longest may be often used

18. Ex. FIFO Page Replacement Working of a FIFO algorithm for a job with four pages(A, B, C, D) as it’s processed by a system with only two available page frames

19. Second Chance Page Replacement • Operation of a second chance • pages sorted in FIFO order • Page list if fault occurs at time 20, A has R bit set(numbers above pages are loading times)

20. The Clock Page Replacement Algorithm

21. Summary • Trade-offs in page table implementation for speed and space • TLB in H/W • Multi-level page tables • Inverted page tables • The difference between page replacement policies • first-in first-out • not-recently-used page • clock & second chance

22. Thursday, March 29 • Assignment #7 Due Monday, April 2 • Continue reading, Chapter 4 – Memory Management Sec 4.4.8 Working Set Sec 4.4.10 Summary of Page Replacement Sec 4.5.1 Modeling Page Repl. Sec 4.8 Segmentation (pg 249-253)