Stuff iThink that will be on the Quiz/Midterm.

1. Stuff iThinkthat will be on the Quiz/Midterm. Page Replacement by Andrew Yee

2. What is Page Replacement? Virtual Memory Memory Management

3. Why Page Replacement? • Cache is limited • Need ways to manage virtual memory in Cache

4. How to Page Replace? • Cache is limited small, you have to figure out ways to choose which pages to keep in cache. Which you think would most likely be used again in the future . • FIFO and LRU are techniques used to determine which pages should be kept cache. • Optimal is the best possible outcome because you use future knowledge which pages will be used again to determine which pages to keep. • 3 Ways to manage Virtual Memory in Cache • Optimal – Best possible outcome • LRU - Least Recently Used • FIFO - First In First Out Think “Stacks”

5. How to Represent Pages in Time? Time increases Frame set that can 3 page reference string. Frame cell that can hold a page reference string. CPU issues the page reference string. Think Animated “Stacks”

6. How to Represent Page Hit and Page Fault (miss)? Time increases Page Hit – Don’t box the frame set Page Fault (miss) – box the frame set Think Animated “Stacks”

7. Optimal Method • What you need? Requires Future Knowledge.

8. Example of Optimal Method 1 2 1 3 2 1 4 2 1 4 2 1 4 2 1 4 2 1 4 2 1 4 2 1 4 5 1 4 5 1 4 5 1 4 5 1 4 5 1 5 –Page Fault (Missed) Each of the rectangle is a frame, and each number is a page. Page fault (miss) is when you cant find the page in cache. A hit is when you do find it. You want to minimize the # of miss. Each cell contains a Page.

9. FIFO – First in First Out Method • How? First Page In First Page Out

10. Example of FIFO Method 1 2 1 3 2 1 4 3 2 4 3 2 1 4 3 2 1 4 2 1 4 2 1 4 5 2 1 4 5 2 4 5 2 1 4 5 1 4 5 9 –Page Fault (Missed) Each of the rectangle is a frame, and each number is a page. Page fault (miss) is when you cant find the page in cache. A hit is when you do find it. You want to minimize the # of miss. Each cell contains a Page.

11. LRU – Least Recently Used • Key to this method is: Past experience may indicate future behavior

12. Example of LRU Method 1 2 1 3 2 1 4 3 2 4 3 2 1 4 3 2 1 4 1 2 4 4 1 2 5 4 1 4 5 1 5 4 1 1 5 4 4 1 5 7 –Page Fault (Missed) Each of the rectangle is a frame, and each number is a page. Page fault (miss) is when you cant find the page in cache. A hit is when you do find it. You want to minimize the # of miss. Each cell contains a Page.

13. Page Fault Comparison

14. Practical Method Comparison

15. Hit Ratio • Hit Ratio = (Total # of Frame Set - # of Page Fault) ---------------------------------------------------- * 100 Total # of Frame Set

16. Hit Ratio

17. Practice Example • Compute the hit ratio of each method: • Optimal, FIFO, LRU

18. Practice Example • Compute the hit ratio of each method: • Optimal 1 3 1 3 6 3 1 4 6 3 Number of Page Fault = 4 Hit ratio = (5 - 4) / 5 * 100 = 20%

19. Practice Example • Compute the hit ratio of each method: • FIFO 1 3 1 3 1 6 3 1 4 6 3 Number of Page Fault = 4 Hit ratio = (5 - 4) / 5 * 100 = 20%

20. Practice Example • Compute the hit ratio of each method: • LRU 1 3 1 3 1 6 3 1 4 6 3 Number of Page Fault = 4 Hit ratio = (5 - 4) / 5 * 100 = 20%