lec12-PageReplaceAndDisks - Paging Policies CSE120...

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1 CSE120 Principles of Operating Systems Prof Yuanyuan (YY) Zhou Lecture 12: Page Replacement & Disks Nov 9, 2010 November 8, 2010 CSE 120 2 Paging Policies Fetch Strategies When should a page be brought into primary (main) memory from secondary (disk) storage. Placement Strategies When a page is brought into primary storage, where is it to be put? Replacement Strategies Which page now in primary storage is to be removed from primary storage when some other page or segment is to be brought in and there is not enough room. November 8, 2010 CSE 120 3 Issue: Eviction Hopefully, kick out a less-useful page Dirty pages require writing, clean pages don’t Hardware has a dirty bit for each page frame indicating this page has been updated or not Where do you write? To “swap space” Goal: kick out the page that’s least useful Problem: how do you determine utility? Kick out pages that aren’t likely to be used again Heuristic: temporal locality exists November 8, 2010 CSE 120 4 Page Replacement Strategies The Principle of Optimality Replace the page that will not be used again the farthest time in the future. Random page replacement Choose a page randomly FIFO - First in First Out Replace the page that has been in primary memory the longest LRU - Least Recently Used Replace the page that has not been used for the longest time LFU - Least Frequently Used Replace the page that is used least often NRU - Not Recently Used An approximation to LRU. Working Set Keep in memory those pages that the process is actively using.
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2 November 8, 2010 CSE 120 5 Belady’s Algorithm Known as the optimal page replacement algorithm because it has the lowest fault rate for any page reference stream/sequence Idea: Replace the page that will not be used for the longest time in the future Problem: Have to predict the future Why is Belady’s useful then? Use it as a yardstick Compare implementations of page replacement algorithms with the optimal to gauge room for improvement If optimal is not much better, then algorithm is pretty good If optimal is much better, then algorithm could use some work Random replacement is often the lower bound November 8, 2010 CSE 120 6 Optimal Example 12 references, 7 faults Hit Ratio: 5/12 Miss Ratio: 7/12 Evict c (farthest in the future) November 8, 2010 CSE 120 7 First-In First-Out (FIFO) FIFO is an obvious algorithm and simple to implement Maintain a list of pages in order in which they were paged in On replacement, evict the one brought in longest time ago Why might this be good? Maybe the one brought in the longest ago is not being used Why might this be bad? Then again, maybe it’s not
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This note was uploaded on 12/09/2010 for the course CSE CSE 120 taught by Professor Yuanyuanzhou during the Spring '10 term at UCSD.

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lec12-PageReplaceAndDisks - Paging Policies CSE120...

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