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LRU-K485

# LRU-K485 - LRU-K Page Replacement Algorithm CSCI 485...

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Unformatted text preview: LRU-K Page Replacement Algorithm CSCI 485 Lecture notes Instructor: Prof. Shahram Ghandeharizadeh. Outline • History • Motivation for LRU-K • Alternatives to LRU-K • LRU-K • Design and implementation • Conclusion History • LRU-K is attributed to Elizabeth J. O’Neil, Patrick E. O’Neil, and Gerhard Weikum: – The LRU-K Page Replacement Algorithm for Database Disk Buffering, ACM SIGMOD 1993, Washington D.C., page 297-306. Least Recently Used (LRU) • When a new buffer page is needed, the buffer pool manager drops the page from buffer that has not been accessed for the longest time. • Originally for patterns of use in instruction logic (Denning 1968). • Limitation: Decides what page to drop from buffer based on too little information (time of last reference). Pseudo-code for LRU LRU (page p) If p is in the buffer then LAST(p) = current time; Else i) Min = current time + 1; ii) For all pages q in the buffer do a) If (LAST(q) < min) victim = q Min = LAST(q) iii) If victim is dirty then flush it to disk Example 1: LRU Limitation • Consider a non-clustered, primary B-tree index on the SS# attribute of the Employee table. – t(Emp) = 20,000 – P(Emp) = 10,000 (2 records per disk page) – lp(I, Emp) = 100 – Workload: queries that retrieve Emp records using exact match predicates using SS# attribute, e.g., SS#=940-98-7555 • If the B-tree is one-level deep (root-node, followed by the 100 leaf pages), pattern of access is: Ir, I1, D1, Ir, I2, D2, Ir, I3, D3, …. • Assume your workload consists of 101 frames, what is the ideal way to assign leaf pages and data pages to these frames? What will LRU do?...
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LRU-K485 - LRU-K Page Replacement Algorithm CSCI 485...

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