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OPERATING SYSTEMS DESIGN AND IMPLEMENTATION Third Edition ANDREW S. TANENBAUM ALBERT S. WOODHULL

Lecture 10, 19 November 2013. OPERATING SYSTEMS DESIGN AND IMPLEMENTATION Third Edition ANDREW S. TANENBAUM ALBERT S. WOODHULL. Chap. 4 Memory Management 4.4 Page Replacement Algorithms 4.5 Design Issues 4.6 Segmentation. 4.4 Page Replacement Algorithms. Page fault forces choice

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OPERATING SYSTEMS DESIGN AND IMPLEMENTATION Third Edition ANDREW S. TANENBAUM ALBERT S. WOODHULL

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  1. Lecture 10, 19 November 2013 OPERATING SYSTEMSDESIGN AND IMPLEMENTATIONThird EditionANDREW S. TANENBAUMALBERT S. WOODHULL Chap. 4 Memory Management 4.4 Page Replacement Algorithms 4.5 Design Issues 4.6 Segmentation

  2. 4.4 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 Tanenbaum & Woodhull, Operating Systems: Design and Implementation, (c) 2006 Prentice-Hall

  3. 4.4.1 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 Tanenbaum & Woodhull, Operating Systems: Design and Implementation, (c) 2006 Prentice-Hall

  4. 4.4.2 Not Recently Used Page Replacement Algorithm • Each page has Reference bit, Modified bit • bits are set when page is referenced, modified • R bits are cleared periodically • 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 Tanenbaum & Woodhull, Operating Systems: Design and Implementation, (c) 2006 Prentice-Hall

  5. 4.4.3 FIFO Page Replacement Algorithm • Maintain a linked list of all pages • in order they came into memory • Page at beginning of list is replaced • Disadvantage • page in memory the longest may be often used, and should not be replaced ! Tanenbaum & Woodhull, Operating Systems: Design and Implementation, (c) 2006 Prentice-Hall

  6. 4.4.4 Second Chance Page Replacement Algorithm Fig. 4.14 Operation of a second chance • pages sorted in FIFO order • If page fault occurs at time 20 and A has R bit set, it is moved to the end of the list (numbers above pages are loading times) Tanenbaum & Woodhull, Operating Systems: Design and Implementation, (c) 2006 Prentice-Hall

  7. 4.4.5 The Clock Page Replacement Algorithm Fig. 4.15 The clock page replacement algorithm Tanenbaum & Woodhull, Operating Systems: Design and Implementation, (c) 2006 Prentice-Hall

  8. 4.4.6 Least Recently Used (LRU) • Assume pages used recently will used again soon • throw out page that has been unused for longest time • Solution 1: keep a linked list of pages • most recently used at front, least at rear • update this list every memory reference !! • Solution 2: ‘timestamp’the page table entry for the page just referenced with a counter • choose page with lowest value counter Tanenbaum & Woodhull, Operating Systems: Design and Implementation, (c) 2006 Prentice-Hall

  9. 4.4.6 Least Recently Used (LRU): solution 3 • Data structure: Use a matrix of n*n bits, where n is the number of the page table entries • Algorithm: If page x is referenced, set all 1 in the x row, and all 0 in the column 0 • Property: The row whose binary value is lowest is the least recently used Fig. 4.16 LRU using a matrix – pages are referenced in the order 0, 1, 2, 3, 2, 1, 0, 3, 2, 3 0 1 2 3 2 1 0 3 2 3 Tanenbaum & Woodhull, Operating Systems: Design and Implementation, (c) 2006 Prentice-Hall

  10. 4.4.7 Simulating LRU in Software • Idea • a counterand a R bit are associated to each page of the page table • the counter roughly keeps track how often each page has been ref. in the near past: • at every clock, all counters are shifted right by 1 bit and the associated R bit is added Fig. 4.17 The aging algorithm simulates LRU in software. Shown are 6 pages for 5 clock ticks, (a) – (e) Tanenbaum & Woodhull, Operating Systems: Design and Implementation, (c) 2006 Prentice-Hall

  11. 4.5.1 The Working Set Model Definition. The working set is the set of pages used by the k most recent memory references, and w(k,t) is the size of the working set at time t. Property. w(k,t) is a monotonically nondecreasing function of k (Hint: larger k means looking further in the past k Fig. 4.18 Tanenbaum & Woo)dhull, Operating Systems: Design and Implementation, (c) 2006 Prentice-Hall

  12. The Working Set Page Replacement Algorithm (i.e. current process time) Fig. 3.20 The working set algorithm R bit is cleared at every ‘clock tick’ (tau is assumed to span several ‘clock ticks’) Tanenbaum, Modern Operating Systems, 3rd ed., (c) 2009, Prentice-Hall

  13. The WSClock Page Replacement Algorithm Fig. 3.21 Operation of the WSClock algorithm 2014 2014 Tanenbaum, Modern Operating Systems, 3rd ed., (c) 2009, Prentice-Hall

  14. Review of Page Replacement Algorithms Tanenbaum, Modern Operating Systems, 3rd ed., (c) 2009, Prentice-Hall

  15. 4.6 Segmentation • Fact. A compiler has many tables that are built up as compilation proceeds, possibly including: • The source text being saved for the printed listing (on batch systems). • The symbol table – the names and attributes of variables. • The table containing integer, floating-point constants used. • The parse tree, the syntactic analysis of the program. • The stack used for procedure calls within the compiler. Tanenbaum & Woodhull, Operating Systems: Design and Implementation, (c) 2006 Prentice-Hall

  16. 4.6 Segmentation: bumping problem Fig. 4.21 One-dimensional address space with growing tables, one table may bump into another Tanenbaum & Woodhull, Operating Systems: Design and Implementation, (c) 2006 Prentice-Hall

  17. 4.6 Segmentation: segmented memory Fig. 4.22 A segmented memory allows each table to grow or shrink, independently of the other tables Tanenbaum & Woodhull, Operating Systems: Design and Implementation, (c) 2006 Prentice-Hall

  18. 4.6 Segmentation: segmentation vs paging Fig. 4.23 Comparison of paging and segmentation Tanenbaum & Woodhull, Operating Systems: Design and Implementation, (c) 2006 Prentice-Hall

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