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week9_wed - CSCC 69H3 Operating Systems Spring 2010...

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CSCC 69H3 Operating Systems Spring 2010 Professor Bianca Schroeder U of T
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Before we start some questions: Anybody still working on A2? Anybody whose group has fallen apart?
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What are possible replacement algorithms? Not-recently-used (NRU) First-in-first-out (FIFO) Least-recently-used (LRU) Least-frequently-used Most-frequently-used Ease of implementation Performance + + - - -
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What are possible replacement algorithms? Not-recently-used (NRU) First-in-first-out (FIFO) Least-recently-used (LRU) Least-frequently-used Most-frequently-used Ease of implementation Performance + ~ + ~ - ++ - - - -
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What are possible replacement algorithms? Not-recently-used (NRU) First-in-first-out (FIFO) Least-recently-used (LRU) Least-frequently-used Most-frequently-used Second chance Ease of implementation Performance + ~ + ~ - ++ - - - - + +
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Second-Chance Idea: FIFO (First-in-first-out) considers only age NRU (Not recently used) considers only usage Maybe we should combine the two! Second chance algorithm: Don’t evict the oldest page if it has been used. Evict the oldest page that has not been used. Pages that are used often enough to keep reference bits set will not be replaced
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Modelling Clock 1st page fault: Advance hand to frame 4, use frame 3 2nd page fault (assume none of these pages are referenced) Advance hand to frame 6, use frame 5 R=1 R=1 R=1 R=0 0 1 2 3 4 5 6 7 8 Physical Frames R=0 R=1
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Fixed vs. Variable Space In a multiprogramming system, we need a way to allocate memory to competing processes Problem: How to determine how much memory to give to each process? Fixed space algorithms Each process is given a limit of pages it can use When it reaches the limit, it replaces from its own pages Local replacement
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