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Lecture23nFull - COT 4600 Operating Systems Spring 2011...

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Click to edit Master subtitle style 8/1/11 Lecture 23 COT 4600 Operating Systems Spring 2011 Dan C. Marinescu Office: HEC 304 Office hours: Tu-Th 5:00-6:00 PM
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Lecture 23 8/1/11 Lecture 23 - Tuesday April 14, 2011 n Last time: ¨ Memory characterization ¨ Multilevel memories management using virtual memory ¨ Adding multi-level memory management to virtual memory n Today: ¨ Optimizations for dynamic address translation – the TBL ¨ The working set ¨ Page replacement algorithms ¨ Performance. Basic concepts regarding queuing systems n Arrival process n Service process n Next Time: ¨ Performance 22
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Lecture 23 8/1/11 Memory management elements at each level 1. The string of references directed at that level. 2. The capacity at that level 3. The bring in policies 1. On demand ° bring the cell to the primary device from the secondary device when it is needed. E.g., demand paging 2. Anticipatory. E.g. pre-paging 4. The replacement policies ¨ ° FIFO ° First in first out ¨ ° OPTIMAL ± what a clairvoyant multi-level memory manager would do. Alternatively, construct the string of references and use it for a second execution of the program (with the same data as input). ¨ ° LRU – Least Recently Used v replace the page that has not been referenced for the longest time. ¨ ° MSU – Most Recently Used i replace the page that was referenced most recently 33
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Lecture 23 8/1/11 The translation look-aside buffer -TLB n TLB - Cache for page table entries. n Function: optimize the dynamic address translation . n Basic idea: keep in a fast memory the address in main memory where a page is loaded; thus avoid to access the page table to translate addresses in the same page. 44
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Lecture 23 8/1/11 Working set and thrashing n Provide a characterization of the locality of a program. n Working set: the set of last Δ pages referenced. n Example Δ=10 ….2 6 1 5 7 7 7 7 5 1 | 6 2 3 4 1 2 3 4 4 4 3 4 3 4 4 4 4 | 1 5 7 3 8 7 6 t1 } ° t2 WS(t1)={1,2,5,6,7} WS(t2)={3,4} n How to select Δ ¨ If too small will not characterize the locality ¨ If too large may overlap two localities n The size of the working set tell us how much memory a program needs. n If the size of the real memory is smaller than the size of the working set the program keeps generating page faults 55
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Lecture 23 8/1/11 Terminology n The main memory consists of blocks of size equal to the page size. n Page replacement policy : strategy to decide which block can be freed to allow a new page to be brought in.
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