9. page_replacement - Silberschatz Galvin and Gagne 2002...

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Silberschatz, Galvin and  Gagne  2002 10.1 Virtual Memory
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Silberschatz, Galvin and  Gagne  2002 10.2 Background Virtual memory  – separation of user logical memory  from physical memory. Only part of the program needs to be in memory for  execution. Logical address space can therefore be much larger  than physical address space. Allows address spaces to be shared by several  processes. Allows for more efficient process creation. Virtual memory can be implemented via: Demand paging  Demand segmentation
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Silberschatz, Galvin and  Gagne  2002 10.3 Demand Paging Bring a page into memory only when it is needed. Less I/O needed Less memory needed  Faster response More users Page is needed   reference to it invalid reference   abort not-in-memory   bring to memory
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Silberschatz, Galvin and  Gagne  2002 10.4 Valid-Invalid Bit With each page table entry a valid–invalid bit is  associated (1   in-memory, 0   not-in-memory) Initially valid–invalid but is set to 0 on all entries. Example of a page table snapshot. During address translation, if valid–invalid bit in page  table entry is 0   page fault. 1 1 1 1 0 0 0 Frame # valid-invalid bit page table
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Silberschatz, Galvin and  Gagne  2002 10.5 Page Fault If there is ever a reference to a page, first reference will trap to  OS   page fault OS looks at another table to decide: Invalid reference   abort. Just not in memory. Get empty frame. Swap page into frame. Reset tables, validation bit = 1. Restart instruction:
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Silberschatz, Galvin and  Gagne  2002 10.6 Steps in Handling a Page Fault
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Silberschatz, Galvin and  Gagne  2002 10.7 What happens if there is no free frame? Page replacement – find some page in memory, but not  really in use, swap it out. algorithm performance – want an algorithm which will result in minimum  number of page faults. Same page may be brought into memory several times.
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Silberschatz, Galvin and  Gagne  2002 10.8 Performance of Demand Paging Page Fault Rate 0    p    1.0 if  p  = 0 no page faults  if  p  = 1, every reference is a fault Effective Access Time (EAT) EAT = (1 –  p ) x memory access p  (page fault overhead)  [swap page out + swap page in+ restart overhead]
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Silberschatz, Galvin and  Gagne  2002 10.9 Demand Paging Example Memory access time = 1 microsecond 50% of the time the page that is being replaced has been  modified and therefore needs to be swapped out.
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