virtual - Virtual Memory s Background s Demand Paging s...

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Virtual Memory Background Demand Paging Page Replacement Allocation of Frames Thrashing
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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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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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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.
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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: Least Recently Used block move auto increment/decrement location
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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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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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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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virtual - Virtual Memory s Background s Demand Paging s...

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