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Virtual Memory (Page Fault (Operating system looks at another table to…
Virtual Memory
Page Fault
If there is a reference to a page, first reference to that page will trap to OS
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Demand Paging
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Optimizations
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Demand page in from program binary on disk, but discard rather than paging out when freeing frame
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Mobile systems
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Instead, demand page from file system and reclaim read-only pages
Least Recetly Used
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Counter implementation
When a page needs to be changed, look at the counters to find smallest value
Every page entry has a counter; every time page is referenced through this entry, copy the clock into the counter
LRU Algorithms
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Reference bit
With each page associate a bit, initially = 0
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Second-chance algorithm
Generally FIFO, plus hardware-provided reference bit
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Virtual-address space
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Enables sparse address spaces with holes left for growth, dynamically linked libraries
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Page Replacement
Use modify (dirty) bit to reduce overhead of page transfers – only modified pages are written to disk
Page replacement completes separation between logical memory and physical memory – large virtual memory can be provided on a smaller physical memory
Prevent over-allocation of memory by modifying page-fault service routine to include page replacement
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Thrashing
If a process does not have “enough” pages, the page-fault rate is very hig
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