Virtual Memory

Virtual Memory

1. Paging Continued

   • Demand Paging
     • Bringing a page into memory only when it's required
       • Less I/O
       • Less memory needed
       • Faster response
       • More users
       • Lazy swapper
         • Pager = Swapper dealing with pages
   • Page Faults
     • Referencing page not in memory
       • 1. OS looks at another table - Invalid reference, or just not in memory
       • 2. Get empty frame from list
       • 3. Swap page into frame
       • 4. Reset tables
       • 5. Set validation bit = v (in memory)
       • 6. Restart instruction which caused page fault
     • Trap to OS
     • Page fault overhead (p)= service page fault interrupt + read page in + restart
       • Effective Access Time = (1-p) x memory access + p x page fault overhead
   • Page Replacement
     • When there is no free frame
       • First In, First Out Algorithm
       • Optimal Algorithm
         • Replace page which won't be used for longest period of time
       • Least Recently Used Algorithm
         • Replace the page that has not been used for the longest period of time
           • Counter Implementation
             • Each page has a counter - Look at counter to determine change
           • Stack Implementation
             • Stack of page numbers in double link form
               • Page referenced - Move it to top, 6 pointers to be changed
                 • Eliminates need to search
         • Second-chance list

2. Allocation of Frames

   • Fixed Allocation
     • Equal Alocation
       • E.g. 100 frames, 5 processes = Each process has 20 frames
     • Proportional Allocation
       • Priorities rather than sizes
         • If a process generates a page fault: replace (lower priority)
   • Priority Allocation
   • Global v Local Allocation
     • Local Replacement
       • Each process selects only from its own allocated frames
     • Global Replacement
       • Process selects a replacement frame from whole set of frames
         • Greater system throughput

3. Thrashing

   • CPU spends more time swapping than executing instructions
     • Number of frames allocated is below minimum number required

4. Working Set

   • Set of pages accessed by a process along a given period

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