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Virtual Memory - Coggle Diagram

- - - - Determining necessary pages
        
        Requires new MMU functionality
    - - OS determines whether it's invalid page or just not in memory
      - OS then finds a free frame
      - OS Swaps page into frame via scheduled disk OP and reset tables
    - - Extreme (pure)
      - Hardware support to improve page faults
    - - Swap space I/O instead of file-system I/O which allows you to copy entire process to swap space at load time then swap in/out of memory (used in old BSD Unix)
      - Demand pages for program binary files, overwrite, swap space with heap memory (current BSD)
- - - - FIFO (Replace oldest(first) frame with new frame
      - LRU (Least Recently Used) (Implemented using a stack or a counter) (requires special slow hardware)
      - LRU Approximation Algorithms
        
        Reference bit for each page that is set once the page has been used, suffers that all pages may be set to 1, in which case the bits are reset
        
        Second-Chance (clock replacement) where the bit starts with 1, then is set to 0, then the respective page is replaced
        
        Enhanced Second-Chance where you use the pair (reference bit, modify bit)
      - Counting (counts the number of each page's uses) (not common)
        
        LFU (Least Frequently Used), suffers that new processes will be caught as least frequently used
        
        MFU (Most Frequently Used), Based on the argument that a page with the smallest count was probably just brought in and has yet to be used
      - Page Buffering
        
        Always keep a pool of free frames
        
        Possibly keep a list of modified pages, when storage idle write those pages
        
        Possibly keep free frame contents undamaged to not load them from disk if referenced against before reuse
      - Adding more frames may actually result in more page faults (Belady's Anomaly)
      - Optimal
        
        "Optimal" where you replace page that will not be used for longest period of time (last out) (predicts future) (impossible)
        
        Used only when comparing different algorithms
    - - Fixed
        
        Equal Allocation
        
        Proportional Allocation
      - Priority
        
        Allocation depends on priority of processes
      - Global vs Local
        
        Global Replacement where a process selects from a set of all frames in memory, processes can trade frames
        
        Local Replacement where each process only uses its own allocated frames, potentially causing underutilized memory
    - - Prevents over-allocation of memory
      - Use modify dirty bit to indicate this page has been changed