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Virtual memory (TLB (Is a cache (Replacement, Other cache design…

- - - - Page fault
        
        Invoke OS
    - - Fetch from page table, update TLB
- - - - Many page faults when process first starts
    - - Effective if pages are stored contiguously in secondary memory
      - Ineffective if pages are not referenced
      - Selective employment
        
        At start of process
        
        At page fault
  - - - Impossible to implement
      - A 'yardstick' for efficiency
    - - Difficult to implement, overhead
    - - Circular buffer, round-robin removal
      - Simple implementation
      - Performs poorly w/ time locality
    - - 'Use'-bit w/ each frame
        
        Set 1 when referenced
        
        Iterate buffer when replacing
        
        If 'Use' == 1, set to 0
        
        If 'Use'== 0, replace and quit
        
        Can exploit 'Modify'-bit
        
        Replacement hierarchy
        
        1) 'Use' == 0, 'Modify' == 0
        
        2) 'Use' == 0, 'Modify' == 1
      - Circular buffer
      - Some overhead
    - - "To be replaced"-lists
        
        Modified page list
        
        Free page list
        
        Can re-fetch unreplaced pages easily
        
        Enables batch writing for modified pages
    - - Cannot be replaced
  - - - Write out pages when selected for replacement
    - - Writes out before replacement?
      - Allows writing in batches
  - - - Global
        
        Replace any page in resident set
        
        Simple, minimal overhead
      - Local
        
        Replace only pages of process w/ page fault
    - - Predetermined # of frames / process
    - - Increase frames for process w/ many page faults
      - Decrease frames for process w/ few page faults
      - SW overhead in assessing process behavior
    - - Too small allocation > high PFR
      - Too large allocation > too few processes in main memory
        
        Increased processor idling
        
        More time wasted swapping
    - - Periodically re-evaluate allocation size
      - Working set strategy
        
        Set of pages referenced in d virtual time units
        
        Virtual time unit = memory reference
        
        Changing of locality
        
        Remove pages from resident set not in working set
        
        Execute only if working set in main memory
        
        Issues
        
        Optimal value for d unknown
        
        Measuring working set for each process impractical
      - PFF
        
        'Use'-bit on pages
        
        On page fault, note virtual time since last one
        
        If d < some threshold, add a page to resident set
        
        Else discard all pages with 'Use' == 0
        
        Shrinks resident set
        
        Set for remaining pages 'Use' == 0
        
        Performs badly when changing localities
      - Variable-interval sampled working set policy
    - - Simple implementation
      - + Page buffering
      - Allocation size changes 'randomly' on replacement
  - - - Process suspension
- - - - 'Present'-bit
      - 'Modify'-bit
      - Other control bits
      - Frame number
    - - Virtual page # > Hash function?
        
        Maps to real memory frame?
      - One entry per real memory frame
      - Entry
        
        Page number
        
        Process ID
        
        Control bits
        
        Valid
        
        Referenced
        
        Modified
        
        Locked
        
        Chain pointer
      - Indexed by frame number
    - - Page number
      - Offset
    - - Root page table
      - User page table
  - - - Page table size
      - Amount of internal fragging
      - Page fault rate
- - - - Segment table / process
      - Page table / segment
  - - - Page number
      - Page offset
  - - - Segment number
      - Page number
      - Offset
    - - Control bits
      - Length
      - Segment base (page table)
    - - 'Present'-bit
      - 'Modify'-bit
      - Other control bits
      - Frame number
- - - - 'Present'-bit
      - 'Modify'-bit
      - Other control bits
      - Length
      - Segment base
    - - Segment number
      - Offset