Virtual Memory

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Virtual Memory

• Minh Chuong

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Methods for Data Transfer

• Paging fixed sized data being swapped
• Segmentation variable sized memory blocks

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Comparison
  Paging Segmentation
Words per address One Two
Programmer Visible? Invisible to application programmer May be visible to application programmer
Replacing a block Trivial (all blocks are the same size) Hard (must find contiguous, variable-size, unused portion of main memory)
Memory use inefficiency Internal fragmentation (unused portion of page) External fragmentation (unused portion of main memory)
Efficient disk traffic Yes (adjust page size to balance access time and transfer time) Not always (small segments may transfer just a few bytes)
 
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Translation Lookaside Buffer

• TLB a special cache used to keep track of
  recently used translation
• use bit monitor usage for replacement
• dirty bit monitor alterations

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Page Table Actions
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Virtual Memory Summary
CPU
virtual address
TLB
Page table
physical address
Cache
DRAM
physical address
disk address
Disk
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Paged Virtual Memory

• Page Tables
• Paging
• Dynamic Address Translation
• Paging Supervisor
• Permanently Resident Pages

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Page Tables

• Used to translate virtual addresses seen by the
  application program into physical addresses

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Paging

• Process of saving inactive virtual memory pages
  to disk and restoring them to real memory when
  required

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Dynamic Address Translation

• Translates address of particular instructions at
  virtual address to corresponding physical address
  (MMU), raises a page fault exception when page
  tables indicate that the virtual memory page
  isnt currently in real memory

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Paging Supervisor

• Part of the OS that creates and manages page
  tables
• When the dynamic address translation hardware
  raises page fault exception, paging supervisor
  searches the paged files (on disk) and reads it
  into physical memory

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Permanently Resident Pages

• Memory that can not be swapped out for secondary
  storage
• Interrupt
• Page tables
• Data buffer
• Timing-dependent kernel/application

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Page Replacement Algorithms

• Local selects for replacement some page that
  belongs to that same process or group of
  processes sharing a memory partition
• Global free to select any page in memory to
  replace

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Algorithms

• Not recently Used
• First-in, First-out
• Second-chance
• Clock
• Least recently used (and variants of it)
• Random
• Ageing

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Not recently used

• Favours keeping pages in memory that have been
  recently used.
• Class 0 not referenced, not modified
• Class 1 not referenced, modified
• Class 2 referenced, not modified
• Class 3 referenced, modified
• Randomly picks from lowest category for removal

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First-in, first-out

• Pages in memory are kept in a queue, pages in the
  front gets replaced first

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Second-chance

• Similar to first-in, first-out except that
  theres a reference bit, if front of queues
  pages reference bit is not set, it will be
  swapped out, else it is moved to the back of the
  queue

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Clock

• Even more efficient version of FIFO
• Has an iterator pointing at oldest page, if
  reference bit is not set, it gets replace, else
  iterator is incremented to next

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Least recently used

• Replaces the least recently used pages in memory

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Random

• Just replaces a random page in memory

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Not frequently used

• Every page has a counter
• All pages that have been referenced gets counter
  incremented
• Page with lowest counter is swapped out

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Ageing

• Descendant of not frequently used
• Pages with closer to present time has higher
  impact as to it being swapped out or not
• Page with lowest counter and oldest references
  get swapped out

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Thrashing

• Shuffling blocks of virtual memory between real
  memory and disks

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• Thank You