Lecture 20 Virtual Memory

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Lecture 20Virtual Memory

• Page-Replacement Algorithms

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Optimal Policy

• Replace page that will not be used for longest
  period of time.
• How do you know this?
• Is impossible!, the OS does not know future
  events
• Why do we present it?
• Used for measuring how well your algorithm
  performs.

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Optimal Policy
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First-in, First Out(FIFO)

• FIFO policy treats the page frames allocated to a
  process as a circular buffer, and pages are
  removed in round-robin style.
• A page fetched into memory a long time ago may
  have now fallen out of use
• It requires a pointer that circles through the
  page frames of the process
• It is simple but not optimal.

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First-in, First Out(FIFO)
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Least Recently Used(LRU)

• Replaces the page in memory that has not been
  referenced for the longest time.
• By the principle of locality, this should be the
  page least likely to be referenced in the near
  future.
• LRU policy does nearly as well as the optimal
  policy
• Problem Difficulty in implementation

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Least Recently Used(LRU)

• Counter implementation
• Every page entry has a counter every time page
  is referenced through this entry, copy the clock
  into the counter.
• When a page needs to be changed, look at the
  counters to determine which are to change.
• Problem Tremendous overhead (even if the
  hardware would support this scheme)

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Least Recently Used(LRU)

• Stack implementation keep a stack of page
  numbers in a double link form
• Page referenced
• move it to the top
• requires 6 pointers to be changed (entries must
  be moved even from the middle of the stack)
• No search for replacement

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Least Recently Used(LRU)
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LRU Approximation AlgorithmsOne bit Hardware
support

• Initially, all bits are cleared (to 0) by the OS
• As a user process executes, the bit associated
  with each page referenced is set (to 1) by the
  hardware
• After some time, we can determine which pages
  have been used and which have not been used by
  examining the reference bits

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LRU Approximation AlgorithmsAdditional-Reference-
bits Hardware support

• We can gain additional ordering information by
  recording the reference bits at regular intervals
• We can keep an 8-bit byte for each page in a
  table in memory
• At regular intervals a timer interrupt transfers
  control to the OS
• The OS shifts the reference bit for each page
  into the high-order bit of its 8-bit byte,
  shifting the other bits right 1 bit, discarding
  the low-order bit
• The page with the lowest number is the LRU page
• Notice that the numbers are not guaranteed to be
  unique

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LRU Approximation AlgorithmsSecond-Chance
Algorithm

• Is a FIFO replacement algorithm
• When a page is first loaded in memory, the use
  bit is set to 0
• When the page is referenced, the use bit is set
  to 1
• When a page is selected
• If its reference bit 0 then we replace this
  page
• If its reference bit 1 then move to to select
  the next FIFO page
• Thus, a page that is given a second chance will
  not replaced until all the other pages are
  replaced (or given second chances)

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Clock Policy
State of buffer just prior to a page replacement
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Clock Policy