Lecture 16: Page Replacement Algorithms Tanenbaum Ch' 4'4

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Lecture 16Page Replacement Algorithms
(Tanenbaum Ch. 4.4)
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First-In-First-Out (FIFO) Algorithm

• Reference string 1, 2, 3, 4, 1, 2, 5, 1, 2, 3,
  4, 5
• 3 frames (3 pages can be in memory at a time per
  process)
• Beladys Anomaly more frames ? more page faults

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FIFO Page Replacement
Problem heavily-used old page will be evicted
from memory.
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Clock Page Replacement Algorithm
Algorithmically identical with another algorithm
called 2nd chance. Only the implementation is
different.
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Clock Algorithm Example
Pictures from Stallings Operating Systems, 6th
edition.
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The 2nd Chance Algorithm

• Holds a FIFO list of pages, each page has a bit
  R1 if referenced.
• Evict the oldest page not referenced in last
  tick.
• Repeat while oldest has R1
• Move to end of list (as if just paged in)
• Set R0
• At worst, all pages have R1, so degenerates to
  regular FIFO.
• Page list if fault occurs at time 20, A has R bit
  set (numbers above pages are loading times)

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Not Recently Used

• The clock algorithm on 2 bits referenced R and
  modified M
• Hardware sets corresponding R on every memory
  reference
• At process start, M and R set to 0
• Each circular pass, R bits set to 0.
• Pages are classified
• not referenced, not modified
• not referenced, modified
• referenced, not modified
• referenced, modified
• NRU removes page from lowest-numbered non-empty
  class

Picture from Stallings Operating Systems, 6th
edition.
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System Behavior Principle of Locality
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Least Recently Used (LRU) Algorithm

• Principle of locality pages used recently will
  be used again soon
• throw out the page that has been unused for
  longest time
• Must keep a linked list of pages
• most recently used at front, least at rear
• update this list every memory reference !!
• Alternatively keep time of last reference in each
  page table entry
• choose page with lowest timestamp
• Reference string 1, 2, 3, 4, 1, 2, 5, 1, 2, 3,
  4, 5

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LRU Page Replacement
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Comparison of 4 Page Replacement Algorithms

• Study published by Baer in 1980.
• 0.25 million memory references in a FORTRAN
  program
• Page size 256 words

From Stallings Operating Systems, 6th edition.
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Working-Set Model

• ? ? working-set window ? a fixed number of page
  references Example 10,000 instruction
• WSSi (working set of Process Pi) total number
  of pages referenced in the most recent ? (varies
  in time)
• if ? too small will not encompass entire locality
• if ? too large will encompass several localities
• if ? ? ? will encompass entire program
• D ? WSSi ? total demand frames
• If D gt m ? Thrashing
• Policy if D gt m, then suspend one process

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Keeping Track of the Working Set

• Approximate with interval timer a reference bit
• Example ? 10,000
• Timer interrupts after every 5000 time units
• Keep in memory 2 bits for each page
• Whenever a timer interrupts copy and sets the
  values of all reference bits to 0
• If one of the bits in memory 1 ? page in
  working set
• Make sure working set in memory before process
  runs

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Working Set as Defined by Window Size
From Stallings Operating Systems, 6th edition.
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Typical Working Set Size
From Stallings Operating Systems, 6th edition.
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Thrashing

• If a process does not have enough pages, the
  page-fault rate is very high. This leads to
• low CPU utilization
• operating system thinks that it needs to increase
  the degree of multiprogramming
• another process added to the system
• Thrashing ? a process is busy swapping pages in
  and out

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Thrashing (Cont.)
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Page-Fault Frequency Scheme

• Establish acceptable page-fault rate
• If actual rate too low, process loses frame
• If actual rate too high, process gains frame

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Animation for Page Replacement Algorithms

• http//gaia.ecs.csus.edu/zhangd/oscal/PagingApple
  t.html