Memory Replacement Policies

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Memory Replacement Policies

• Stallings 8.1,8.2 (Ch22)
• Lawrence Angrave

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Demand Paging Example
VM
fault
ref
Load M
Page table
Free frame
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Concepts this Lecture

• Terminology
• Demand Paging
• Replacement Issues
• Replacement Strategies

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1 Terminology

• Reference string the memory reference sequence
  generated by a program.
• Paging moving pages to (from) disk
• Optimal the best (theoretical) strategy
• Eviction throwing something out
• Pollution bringing in useless pages/lines

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2 Demand Paging - Paging Policies

• Fetch Strategies
• When should a page be brought into primary (main)
  memory from secondary (disk) storage?
• Placement Strategies
• When a page is brought into primary storage,
  where is it to be put?
• Replacement Strategies
• Which page now in primary storage is to be
  removed from primary storage when some other page
  or segment is to be brought in and there is not
  enough room?

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3 Replacement Issues
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Page Replacement Algorithm (review)

• find a free page frame
• if free page frame use it
• otherwise, select a page frame using the page
  replacement algorithm
• write the selected page to the disk and update
  any necessary tables
• find location of page on disk
• read the requested page from the disk.
• restart the user process.

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Page Replacement Issues 1/2

• if no frames are free, a disk read and write of
  page is required
• if the page to be replaced has not been changed
  since it was read in, it can be discarded and
  does not need to be copied out to secondary
  storage.
• read-only pages are never written but may be
  shared!

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Page Replacement Issues 2/2

• a dirty bit is set in the page table by hardware
  to indicate that the page has been modified.
• need to minimize page faults.
• reference string is the memory references
  generated by a program.

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Replacement Strategies 4
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Page Replacement Strategies 1/2

• optimal
• replace the page that will not be used again the
  farthest time in the future.
• random
• choose a page randomly
• FIFO - first in first out
• replace the page that has been in primary memory
  the longest

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Page Replacement Strategies 2/2

• LRU - least recently used
• replace the page not used for the longest time
• LFU - least frequently used
• replace the page that is used least often
• NUR - not used recently
• an approximation to LRU
• working set
• keep in memory those pages that the process is
  actively using.

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Principal of Optimality

• provides a basis for comparison with other
  schemes.
• is difficult to implement because of trying to
  predict the reference string for a program.
• but sometimes compiler technology can help by
  providing hints.

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Principal of Optimality

• if the reference string can be predicted
  accurately, then don't use demand paging but
  should use pre-paging instead.
• because this would allow paging activity of
  pages needed in the future to be overlapped with
  computation.

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Optimal Example
12 references, 7 faults
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FIFO
12 references, 9 faults
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Paging Behavior with IncreasingNumber of Page
Frames
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Belady's Anomaly (for FIFO)
As the number of page frames increase, so does
the fault rate.
12 references, 10 faults
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LRU
12 references, 10 faults
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Least Recently Used Issues

• does not suffer from Belady's anomaly
• use time
• record time of reference with page table entry
• use counter as clock
• search for smallest time.
• use stack
• remove reference of page from stack (linked list)
• push it on top of stack
• both approaches require large processing
  overhead, more space, and hardware support.

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LRU and Anomalies
Anomalies cannot occur, why?
12 references, 8 faults
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NUR A LRU Approximation

• additional reference bits
• a register is kept per page
• a one bit is set in the register if the page is
  referenced
• the register is shifted by one after some time
  interval
• 00110011 would be accessed more recently than
  00010111
• the page with register holding the lowest number
  is the least recently used.
• the value may not be unique. use fifo to resolve
  conflicts.

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Second Chance

• second chance algorithm uses a register size one
  a reference bit.
• the page table entry has a reference bit
• initially, the reference bit for a page is set to
  0
• when a page is referenced, the page is set to 1
• pages are kept in FIFO order using a circular
  list.
• select head of FIFO

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Second Chance

• if page has reference bit set, reset bit and
  select next page in FIFO list.
• keep processing until reach page with zero
  reference bit and page that one out.
• system v, r4 uses a variant of second chance.

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Second Chance Example
12 references, 9 faults
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Page Classes

• 1.(0,0) neither referenced nor dirtied
• 2.(0,1) not referenced (recently) but dirtied
• 3.(1,0) referenced but clean
• 4.(1,1) referenced and dirtied
• select a page from lowest class
• if conflict, use random or FIFO.

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Summary

• Fetch, placement, page replacement.
• Demand paging algorithm.
• Simple page replacement policies.
• Optimal
• FIFO
• LRU
• NUR
• Second chance, page classes