Operating Systems

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Operating Systems

• Prof. Navneet Goyal
• Department of Computer Science Information
  Systems
• BITS, Pilani

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Topics for Today

• Thrashing
• Working Set

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Thrashing

• Swapping out a piece of a process just before
  that piece is needed
• The processor spends most of its time swapping
  pieces rather than executing user instructions

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Principle of Locality

• Program and data references within a process tend
  to cluster
• Only a few pieces of a process will be needed
  over a short period of time
• Possible to make intelligent guesses about which
  pieces will be needed in the future
• This suggests that virtual memory may work
  efficiently

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Locality In A Memory-Reference Pattern
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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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Demand Paging and Thrashing

• Why does demand paging work?Locality model
• Process migrates from one locality to another
• Localities may overlap
• Why does thrashing occur?? size of locality gt
  total memory size

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Working-Set Model

• ? ? working-set window ? a fixed number of page
  references Example 10,000 instruction
• WSSi (working set size 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 of the processes

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Working-set model
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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
• Why is this not completely accurate?
• Improvement 10 bits and interrupt every 1000
  time units

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Page Size

• Smaller page size, less amount of internal
  fragmentation
• Smaller page size, more pages required per
  process
• More pages per process means larger page tables
• Larger page tables means large portion of page
  tables in virtual memory
• Secondary memory is designed to efficiently
  transfer large blocks of data so a large page
  size is better

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Page Size

• Small page size, large number of pages will be
  found in main memory
• As time goes on during execution, the pages in
  memory will all contain portions of the process
  near recent references. Page faults low.
• Increased page size causes pages to contain
  locations further from any recent reference.
  Page faults rise.

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Frame Allocation Issues

• Smaller no. of frames to a process more
  processes can reside in MM increases
  probability that OS will find at least one ready
  process
• Despite Principle of Locality, Page faults rate
  will be high
• Beyond a certain size, additional allocation will
  have no noticeable effect on page fault rate
  because of Principle of Locality

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Frame Allocation Schemes

• Fixed Allocation
• Equal Allocation
• Proportional Allocation
• Priority Allocation

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Fixed Allocation
Equal allocation e.g., if 100 frames and 5
processes, give each 20 pages. Proportional
allocation Allocate according to the size of
process.
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Priority Allocation

• Use a proportional allocation scheme using
  priorities rather than size.
• If process Pi generates a page fault,
• select for replacement one of its frames.
• select for replacement a frame from a process
  with lower priority number.

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Global vs. Local Allocation

• Global replacement process selects a
  replacement frame from the set of all frames one
  process can take a frame from another.
• Local replacement each process selects from
  only its own set of allocated frames.