Virtual Memory: PAGING

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Tutorial 8

• Virtual Memory PAGING
• presented by
• Antonio Maiorano
• Paul Di Marco

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Paging

• Primary Memory (RAM) is divided into fixed size
  partitions we call frames
• Processes also divided into same sized partitions
  we call pages
• Result almost no fragmentation

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Process Location

• Most of the process is on Secondary Memory (HD)
• Some of the process pages are loaded into frames
  in RAM
• In RAM, the frames allocated to a process need
  not be contiguous
• Uses a page table to map pages to frames

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Whats the point?

• Many more processes can run at the same time
• Works well because
• Reference locality behaviour
• Well-supported by hardware

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How does it work?

• For each memory reference, Paging System must be
  able to translate each virtual address into a
  physical address at run-time
• Address ltpage/frame num, offsetgt
• Ex 32 bits lt24 bits, 8 bitsgt lt16777216
  pages, 256 locations per pagegt

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In the CPU
Page
Offset
Virtual Address
Missing Page
MAP
Frame
Offset
Physical Address
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Page Reference Stream

• For a given process, the page reference stream is
  the list of virtual page numbers ordered
  according to when they are referenced
• Example Process with 5 pages 1, 2, 3, 1, 2,
  3, 4, 5

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Static Paging Algorithms

• When process starts, it is allocated a fixed
  number of frames in RAM
• Paging Policies defines how pages will be
  loaded/unloaded into frames allocated to the
  process

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3 Basic Paging Policies

1. Fetch Policy determines when a page should be
   loaded into RAM
2. Replacement Policy if all frames are full,
   determines which page should be replaced
3. Placement Policy determines where a fetched page
   should be placed in RAM

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Demand Paging

• Since the page reference stream is not known
  ahead of time, we cant pre-fetch pages
• We know which page to load during run-time so we
  load them on demand
• Result concentrate on replacement policies (3)

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Demand Paging Algorithms

• Random, Optimal,
• LRU, LFU, FIFO

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Random Replacement

• Choose a page at random to be the victim page
• Simple to implement, but is the worst algorithm
• No knowledge is taken into account!
• Each page thus has same probability of being the
  victim

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Optimal Algorithm

• Best algorithm however requires knowledge of the
  future page references (!)
• Always results in least possible page faults
• Replace the page that will be referenced the
  latest from now

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Optimal Algorithm Example

• The process reference stream is
• 0 1 2 3 0 1 2 3 0 1 2 3 4 5 6 7
• The process is allocated 3 frames

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Least Recently Used (LRU)

• Replace the page which was last referenced the
  longest time ago

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Least Frequently Used (LFU)

• Replace the page which was least frequently
  referenced (since the beginning of the reference
  stream)

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First In First Out (FIFO)

• Replace the page that has been in memory longest

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Comparison of the samples

• For the same reference stream and number of
  available frames
• Optimal caused 10 page faults
• LRU caused 16 page faults
• LFU caused 12 page faults
• FIFO caused 16 page faults