Lecture 8: Virtual Memory

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Lecture 8 Virtual Memory

• Operating System
• Fall 2006

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Two characteristics of paging and segmentation

• Memory references are dynamically translated into
  physical addresses at run time
• A process may be swapped in and out of main
  memory such that it occupies different regions
• A process may be broken up into pieces that do
  not need to located contiguously in main memory
• All pieces of a process do not need to be loaded
  in main memory during execution

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Virtual Memory

• It is not necessary that all of the pages or all
  of the segments of a process be in main memory
  during execution. As long as the piece holding
  the next instruction and the data to be accessed
  are in main memory, then execution may proceed.
• Use page table to do address translation. If the
  page is not in memory, it generates a page fault
  interrupt, the OS will bring the page from disk
  into main memory. When this is done, resume
  execution.

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Advantages of Virtual Memory

• More processes may be maintained in main memory
• Only load in some of the pieces of each process
• With so many processes in main memory, it is very
  likely a process will be in the Ready state at
  any particular time
• A process may be larger than all of main memory.
  Programs become portable across different
  platforms.

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Types of Memory

• Real memory
• Physical Main memory
• Virtual memory
• Programmer perceived memory
• Memory on disk
• Allows for effective multiprogramming and
  relieves the user of tight constraints of main
  memory

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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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Support Needed for Virtual Memory

• Hardware must support paging and segmentation
• Operating system must be able to management the
  movement of pages and/or segments between
  secondary memory and main memory

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Paging

• Each process has its own page table
• Each page table entry contains the frame number
  of the corresponding page in main memory
• Presence Bit A bit is needed to indicate whether
  the page is in main memory or not
• Modify Bit
• Another bit is needed to indicate if the page has
  been altered since it was last loaded into main
  memory
• If no change has been made, the page does not
  have to be written to the disk when it needs to
  be swapped out

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Page Table Entries
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Translation Lookaside Buffer

• Contains page table entries that have been most
  recently used
• Functions same way as a memory cache

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Paging Hardware With TLB
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Structure of the Page Table

• Hierarchical Paging
• Hashed Page Tables
• Inverted Page Tables

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Hierarchical Page Tables

• Break up the logical address space into multiple
  page tables
• A simple technique is a two-level page table

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Two-Level Page-Table Scheme
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Address-Translation Scheme
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Memory Protection

• Memory protection implemented by associating
  protection bit with each frame
• Valid-invalid bit attached to each entry in the
  page table
• valid indicates that the associated page is in
  the process logical address space, and is thus a
  legal page
• invalid indicates that the page is not in the
  process logical address space

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Valid (v) or Invalid (i) Bit In A Page Table
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Segmentation

• May be unequal, dynamic size
• Simplifies handling of growing data structures
• Allows programs to be altered and recompiled
  independently
• Lends itself to sharing data among processes
• Lends itself to protection

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Segment Tables

• corresponding segment in main memory
• Each entry contains the length of the segment
• A bit is needed to determine if segment is
  already in main memory
• Another bit is needed to determine if the segment
  has been modified since it was loaded in main
  memory

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Segment Table Entries
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Segmentation Hardware
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Combined Paging and Segmentation

• Paging is transparent to the programmer
• Paging eliminates external fragmentation
• Segmentation is visible to the programmer
• Segmentation allows for growing data structures,
  modularity, and support for sharing and
  protection
• Each segment is broken into fixed-size pages

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Combined Segmentation and Paging
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OS Supports for Virtual Memory

• Virtual Memory not all pages of a process are in
  main memory
• OS needs to decide on the following issues
• Fetch Policy
• Placement Policy
• Replacement Policy

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Fetch Policy

• Fetch Policy
• Determines when a page should be brought into
  memory
• Demand paging bring pages into main memory only
  when it is needed
• Many page faults when process first started
• Less I/O needed
• Less memory needed
• Faster response
• More users
• Prepaging brings in more pages then needed even
  though it is not needed now.
• Faster to bring in several pages than one at a
  time
• More efficient to bring in pages that reside
  contiguously on the disk

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Placement Policy

• Decides where a process piece reside in main
  memory
• For paging system, it is a trivial issue
• For segmentation system, use first-fit or
  best-fit to look for a hole.

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Replacement Policy

• Determines which page to replace when a new page
  needs to be brought in and there is no empty page
  frame around
• Page removed should be the page least likely to
  be referenced in the near future
• Most policies predict the future behavior on the
  basis of past behavior

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Replacement Policy

• Frame Locking
• If frame is locked, it may not be replaced
• Kernel of the operating system
• Control structures
• I/O buffers
• Associate a lock bit with each frame

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Replacement Algorithms

• Beladys Optimal Algorithm
• Least Recently Used Algorithm (LRU)
• First-in-first-out Algorithm (FIFO)
• Clock (approximation of LRU)

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Beladys Optimal Algorithm

• Optimal policy
• Selects for replacement that page for which the
  time to the next reference is the longest
• Impossible to have perfect knowledge of future
  events

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Least Recently Used (LRU)

• Replaces the page 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
• Each page could be tagged with the time of last
  reference. This would require a great deal of
  overhead.

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First-in, first-out (FIFO)

• Treats page frames allocated to a process as a
  circular buffer
• Pages are removed in round-robin style
• Simplest replacement policy to implement
• Page that has been in memory the longest is
  replaced
• These pages may be needed again very soon
• LRU performs better than FIFO, but difficult to
  implement.

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Clock Policy

• Additional bit called a use bit
• 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 it is time to replace a page, the first
  frame encountered with the use bit set to 0 is
  replaced.
• During the search for replacement, each use bit
  set to 1 is changed to 0

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Further Refinement for Clock Policy

• Two bits
• Used bit u bit1 when used
• Modify bit m bit1 when the page is written
• Four states
• u0m0 not used, not modified
• u1m0 used, not modified
• u0m1 not used, modified
• u1m1 used, modified

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Further Refinement for Clock Policy

• Algorithm
• Beginning at the current position of the ptr,
  scan the frame buffer. During this scan, make no
  changes to the use bit. The first frame
  encountered with (u0m0) is selected for
  replacement.
• If step 1 fails, scan again, looking for the
  frame with (u0m1). The first such frame
  encountered is selected for replacement. During
  this scan, set the use bit to 0 on each frame
  that is bypassed.
• If step 2 fails, the ptr should have returned to
  its original position and all of the frames in
  the set will have a use bit of 0. Repeat step 1,
  and if necessary, step 2. This time, a frame will
  be found for the replacement.

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End of lecture 8
Thank you!