Kautalya Mishra

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VIRTUAL MEMORY

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  Kautalya Mishra

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Memory Hierarchy
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Before Virtual Memory ?

• Programmers when confronted with the problem of
  having programs that were too big to fit in to
  the available memory came up with a solution of
  splitting the program in to pieces called
  Overlays.
• Overlay 0 would start running first followed by
  the next overlay. The overlays were kept on the
  disk and swapped in and out of the memory by the
  operating system dynamically.
• Although the work of swapping Overlays in and out
  was done by the system, the LABORIOUS work of
  splitting the program in to Overlays had to be
  done by the programmer.

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Virtual Memory

• The basic idea of Virtual Memory is to keep only
  those parts of the program currently in use in
  the memory and the rest on the disk drive.
• For example a 16M program can run on a 4M
  machine by carefully choosing which 4M to keep in
  memory at each instant, with pieces of the
  program being swapped between disk and memory as
  needed.
• It can work in a Multiprogramming system, with
  bits and pieces of many programs brought to the
  memory at once by the Operating System. While a
  program is waiting for part of itself to be
  brought in, it is waiting for I/O and cannot run,
  so the CPU can be given to another process, the
  same way as for any other multiprogramming system.

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Virtual Memory implemented using PAGING
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• The position and function of the MMU

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Virtual Address Space
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Internal operation of a MMU
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Page Faults

• When a page being referred to by the virtual
  address is not loaded as a page frame in to the
  memory, a page fault is occurred which causes the
  CPU to trap the Operating System.
• The Operating System removes a page frame from
  the memory and loads the new page in to the freed
  location at the same time making appropriate
  changes to the page table.
• There are a number of page replacement algorithms
  available such as The Optimal Page Replacement
  Algo, The Not Recently Used Page Replacement
  Algo, FIFO etc.

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• Problems faced with Virtual Memory
  implementation
• Possibly large page table
• Thrashing
• Solution
• Use multilevel page tables.
• Use TLB (Translation Lookaside Buffer) or
  sometimes called Associative Memory.
• Page fault rates can be reduced by using a
  Working Set Model approach or a Prepaging
  approach.
• Increase RAM!

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Advances Made ?

• Some advanced systems give programmers some
  control over the memory map so that they can used
  it in a non-traditional way. It allows for the
  possibility of sharing the same memory space
  between two or more processes.
• Distributed Shared Memory The idea here is to
  allow multiple processes over a network to share
  a set of pages, possibly, but not necessarily, as
  a single shared linear address space

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References

• Operating Systems Design and Management Andrew
  S. Tanenbaum and Albert S. Woodhull
• http//computer.howstuffworks.com/virtual-memory.h
  tm
• http//en.wikipedia.org/wiki/Virtual_memory