Virtual Memory Paging

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Virtual Memory / Paging

• Stewart Blakeway
• FML 208
• blakews_at_hope.ac.uk
• 0151 291 3113

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Session Aims

• Understand what Paging is
• Why Paging is used
• Characteristics and Implications of Paging

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Available RAM

• All computers have a limited amount of RAM
• Typically you can expect 256, 512, 1024 or 2048
  Meg of RAM in your computer
• This is NOT a lot in comparison to how much
  Memory the Operating System, Applications and
  Devices demand

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Main Memory

• Main Memory is essential for Processes to exist
• This is where instructions reside which are
  interpreted by the CPU

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Single Process System
Memory
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Fixed Memory Partition
1000k
Partition 3 400k
600k
Partition 2 300k
300k
Partition 1 200k
100k
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Variable Memory Partition
1000k
600k
300k
100k
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Variable Memory Partition
1000k
600k
300k
100k
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Variable Memory Partition with Compaction
1000k
600k
300k
100k
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Simple Paging
Process A
Process B
Process C
Memory Space
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Simple Paging
Process A
Process C
Memory Space
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Simple Paging
Process A
Process D
Process C
Memory Space
Process E
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Main Memory

• In addition to holding instructions RAM is also a
  workspace, a transient storage repository.
• space must also me made available for
• Operating System Code
• Operating System Data
• User Program Code
• User Program Data
• Video Storage Space
• And so-forth

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Imagine

• Loading Microsoft Windows
• Microsoft Internet Explorer
• An email Application
• Microsoft Word
• And Listening to Music whilst you do all that
  multi-tasking!

It doesnt stop there. You need memory for your
hardware. For example you require memory to
display images on your monitor
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Memory

• RAM is very quick, however it is very small.
• Each time a PROCESS starts it REQUESTS an amount
  of RAM
• The amount of total RAM requested is far too
  great in comparison to the amount of Physical RAM
  available in Multiprogramming Operating Systems

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Virtual Memory

• The Operating System has a Memory Management Unit
  MMU that translates virtual addresses to physical
  addresses in RAM.
• This is known as Address Translation

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Swapping

• When a running program references an address of a
  page which is not resident in RAM, the page has
  to be located in secondary storage and loaded
  into RAM.
• If RAM is full the resident page has to be
  swapped out to create space for the new page
• This is know as SWAPPING

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Swapping

• Whenever the system requires a component that is
  currently residing in Virtual Memory rather than
  in physical RAM the MMU determines which item or
  items in RAM are not being used at present and
  moves them to the hard disk virtual memory and
  loads the needed component into the just vacated
  physical RAM.

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Question

• What is swapped in and out of RAM ?

Answer PAGES
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Principal Problems to be Handled by the MMU

• To provide memory space to enable several
  processes to be executed at the same time
• To provide a satisfactory level of performance
  for the users
• To protect each process from each other
• Where desired, to enable sharing of memory space
  between processes
• To make addressing of memory space as transparent
  as possible for the programmer

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Question

• Can several processes be executed at the same
  time?

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The Operating System

• Virtual Memory is mapped by the Operating System
  to physical memory
• This mapping is performed in chunks, called
  pages, which are usually 4K each
• This mapping is performed with help from the
  hardware -- the CPU has built in support for
  virtual memory to physical memory address
  mappings.

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Memory Mapping

• It's this mapping that lets processes run in
  isolated memory spaces, so as to not interfere
  with other processes.
• This whole scheme was introduced in Intel CPUs
  with Intel 286/386 processors, and it's called
  Protected Mode.

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Virtual Memory

• This virtual memory to physical memory address
  mapping lets the operating system pretend that
  there is more memory available than actual
  physical memory in the machine.

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Question

• What is Virtual Memory?

Virtual Memory is space on the hard drive that is
mapped out and used to supplement the physical
RAM memory installed in the computer.
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Page File

• The area of the hard disk that stores the RAM
  image is called a page file. It holds pages of
  RAM on the hard disk, and the Operating System
  moves data back and forth between the page file
  and RAM.
• Previous versions of Windows called this a swap
  file and had a .SWP extension.

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Key Points about Virtual RAM

• The key fact to remember about virtual memory is
  that it exists to compensate for the lack of
  physical RAM.  Therefore the relationship between
  them is an inverse one
• More physical RAM means a reduced need for
  virtual memory
• Reducing the amount of physical RAM will mean an
  increased need for virtual memory.

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Thrashing

• Hard Disks are incredibly slow in comparison to
  RAM
• When there is a limited amount of RAM, Virtual
  Memory is accessed very frequently and this
  causes Thrashing
• Thrashing is the Operating System constantly
  swapping PAGES back and forth between RAM and the
  hard disk.
• Thrashing slows down your computer considerably.

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Total Size of the Page File

• The size of the page file is based on how much
  RAM is installed in the computer.
• By default, Windows XP creates a page file which
  is 1.5 times the amount of installed RAM
• The page file is placed on the hard drive in the
  same folder as XP

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Page Faults

• Only those parts of the program and data that are
  currently in active use need to be held in
  physical RAM. Other parts are then held in a page
  file
• When a program tries to access some address that
  is not currently in physical RAM, it generates an
  interrupt, called a Page Fault.
• This asks the system to retrieve the page
  containing the address from the page file
• This a valid page fault normally happens
  quite invisibly. If there is pressure on space in
  RAM, then parts of code and data that are not
  currently needed can be paged out in order to
  make room

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Question

• What is never paged out?

The Kernel remains resident in RAM, This is never
paged out.
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Invalid Page Faults

• Sometimes, through program or hardware error, the
  page is not there.
• The system then has an Invalid Page Fault
  error.
• This will be a fatal error if detected in a
  program
• The Application is normally shut down by the
  Operating System

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Improving Performance

• The page file is a reserved section of the hard
  drive where data may be written and retrieved as
  needed. Since the paging file and operating
  system files are by default located on the same
  drive, parallel access to both locations is
  impossible. One or the other has to wait, slowing
  down overall system performance. Moving the
  page file to another drive will improve
  performance.

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Question

• How else could you improve performance?

Installing more RAM
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Memory Management in Windows

• Processes
• Processes
• Memory Usage
• Virtual Memory Usage
• Performance
• Commit Charge
• Physical Memory
• Kernel Memory

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Windows Task Manager
Virtual Memory Usage
Process Name
Physical Memory Usage
Each Process has a Virtual Amount of memory Limit
of 4GB
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Physical Memory
Commit Charge
Kernel Memory
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Question

• What is Fragmentation?

The scattering of parts of a file throughout a
disk, eg when the operating system breaks up the
file and fits it into the spaces left vacant by
previously deleted files.
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Non-Contiguous Page Files

• Paging files are normally created when a drive is
  relatively empty and finding a large contiguous
  block of space is not a problem.
• A page file is dynamic by default, it can be
  expanded and contracted depending on the amount
  of extra virtual memory that's needed.
• If the initial block of drive space that was
  allocated at setup becomes surrounded by
  additional files that have been saved to the
  drive, a split page file can occur when the
  operating system expands it past the initial
  size.

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Question

• How do you defragment files?

Windows comes with a utility called Disk
Defragmenter which will reorganise the files
stored on disk
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Question

• What creates the Page File?
• The CPU
• The RAM
• The Operating System
• Its created by magic

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What is loaded in RAM?

• The Non-Paged area. Parts of the System which are
  so important that they may never be paged out -
  the area of RAM used for these in XP is called
  the Non-Paged area. This mainly contains core
  code of the system.
• The Page Pool which can be used to hold
• Program code,
• Data pages that have had actual data written to
  them
• A basic amount of space for the file cache (known
  in Windows 9x systems as Vcache) of files that
  have recently been read from or written to hard
  disk.
• Any remaining RAM will be used to make the file
  cache larger.

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Splitting the Memory

• This Virtual address space is split up into
  chunks, commonly 4K in size, called pages.
• The physical memory is also split up into chunks,
  also commonly 4K in size, called frames

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Page Table

• A page table is the data structure used by a
  virtual memory system in a computer operating
  system to store the mapping between virtual
  addresses and physical addresses
• The page table is stored in physical RAM

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Addressing

• How addressing works will be looked at in future
  Lectures

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So What do we Know?

• RAM is Limited
• Virtual Memory gives the impression that there is
  more RAM than Physical RAM
• Every Process and Device requires RAM
• The Kernel is never Paged Out
• Paging is Managed by the MMU
• Page Faults are normal interrupts
• Invalid Page Faults are bad
• How to view Memory Allocation and Resources in
  Windows

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Any Questions?