Chapter 9: Virtual Memory

1
Chapter 9 Virtual Memory
2
Chapter 9 Virtual Memory

• Background
• Demand Paging
• Copy-on-Write
• Page Replacement
• Allocation of Frames
• Thrashing
• Memory-Mapped Files
• Allocating Kernel Memory
• Other Considerations
• Operating-System Examples

3
Objectives

• To describe the benefits of a virtual memory
  system
• To explain the concepts of demand paging,
  page-replacement algorithms, and allocation of
  page frames
• To discuss the principle of the working-set model

4
Background

• Virtual memory separation of user logical
  memory from physical memory.
• Only part of the program needs to be in memory
  for execution
• Logical address space can therefore be much
  larger than physical address space
• Allows address spaces to be shared by several
  processes
• Allows for more efficient process creation
• Virtual memory can be implemented via
• Demand paging
• Demand segmentation

5
Virtual Memory That is Larger Than Physical Memory
?
6
Virtual-address Space
7
Shared Library Using Virtual Memory
8
Demand Paging

• Bring a page into memory only when it is needed
• Less I/O needed
• Less memory needed
• Faster response
• More users
• Page is needed ? reference to it
• invalid reference ? abort
• not-in-memory ? bring to memory
• Lazy swapper never swaps a page into memory
  unless page will be needed
• Swapper that deals with pages is a pager

9
Transfer of a Paged Memory to Contiguous Disk
Space
10
Valid-Invalid Bit

• With each page table entry a validinvalid bit is
  associated(v ? in-memory, i ? not-in-memory)
• Initially validinvalid bit is set to i on all
  entries
• Example of a page table snapshot
• During address translation, if validinvalid bit
  in page table entry is I ? page fault

Frame
valid-invalid bit
v
v
v
v
i
.
i
i
page table
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Page Table When Some Pages Are Not in Main Memory
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Page Fault

• If there is a reference to a page, first
  reference to that page will trap to operating
  system
• page fault
• Operating system looks at another table to
  decide
• Invalid reference ? abort
• Just not in memory
• Get empty frame
• Swap page into frame
• Reset tables
• Set validation bit v
• Restart the instruction that caused the page fault

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Page Fault (Cont.)

• Restart instruction
• block move
• auto increment/decrement location

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Steps in Handling a Page Fault
15
Performance of Demand Paging

• Page Fault Rate 0 ? p ? 1.0
• if p 0 no page faults
• if p 1, every reference is a fault
• Effective Access Time (EAT)
• EAT (1 p) x memory access
• p (page fault overhead
• swap page out
• swap page in
• restart overhead
• 
  )

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Demand Paging Example

• Memory access time 200 nanoseconds
• Average page-fault service time 8 milliseconds
• EAT (1 p) x 200 p (8 milliseconds)
• (1 p) x 200 p x 8,000,000
• 200 p x 7,999,800
• If one access out of 1,000 (ie. p 0.001) causes
  a page fault, then
• EAT 8.2 microseconds.
• This is a slowdown by a factor of 40!!

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Process Creation

• Virtual memory allows other benefits during
  process creation
• - Copy-on-Write
• - Memory-Mapped Files (later)

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Copy-on-Write

• Copy-on-Write (COW) allows both parent and child
  processes to initially share the same pages in
  memoryIf either process modifies a shared page,
  only then is the page copied
• COW allows more efficient process creation as
  only modified pages are copied
• Free pages are allocated from a pool of
  zeroed-out pages

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What happens if there is no free frame?

• Page replacement find some page in memory, but
  not really in use, swap it out
• algorithm
• performance want an algorithm which will result
  in minimum number of page faults
• Same page may be brought into memory several times

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Page Replacement

• Prevent over-allocation of memory by modifying
  page-fault service routine to include page
  replacement
• Use modify (dirty) bit to reduce overhead of page
  transfers only modified pages are written to
  disk
• Page replacement completes separation between
  logical memory and physical memory large
  virtual memory can be provided on a smaller
  physical memory

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Need For Page Replacement
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Basic Page Replacement

1. Find the location of the desired page on disk
2. Find a free frame - If there is a free
   frame, use it - If there is no free frame,
   use a page replacement algorithm to select a
   victim frame
3. Bring the desired page into the (newly) free
   frame update the page and frame tables
4. Restart the process

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Page Replacement
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Page Replacement Algorithms

• Want lowest page-fault rate
• Evaluate algorithm by running it on a particular
  string of memory references (reference string)
  and computing the number of page faults on that
  string
• In all our examples, the reference string is
• 1, 2, 3, 4, 1, 2, 5, 1, 2, 3, 4, 5

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Graph of Page Faults Versus The Number of Frames
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First-In-First-Out (FIFO) Algorithm

• Reference string 1, 2, 3, 4, 1, 2, 5, 1, 2, 3,
  4, 5
• 3 frames (3 pages can be in memory at a time per
  process)
• 4 frames
• Beladys Anomaly more frames ? more page faults

1
1
4
5
2
2
1
3
9 page faults
3
3
2
4
1
1
5
4
2
2
1
10 page faults
5
3
3
2
4
4
3
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FIFO Page Replacement
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FIFO Illustrating Beladys Anomaly
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Optimal Algorithm

• Replace page that will not be used for longest
  period of time
• 4 frames example
• 1, 2, 3, 4, 1, 2, 5, 1, 2, 3, 4, 5
• How do you know this? MUST CONSIDER THE FUTURE!
• Used for measuring how well your algorithm
  performs

1
4
2
6 page faults
3
4
5
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Optimal Algorithm

• Replace page that will not be used for longest
  period of time
• 4 frames example
• 1, 2, 3, 4, 1, 2, 5, 1, 2, 3, 4, 5
• How do you know this?
• Used for measuring how well your algorithm
  performs

1
4
Choose to replace 4 by 5 since the time until 4
will be used is the longest.
2
6 page faults
3
4
5
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Optimal Algorithm

• Replace page that will not be used for longest
  period of time
• 4 frames example
• 1, 2, 3, 4, 1, 2, 5, 1, 2, 3, 4, 5
• How do you know this?
• Used for measuring how well your algorithm
  performs

1
4
2
6 page faults
Since 1 (and 2, 3) is not included in the future
schedule, assume it is complete and the frame is
available to reload 4.
3
4
5
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Optimal Page Replacement
33
Least Recently Used (LRU) Algorithm

• Reference string 1, 2, 3, 4, 1, 2, 5, 1, 2, 3,
  4, 5
• Counter implementation
• Every page entry has a counter every time page
  is referenced through this entry, copy the clock
  into the counter
• When a page needs to be changed, look at the
  counters to determine which are to change

1
1
5
1
1
2
2
2
2
2
5
4
4
3
5
3
3
3
4
4
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Least Recently Used (LRU) Algorithm

• Reference string 1, 2, 3, 4, 1, 2, 5, 1, 2, 3,
  4, 5
• Counter implementation
• Every page entry has a counter every time page
  is referenced through this entry, copy the clock
  into the counter
• When a page needs to be changed, look at the
  counters to determine which are to change

1
1
5
1
1
2
2
2
2
2
At the time 5 is entered, we most recently used
2, 1 and 4. Hence, 3 was least recently used and
is replaced.
5
4
4
3
5
3
3
3
4
4
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LRU Page Replacement
36
LRU Algorithm (Cont.)

• Stack implementation keep a stack of page
  numbers in a double link form
• Page referenced
• move it to the top
• requires 6 pointers to be changed
• No search for replacement

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Use Of A Stack to Record The Most Recent Page
References
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LRU Approximation Algorithms

• Reference bit
• With each page associate a bit, initially 0
• When page is referenced bit set to 1
• Replace the one which is 0 (if one exists)
• We do not know the order, however
• Second chance
• Need reference bit
• Clock replacement
• If page to be replaced (in clock order) has
  reference bit 1 then
• set reference bit 0
• leave page in memory
• replace next page (in clock order), subject to
  same rules

39
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.)
41
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

42
Working-Set Model

• ? ? working-set window ? a fixed number of page
  references Example 10,000 instruction
• WSSi (working set 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
44
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

45
Page-Fault Frequency Scheme

• Establish acceptable page-fault rate
• If actual rate too low, process loses frame
• If actual rate too high, process gains frame

46
Buddy System

• Allocates memory from fixed-size segment
  consisting of physically-contiguous pages
• Memory allocated using power-of-2 allocator
• Satisfies requests in units sized as power of 2
• Request rounded up to next highest power of 2
• When smaller allocation needed than is available,
  current chunk split into two buddies of
  next-lower power of 2
• Continue until appropriate sized chunk available

47
Buddy System Allocator
48
End of Chapter 9
49
Chapter 10 File-System Interface
50
Chapter 10 File-System Interface

• File Concept
• Access Methods
• Directory Structure
• File-System Mounting
• File Sharing
• Protection

51
Objectives

• To explain the function of file systems
• To describe the interfaces to file systems
• To discuss file-system design tradeoffs,
  including access methods, file sharing, file
  locking, and directory structures
• To explore file-system protection

52
File Concept

• Contiguous logical address space
• Types
• Data
• numeric
• character
• binary
• Program

53
File Structure

• None - sequence of words, bytes
• Simple record structure
• Lines
• Fixed length
• Variable length
• Complex Structures
• Formatted document
• Relocatable load file
• Can simulate last two with first method by
  inserting appropriate control characters
• Who decides
• Operating system
• Program

54
File Attributes

• Name only information kept in human-readable
  form
• Identifier unique tag (number) identifies file
  within file system
• Type needed for systems that support different
  types
• Location pointer to file location on device
• Size current file size
• Protection controls who can do reading,
  writing, executing
• Time, date, and user identification data for
  protection, security, and usage monitoring
• Information about files are kept in the directory
  structure, which is maintained on the disk

55
File Operations

• File is an abstract data type
• Create
• Write
• Read
• Reposition within file
• Delete
• Truncate
• Open(Fi) search the directory structure on disk
  for entry Fi, and move the content of entry to
  memory
• Close (Fi) move the content of entry Fi in
  memory to directory structure on disk

56
Open Files

• Several pieces of data are needed to manage open
  files
• File pointer pointer to last read/write
  location, per process that has the file open
• File-open count counter of number of times a
  file is open to allow removal of data from
  open-file table when last processes closes it
• Disk location of the file cache of data access
  information
• Access rights per-process access mode information

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File Types Name, Extension
58
Access Methods

• Sequential Access
• read next
• write next
• reset
• no read after last write
• (rewrite)
• Direct Access
• read n
• write n
• position to n
• read next
• write next
• rewrite n
• n relative block number

59
Sequential-access File
60
Simulation of Sequential Access on a
Direct-access File
61
Example of Index and Relative Files
62
Directory Structure

• A collection of nodes containing information
  about all files

Directory
Files
F 1
F 2
F 3
F 4
F n
Both the directory structure and the files reside
on disk Backups of these two structures are kept
on tapes
63
A Typical File-system Organization
64
Operations Performed on Directory

• Search for a file
• Create a file
• Delete a file
• List a directory
• Rename a file
• Traverse the file system

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Organize the Directory (Logically) to Obtain

• Efficiency locating a file quickly
• Naming convenient to users
• Two users can have same name for different files
• The same file can have several different names
• Grouping logical grouping of files by
  properties, (e.g., all Java programs, all games,
  )

66
Single-Level Directory

• A single directory for all users

Naming problem Grouping problem
67
Two-Level Directory

• Separate directory for each user

• Path name
• Can have the same file name for different user
• Efficient searching
• No grouping capability

68
Tree-Structured Directories
69
Tree-Structured Directories (Cont)

• Efficient searching
• Grouping Capability
• Current directory (working directory)
• cd /spell/mail/prog
• type list

70
Tree-Structured Directories (Cont)

• Absolute or relative path name
• Creating a new file is done in current directory
• Delete a file
• rm ltfile-namegt
• Creating a new subdirectory is done in current
  directory
• mkdir ltdir-namegt
• Example if in current directory /mail
• mkdir count

mail
prog
copy
prt
exp
count
Deleting mail ? deleting the entire subtree
rooted by mail
71
File Sharing

• Sharing of files on multi-user systems is
  desirable
• Sharing may be done through a protection scheme
• On distributed systems, files may be shared
  across a network
• Network File System (NFS) is a common distributed
  file-sharing method

72
File Sharing Multiple Users

• User IDs identify users, allowing permissions and
  protections to be per-user
• Group IDs allow users to be in groups, permitting
  group access rights

73
Protection

• File owner/creator should be able to control
• what can be done
• by whom
• Types of access
• Read
• Write
• Execute
• Append
• Delete
• List

74
Access Lists and Groups

• Mode of access read, write, execute
• Three classes of users
• RWX
• a) owner access 7 ? 1 1 1 RWX
• b) group access 6 ? 1 1 0
• RWX
• c) public access 1 ? 0 0 1
• Ask manager to create a group (unique name), say
  G, and add some users to the group.
• For a particular file (say game) or subdirectory,
  define an appropriate access.

owner
group
public
chmod
761
game
Attach a group to a file chgrp G
game
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A Sample UNIX Directory Listing
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End of Chapter 10