Final Examination

1
Final Examination

• Class CS2200 Section A
• Instructor Leahy
• Date Monday, May 2, 2005
• Time 800-1050 p.m.
• Location CoC 17

2
Final Examination

• Class CS2200 Section B
• Instructor Ramachandran
• Date Wednesday, May 4, 2005
• Time 800-1050 p.m.
• Location CoC 17

3
CS 2200

• Presentation 34
• Input/Output
• The Condensed Version

4
Our Road Map
Processor
Memory Hierarchy
I/O Subsystem
Parallel Systems
Networking
5
Key Concepts

• History
• Characterization
• Basic Techniques
• Memory Mapped
• Polling/Interrupts
• DMA
• IO Processor

6
Key Concepts

• Disks
• Technology
• Scheduling
• File Systems

7
First there was I/O...

• then computers?

8
Roots of I/O Devices

• Telecommunications
• Smoke signals
• Drums
• Optical telegraphy
• Electrical telegraphy
• Wireless telegraphy
• Teletype
• Television
• etc.

9
Roots of I/O Devices

• Control/Tabulation
• Jacquards Punched Cards
• Holleriths tabulating machines
• IBM
• Other technologies
• Magnetic recording
• Wire
• Tape

10
Characterization

• Behavior Input, Output, Storage
• Partner Human, Machine
• Data Rate Peak rate
• Character stream or block (Terminal vs. Disk)
• Sequential or random-access (Modem vs. CD-ROM)
• Synchronous or Asynchronous (Tape vs. keyboard)
• Shareable or dedicated (Keyboard vs. tape)

11
Types

• Device Behavior Partner DataRate
  kb/s
• Keyboard I Human 0.01
• Mouse I Human 0.02
• Voice Input I Human 0.02
• Scanner I Human 400
• Voice Output O Human 0.6
• Line Printer O Human 1
• Laser Printer O Human 200
• Graphics Display O Human 60,000
• Modem IO Machine 8
• Network IO Machine 6,000
• Floppy Disk S Machine 100
• Optical Disk S Machine 1,000
• Magnetic Tape S Machine 2,000
• Magnetic Disk S Machine 10,000

12
Memory Mapped I/O

• Saw this in P2
• Device controller
• Interface between bus and device
• Looks like memory locations
• Registers can be in/out or both
• Some systems have dedicated I/O instructions

13
Polling vs. Interrupts

• Simple to output one word/byte
• But how do we know when we can output again?
• How do we know when there is input?
• Polling Used when very high likelihood of
  finding something
• Interrupt driven Much more efficient
• But...

14
DMA

• Direct Memory Access
• Using sophisticated general-purpose processor to
  manage I/O inefficient use of powerful resource
  especially for large blocks of data.
• Solution Add enough processing power to device
  controller (and possibly bus controller) to allow
  direct transfer between device and memory.

15
DMA
N
Processor to Controller Move N bytes from disk
location X to memory location M
16
DMA
N
Controller Read X into buffer BP 0 while(N gt
0) MemM BufferBP Interrupt
processor
17
DMA
N
Why is this efficient?
18
DMA
Processor
Memory
C
N
Processor can run out of cache
19
I/O Processor

• For very high volumes of data specialized
  processors are used which actually use memory to
  store their control programs.
• Multiplexor channel (character oriented)
• Terminals
• Block multiplexor (block oriented)
• Tape Drives
• Selector channel (high speed block oriented)
• Disk drive

20
Magnetic Disks

• Fixed head drums
• Disks
• Removable disk packs
• Floppy disk
• Invented for IBM Field Engineers
• Contact
• Slow speed

21
Magnetic Disks

• Hard disk
• Higher speed
• Larger
• Higher Density
• Multiplatter
• Flying heads!

22
Disk Terminology
(2 surfaces per platter)
Cylinder Track 'x' on all platters/surfaces
23
Magnetic Disks
24
Steps to Read/Write

• Drive must move heads to desired track
• Seek time
• Must wait for data to be read to arrive under
  heads
• Rotational Latency
• Data read into or written from buffer on
  controller

25
Magnetic Disks

• Hard disk
• Higher speed (3600 - 7200)
• Larger
• Higher Density
• Multiplatter
• Flying heads!
• Performance
• Seek time (8-20 ms or faster)
• Rotational latency (4-8 ms)
• Transfer rate 2-40 MB/sec

26
Magnetic Disks

• Originally all tracks had same number of sectors.
  No longer true.
• Disk rotational speeds increasing
• 12,000 RPM!?!?!?
• seek time, rotational latency, data transfer rate
• Typical numbers
• 20 surfaces, 1000 tracks, 128 sectors

4 GB!!!
27
Disk Calculations

• A disk has 3 platters.
• Each platter has 100 tracks.
• Each track has 20 sectors.
• Each sector has 256 bytes
• How many bytes per cylinder?
• 3 x 20 x 256
• 3 x 2 x 20 x 256
• 3 x 20 x 100 x 256
• 2 x 20 x 256

28
Disk Scheduling

• FCFS
• SSTF
• SCAN
• C-SCAN
• LOOK
• C-LOOK

29
Storage Abstraction

• Process is the OS abstraction for processor
• Data structures are the language abstraction for
  memory
• Consider a program
• Text, and data structures are meaningful for the
  life of the program (i.e. when it is executing as
  a process)
• What about state that a program wants to leave
  behind after execution as a process terminates?
• Magnetic/optical store on the hardware side
• An abstraction of this hardware presented by the
  OS

30
Device independent I/O

• What abstraction should the OS provide?
• Virtual devices with generic API
• open, seek, read, write, close
• Advantages
• programming API is device independent
• easy to add new devices
• Step towards such a device independent I/O
• File systems...

31
File System

• File A Collection of info with attributes
• Data (the actual user info)
• Attributes name, access rights, physical
  representation, logical representation,
  accounting info,
• Where are the attributes?
• Can be part of the file, and/or
• Physical representation kept in directory

32

• File names
• Single level (Univac EXEC 8, circa late 1970s,
  DEC RT-11)
• Unique name for each filerestrictive
• Single directory to searchexpensive
• Two-level (DEC TOPS-10, RSX-11M circa early 80s)
• Top level user/project
• Directory tells how to get to an individual
  user/project
• Example dp033,077test.dat
• Directory was named dp0,0033077.dir
• Second level file name unique to user/project

33

• File names
• Hierarchical (Multics, Unix, NT)
• A file name made of several parts
  (/users/leahy/grades)
• Each part unique with respect to previous parts
• Directory?
• Just another file that contains info about files
  at next lower levels

34

• Other naming issues
• Aliases
• Hard links
• Deletion of a file
• Symbolic links
• File name extensions
• Mandatory (TOPS 10)
• Optional (Unix, NT)
• Version numbers
• Write implicitly creates new versions.
• (RSX-11M)
• Purging
• Write over-writes previous version (Unix, NT)

35
Access Rights

• Permissions on a file
• Read, write, execute, delete, change permissions
• Change permissions
• Who can?
• Who should?
• Storing access rights
• One per user per file feasible?
• Compromise User/Group/Other

36
Physical Representation of File

• Goals
• Fast sequential access (SA)
• Fast random access (RA)
• Ability to grow the file
• Easy allocation of storage
• Data structure in the file system
• A table that maps file name to disk address
• Free list of disk blocks

37
Allocation Strategies

• 1. Fixed contiguous regions
• Regions
• One track
• One cylinder
• Contiguous range of cylinders
• Characteristics
• Data structures (on the disk at a well-known
  place)
• Free list bit map of free cylinders
• Directory mapping table (file name to cylinder
  address)
• SA and RA quick allocation is quick as well
• Cannot grow file size above allocation
• DEC RT-11

-(
38
Allocation Strategies

• 2. Contiguous regions with overflow areas
• Secondary area for growth (also contiguous)
• Characteristics
• RA requires some computation
• SA as fast as 1.

39
Allocation Strategies

• 3. Linked allocation
• File divided into (sector sized) blocks
• Each block points to next one disk becomes a
  giant linked list!
• Characteristics
• Data structures (on the disk at a well-known
  place)
• free-list (a linked list) mapping table (file
  name to starting disk block)
• SA slow due to seek time RA pointer chasing
• Growth easy allocation is expensive

40
Allocation Strategies

• 4. File Allocation Table (FAT) MS-DOS
• At the beginning of each partition, a table that
  contains one entry for each disk block in that
  partition (0 indicates it is free)
• A file occupies a number of entries in the FAT
• Each entry points to the next entry in the FAT
  for SA (-1 indicates it is the last block)
• Characteristics
• FAT is the data structure efficient for
  allocation less chance of screwing up as in (3)
• SA requires FAT lookup (can be alleviated by
  caching the FAT)
• RA requires some computation
• Limit on size of partition (this is BAD!) growth
  easy

41
Allocation Strategies

• 5. Indexed Allocation
• File represented by an index block (on the disk),
  a table of disk blocks for the file
• Characteristics
• Data structures
• Mapping table (file name to index block)
• Free list (can be a bit map of available disk
  blocks)
• Limit on the size of the file
• Wasted space
• Could have mechanism for additional index blocks
• OR...

42
Allocation Strategies

• 6. Multilevel Indexed Allocation
• File represented by an index block (on the disk),
  an indirection table of disk blocks for that file
  
• One level indirection
• Two level indirection
• Triple indirection

43
Multilevel Indexed Allocation
44
Allocation Strategies

• 7. Hybrid (BSD, Unix)
• Combination of (5) and (6)
• Each file represented by an i-node (index node)
• Index to first n disk blocks, plus
• A single indirect index, a double indirect index,
  a triple indirect index
• Characteristics
• SA requires i-node reference
• overhead reduced by in-memory cache
• RA requires some computation but much quicker
  than (3)
• Growth is easy
• Allocation overhead same as (3)

45
Unix Inode
46
Logical Representation

• FS may treat a file as an un-interpreted stream
  of bytes (Unix approach)
• Structured files of a user-defined record (not in
  Unix or NT)
• Only feasible if FS implemented at user level
• What happens when you edit a file?
• Growth of a file is it always at the end of the
  file?
• How does the storage system handle insertions in
  the middle of a file?

47
Questions?
48
(No Transcript)