1. I/O Management and Disk Scheduling
Chapter 8
Advanced Operating System

2. Test
In your opinion, what multimedia applications or tasks, if any, might be candidates for inclusion in a near-future operating system

3. Categories of I/O Devices
Human readable
Used to communicate with the user
Printers
Video display terminals
Display
Keyboard
Mouse

4. Categories of I/O Devices
Machine readable
Used to communicate with electronic equipment
Disk and tape drives
Sensors
Controllers
Actuators

5. Categories of I/O Devices
Communication
Used to communicate with remote devices
Digital line drivers
Modems

6. Differences in I/O Devices
Data rate
May be differences of several orders of magnitude between the data transfer rates

8. Differences in I/O Devices
Application
Disk used to store files requires file management software
Disk used to store virtual memory pages needs special hardware and software to support it
Terminal used by system administrator may have a higher priority

9. Differences in I/O Devices
Complexity of control
Unit of transfer
Data may be transferred as a stream of bytes for a terminal or in larger blocks for a disk
Data representation
Encoding schemes
Error conditions
Devices respond to errors differently

10. Performing I/O Programmed I/O Interrupt-driven I/O
Process is busy-waiting for the operation to complete
Interrupt-driven I/O
I/O command is issued
Processor continues executing instructions
I/O module sends an interrupt when done

11. Performing I/O Direct Memory Access (DMA)
DMA module controls exchange of data between main memory and the I/O device
Processor interrupted only after entire block has been transferred

12. Programmed I/O No interrupts occur
Processor is kept busy checking status
procedure readString (var s);
repeat
Send I/O command “go read a word”
Read I/O status
until I/O done
Read word from I/O module
Write word into memory
until finished reading ……
I/O are much slower than the CPU. It is very inefficient for the CPU to wait for I/O completion in a tight loop. (busy waiting).

13. Interrupt-Driven I/O
No busy waiting. Processor can proceed to do other things when I/O is in progress
When I/O is done, the CPU is interrupted
Still consumes a lot of processor time because every word read or written passes through the processor

14. Interrupt-Driven I/O procedure readString (var s); repeat
Send I/O command “go read a word” …
now, the CPU will do something else
don’t bother checking I/O status
until finished reading ……
When the I/O module finished reading the word, it interrupts the CPU. The CPU will execute an interrupt handler to move the word to memory.
/* interrupt handler */
Read word from I/O module
Write word into memory
return
Afterwards, control is returned to the program which continues to read the next word.
Interrupt-driven I/O is still inefficient in data transfer of large amount because the CPU has to transfer the data word by word between I/O module and memory.

15. Direct Memory Access
Processor grants I/O module authority to read from or write to memory
DMA module controls exchange of data between main memory and the I/O device
processor interrupted only after entire block has been transferred
The processor is only involved at the beginning and end of the transfer

16. DMA procedure readString (var s); Request the DMA module
The DMA module starts reading each word of the data and save them in the memory.
procedure readString (var s);
Request the DMA module
to read some data …
now, the CPU will do something else
don’t bother checking I/O status
DMA I/O is more efficient in large data transfer because the interaction with the I/O module and data transfer between I/O module and memory are performed by the DMA module.
After the DMA has transferred all the data requested to the memory, it notifies that it has finished the I/O by sending the CPU an interrupt

17. Direct Memory Access
Processor delegates I/O operation to the DMA module
DMA module transfers data directly to or form memory
When complete DMA module sends an interrupt signal to the processor

18. DMA

19. DMA Configurations

20. DMA Configurations

21. Operating System Design Issues
Efficiency
Most I/O devices extremely slow compared to main memory
Use of multiprogramming allows for some processes to be waiting on I/O while another process executes
I/O cannot keep up with processor speed
Swapping is used to bring in additional Ready processes which is an I/O operation

22. Operating System Design Issues
Generality
Desirable to handle all I/O devices in a uniform manner
Hide most of the details of device I/O in lower-level routines so that processes and upper levels see devices in general terms such as read, write, open, close, lock, unlock

23. I/O Buffering Reasons for buffering
Processes must wait for I/O to complete before proceeding
Certain pages must remain in main memory during I/O

24. I/O Buffering Block-oriented Stream-oriented
Information is stored in fixed sized blocks
Transfers are made a block at a time
Used for disks and tapes
Stream-oriented
Transfer information as a stream of bytes
Used for terminals, printers, communication ports, mouse and other pointing devices, and most other devices that are not secondary storage

25. Single Buffer
Operating system assigns a buffer in main memory for an I/O request
Block-oriented
Input transfers made to buffer
Block moved to user space when needed
Another block is moved into the buffer
Read ahead

26. Single Buffer Block-oriented
User process can process one block of data while next block is read in
Swapping can occur since input is taking place in system memory, not user memory
Operating system keeps track of assignment of system buffers to user processes

27. Single Buffer Stream-oriented Used a line at time
User input from a terminal is one line at a time with carriage return signaling the end of the line
Output to the terminal is one line at a time

28. I/O Buffering

29. Double Buffer Use two system buffers instead of one
A process can transfer data to or from one buffer while the operating system empties or fills the other buffer

30. Circular Buffer More than two buffers are used
Each individual buffer is one unit in a circular buffer
Used when I/O operation must keep up with process

31. Disk Performance Parameters
To read or write, the disk head must be positioned at the desired track and at the beginning of the desired sector
Seek time
Time it takes to position the head at the desired track
Rotational delay or rotational latency
Time its takes for the beginning of the sector to reach the head

32. Disk Performance Parameters
Access time
Sum of seek time and rotational delay
The time it takes to get in position to read or write
Data transfer occurs as the sector moves under the head

33. Disk Scheduling Policies
Seek time is the reason for differences in performance
For a single disk there will be a number of I/O requests
If requests are selected randomly, we will poor performance

34. OS has to read these tracks: 2,3,4,5,8.
Disk Scheduling
At runtime, I/O requests for disk tracks come from the processes
OS has to choose an order to serve the requests
Process A reads tracks 2, 5
OS has to read these tracks: 2,3,4,5,8.
Process B reads tracks 3, 5
Process C reads tracks 8, 4

35. Disk Scheduling
The order that the read/write head is moved to satisfy several I/O requests
determines the total seek time
affects performance
the OS cannot change the rotational delay or transfer time, but it can try to find a ‘good’ order that spends less time in seek time.
If requests are selected randomly, we will get the worst possible performance...

36. Disk Scheduling Policy
FIFO: fair, but near random scheduling
SSTF: possible starvation
SCAN: favor requests for tracks near the ends
C-SCAN
FSCAN: avoid “arm stickiness” in SSTF, SCAN and C-SCAN

37. First-in-first-out, FIFO
process requests in the order that the requests are made
fair to all processes
approaches random scheduling in performance if there are many processes
Read write head starting at track 8, in the direction of increasing track number 8 1 2 7 10
Pending read/write request for a track

38. Shortest Service Time First, SSTF
select the disk I/O request that requires the least movement of the disk arm from its current position
always choose the minimum seek time
new requests may be chosen before an existing request 8 1 2 7 10

39. SCAN
arm moves in one direction only, satisfying all outstanding requests until there is no more requests in that direction. The service direction is then reversed.
favor requests for tracks near the ends 8 1 2 7 10

40. C-SCAN restrict scanning to one direction only
when the last track has been visited in one direction, the arm is returned to the opposite end of the disk and the scan begins again. 8 1 2 7 10

41. FSCAN
“Arm stickiness” in SSTF, SCAN, C-SCAN in case of repetitive requests to one track
FSCAN uses two queues. When a scan begins, all of the requests are in one of the queues, with the other empty. During the scan, all new requests are put into the other queue.
Service of new requests is deferred until all of the old requests have been processed.

42. Disk Cache Buffer in main memory for disk sectors
Contains a copy of some of the sectors on the disk

43. Disk Cache, Hit and Miss
When an I/O request is made for a particular sector, the OS checks whether the sector is in the disk cache.
If so, (cache hit), the request is satisfied via the cache.
If not (cache miss), the requested sector is read into the disk cache from the disk.

44. Least Recently Used
The block that has been in the cache the longest with no reference to it is replaced
The cache consists of a stack of blocks
Most recently referenced block is on the top of the stack
When a block is referenced or brought into the cache, it is placed on the top of the stack

45. Least Recently Used
The block on the bottom of the stack is removed when a new block is brought in
Blocks don’t actually move around in main memory
A stack of pointers is used

46. Least Frequently Used
The block that has experienced the fewest references is replaced
A counter is associated with each block
Counter is incremented each time block accessed
Block with smallest count is selected for replacement
Some blocks may be referenced many times in a short period of time and the reference count is misleading

47. Hybrid
and
Open source

48. Windows Supports a wide range of computer hardware
Large library software
Unstable: blue screen of death (Except windows XP)
Requires a system reboot when facing errors
Secure operating system
Target of Viruses and spyware

49. Windows Microsoft’ s reaction Windows TCO and Security
Windows Genuine Advantage Notifications
Internet Explorer has lost market share to Firefox and Opera
Internet Explorer: Windows
Firefox: Windows, Linux
Opera: Windows, Linux

50. Windows Total cost of ownership (TCO)
Cost of computer and software
Maintenance
Training
Technical support
Hardware and software upgrade
Windows has a lower TCO: “Get the Facts”

51. Windows Security Windows Vista (Longhorn)
Closed-source is invisible for crackers
Windows Vista (Longhorn)
High security
Easy to management
Detecting hardware feature problem
Faster start up time

52. Linux Cost less More secure: Open source OS for networking
Not viruses target

53. Technical Comparison
Kernel space: The central module of an operating system. It is the part of the operating system that loads first, and it remains in main memory. Because it stays in memory, it is important for the kernel to be as small as possible while still providing all the essential services required by other parts of the operating system and applications. Typically, the kernel is responsible for memory management, process and task management, and disk management.
Windows: file system, Internet explorer, windows media player, GUI
Linux: only file system
Memory management
Windows: Swap file
Linux: avoiding swapping and allocating memory

54. Technical Comparison Swapping
To replace pages or segments of data in memory.
Swapping is a useful technique that enables a computer to execute programs and manipulate data files larger than main memory.
The operating system copies as much data as possible into main memory, and leaves the rest on the disk.
When the operating system needs data from the disk, it exchanges a portion of data (called a page or segment ) in main memory with a portion of data on the disk.

55. Technical Comparison Stability Device Driver
Windows: blue screen of death (instability)
Linux: More stability
Common source of instability is due to bugs in various device drivers
Device Driver
Windows: run in kernel space
Linux: run in user space