I/O Management and Disk Scheduling

1
Lecture 10

• I/O Management and Disk Scheduling

2
I/O ?? ?? (1)

• Human readable
• ??? ???? ???? ?? ???? ??
• ???
• ??? ????? ???
• ????? ??
• ???
• ???

3
I/O ?? ?? (2)

• Machine readable
• ?? ?? ??? ???? ?? ???? ??? ??
• ??? ? ??? ????
• I/O Controllers
• Sensors
• Actuators

4
I/O ?? ?? (3)

• Communication
• ?? ??? ???? ?? ???? ??
• Digital line drivers
• Modems

5
I/O ??? ??? (1)

• Data rate
• ??? ?? ?? I/O ???? ???? ?? ?? ??
• I/O ???? ??? ?? ??? ? ??? ?? ? ??? ? ? ??

6
(No Transcript)
7
I/O ??? ??? (2)

• Application(??)
• ??? ???? ???? ????? ???? ?? ?? ?????? ??
• ?? ??? ???? ???? ????? ???? ??? ????? ?????? ??
• ??? ???? ???? ???(??)? ?? ????? ? ?? ?? ??? ???

8
I/O ??? ??? (3)

• 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

9
I/O ?? ?? (1)

• Programmed 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
• 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

10
I/O ?? ?? (2)

• I/O ??? ??

???? ?? ???? ??
????? ?? I/O? ???? ?? Programmed-I/O Interrupt-driven I/O
I/O? ???? ?? ?? DMA
11
I/O ??? ?? ?? (1)

• Processor directly controls a peripheral device
• Controller or I/O module is added
• Processor uses programmed I/O without interrupts
• Processor does not need to handle details of
  external devices
• Controller or I/O module with interrupts
• Processor does not spend time waiting for an I/O
  operation to be performed
• Direct Memory Access
• Blocks of data are moved into memory without
  involving the processor
• Processor involved at beginning and end only

12
I/O ??? ?? ?? (2)

• I/O module is a separate processor
• I/O Channel
• I/O processor
• I/O module has its own local memory
• Its a computer in its own right

13
DMA(Direct Memory Access) (1)

• DMA(Direct Memory Access)
• ?????? I/O ?? ??? ?? ???? ??
• DMA ??? I/O? ???? ?? ??? ??? ????
• ??? ??? ?? ???? ???? ???? ?? CPU??? ??? ??? ????
• Cycle Stealing(??? ???)
• ????? ??? ???? ?? ?? ??? ??
• ????? ????? ??? ?? ????? ?? ??? ?? ???? ???
• No interrupts occur
• ???? ??? ?? ????? ??

14
DMA(Direct Memory Access) (2)
15
DMA(Direct Memory Access) (3)

• Cycle stealing? CPU? ?? ??? ?????

16
DMA(Direct Memory Access) (4)

• DMA ?? ??
• DMA ??? I/O ??? ??? ??? ???? ??
• ??-DMA ??(a)
• ??? ?? ??? ???? ? ??? ??? ???? ?? ? ?? ??? ?? ???
  ??
• DMA? I/O ??? ???? ??
• ??-DMA ??(b)
• DMA ??? I/O ????? ??? ??? ?? ? ???? ?? ??? ?? ??
  ??
• DMA ??? I/O ????? ?? ??
• ??? I/O ??? ???? DMA ??? ???? ??
• ??-DMA ??(c)
• DMA ??? I/O ????? ??? ??? ??? ??? ???? ??

17
DMA(Direct Memory Access) (5)
18
DMA(Direct Memory Access) (6)
19
DMA(Direct Memory Access) (7)
20
I/O ?? ?? ?? (1)

• ???(Efficiency)
• ???? I/O ??? ????? ? CPU? ???? ??? ?? ???
• ?? ??? ?? ????? ??????? ??? ??
• ?? ?????? I/O ??? ???? ??? ?? ????? ????? ??
• ??? I/O ??? ??? CPU ??? ???? ?? ????? ?? ???
  ????? ???? ???(swapping)? ???? ??? ??
• Swapping? I/O? ??
• ??? ?? ??? ???? ???? I/O? ??

21
I/O ?? ?? ?? (2)

• ???(Generality)
• ???? ??? ?? ??? ?? ??? ???? ?? I/O ??? ??? ??
  ?????
• ??? ??? ???? ?? ??? ???? ???? ???? ????? ???
• ??? ?? ???
• ????? ?? ????? ???? ???? ??? ??
• ????? ?? ????? I/O ??? ??? ???? ??? ?????? ?? ???
  I/O ??? ??? ??? ?? ? ??? ??

22
I/O ?? ?? ?? (3)

• I/O ??? ??? ??
• ??? ?? ??? ??? I/O ??? ??
• ?? ??? I/O ??? ?? ???? ??? ??
• ?? ??? ???? I/O ?? ?? ??? ?? ?? ??? ???? ??
• ??? ?????? ?????? ?? ??? ??? ?? ?? ??? ??? ???
  ??? ??
• I/O ??? ??? ?? ?

23
(No Transcript)
24
I/O Buffering (1)

• I/O ???? ???
• ????? ????? ?? I/O? ??? ??? ?? ??? ??
• ??? ???? ?? ???? ????? I/O ?? ??? ??? ? ??
• I/O? ???? ?? ????? ???? ?? ????? ???? ? ??
• ?? ???? ????? ??

25
I/O Buffering (2)

• I/O ???? ??? ??? ?? ??? ??
• ?? ??(Block-oriented) I/O ??
• ?? ??? ??? ?? ??? ??? ??
• ??? ??? ??? ??
• ??? ??, ??? ?? ?
• ??? ??(Stream-oriented) I/O ??
• ??? ??? ??? ???? ??
• ???, ???, ?? ??, ??? ?

26
I/O Buffering (3)
27
I/O Buffering (4)
28
Single Buffer (1)

• ????? I/O ??? ?? ?????? ??? ??? ??? ??? ????
• ?? ?? ??
• ?? ??? ??? ??? ?????
• ????? ??? ?? ???? ??? ???? ????? ????? ?? ??? ??
• Read ahead or Anticipated input
• ????? ?? ??? ???? ?? ??? ??? ??
• ??? ?????? ??? ???? ????? ??? ???? ??? ????? ????
  ? ??
• ????? ????? ??? ??? ???? ?? ??? ?????? ?? ??? ????

29
Single Buffer (2)

• ??? ?? ??
• ??? ??? ???? ? ???? ??
• ???? ?? ??? ??? ??? ????? ? ??? ??
• ??? ??? ?? ??? ??
• ????? ? ??? ???? ??? ??? ?? ??

30
Double Buffer

• ? ?? ??? ??? ??
• ????? ??? ??? ???? ??? ??? ????? ?? ??? ???? ????
  ??
• ?? ????? ?? ??

31
Circular Buffer

• ??? ??? ???? ? ? ??? ??? ???? ??
• I/O ??? ?? ?? ????? ???? ?? ?? ???? ??
• ?? ??? ? ??? ??? ?? ??? ??
• I/O ??? ????? ??? ??? ??? ??

32
??? ??

• ??? ??
• ???? ?? ?? ??
• ??? ????? ??? I/O ???? ??? ?? ??? ?? ? ??
• CPU ?? ?????? ???? ??? ??? ?? ??
• ?? ??? ?? ??? ?? ??? ?? ??? ???

33
Disk Data Layout
Sectors
Tracks
Inter-sector gap
Inter-track gap
34
Disk Layout Using Constant Angular Velocity
Track 0, Sector 0
Track 2, Sector 7
35
??? ?? ?? (1)

• ??? ??? ??
• ??? ???? ???? ??? ??? ??? ???? ???? ??? ??? ??
  ??? ????
• ?? ??? ?? ??? ??? ?? ???? ?? ????

36
??? ?? ?? (2)

• ?? ??(Seek time)
• ??? ??? ??? ????? ??? ??
• ??? ?????? ????? ???? ??
• ?? ?? ??(Rotational delay or rotational latency)
• ??? ??? ??? ???? ??? ????? ??? ??
• ??? ?? ??(Access time)
• ?? ??? ?? ?? ??? ?
• ??? ???? ?? ??? ??? ????? ??? ??

37
Timing of a Disk I/O Transfer
38
??? ???? ?? (1)

• ?? ??(Seek time)? ??? ??? ?? ??? ??
• ??? ???? ?? ??? ??? ??? ??
• ??? ??? ?? ??? ??? ?? ?? ??? ????? ??? ??? ??? ??
• ??? ??? ????? ???? ????? (random scheduling) ???
  ??? ??? ?? ??? ?? ??
• ??? ??? ???? ?? ??? ????? ??

39
??? ???? ?? (2)

• ????(First-in, first-out FIFO)
• ????? ??? ??? ??? ???? ??
• ?? ????? ?? ????
• ??? ??? ??? ?? ???? ?? ?? ???? ??? ?????

40
??? ???? ?? (3)

• ?? ??(Priority)
• ???? ??? ??? ??? ????? ?? ??? ????? ?? ??? ????
  ??
• ?? ??? ?? ???? ???? ??? ??? ?? ?? ??? ??
• ? ?? ?? ??? ??

41
??? ???? ?? (4)

• ?? ??(Last-in, first-out LIFO)
• ?? ??? ??? ??? ?? ??
• ???? ?? ????? ?? ??? ??
• ????? ???? ?????? ??? ?? ??? ??? ??
• ???? ????? ?? ??? ?? ??
• ? ??? ??? ??? ?? ??? ?? ?? ??? ??? ??? ?? ??? ???
  ???? ??

42
??? ???? ?? (5)

• Shortest Service Time First(SSTF)
• ??? ??? ?? ????? ??? ?? ???? ????? ??? ??? ??? ??
  ??
• ?? ??? ?? ??? ???? ??? ??? ??

43
??? ???? ?? (6)

• SCAN
• ????, LIFO, SSTF ?? ???? ????? ??? ???? ??
• ?? ???? ??? ??? ??? ???? ?? ?????? ?? ???? ?? ???
  ??? ? ??
• ??? ??? ?? ? ???? ????? ?? ??? ???? ??? ??? ???
  ????? ???? ??? ??? ?? ??? ? ??? ???? ?? ??? ????
  ???? ??
• ? ???? ???? ??? ?? ???? ????? ?????
• ?? ???? ??? ??? ???? ??? ?? ?? ??? ??? ??

44
??? ???? ?? (7)

• C-SCAN
• SCAN ???? ??? ??? ?? ?? ????? ????? ?????? ??
• ? ???? ????? ??? ??? ??? ??? ???? ?? ??? ???? ??
  ?? ???? ????
• SCAN ??? ?? ? ??? ???? ??? ?? ???? ???? ??? ????
  ??

45
??? ???? ?? (8)

• SSTF, SCAN, C-SCAN ????? ? ??(Arm stickness)
  ??? ??? ? ??
• ??? ???? ? ??? ?? ?? ???? ????? ??? ?? ?? ?? ????
  ?? ? ??
• N-step-SCAN
• Segments the disk request queue into subqueues of
  length N
• Subqueues are process one at a time, using SCAN
• New requests added to other queue when queue is
  processed
• FSCAN
• Two queues
• One queue is empty for new request

46
??? ???? ???? ??(1)
47
??? ???? ???? ??(2)

• ??? ???? ????? ?? ??
• ?? ?? ?? ??
• ???? 200 ?? ???? ??
• ??? ?? ?? ??? ???? ??
• ?? ??? 55, 58, 39, 18, 90, 160, 150, 38, 184 ???
  ??
• ?? ???? ????
• FIFO
• SSTF
• SCAN
• C-SCAN

48
??? ???? ???? ??(3)

• ??? ?? ?? ?? ??

49
??? ???? ???? ??(4)
50
RAID 0 (non-redundant)
51
(No Transcript)
52
RAID 1 (mirrored)
53
RAID 2 (redundancy through Hamming code)
54
RAID 3 (bit-interleaved parity)
55
RAID 4 (block-level parity)
56
RAID 5 (block-level distributed parity)
57
RAID 6 (dual redundancy)
58
Disk Cache

• Buffer in main memory for disk sectors
• Contains a copy of some of the sectors on the
  disk

59
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

60
Least Recently Used

• The block on the bottom of the stack is removed
  when a new block is brought in
• Blocks dont actually move around in main memory
• A stack of pointers is used

61
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 then not needed any more

62
UNIX SVR4 I/O
63
Windows 2000 I/O