Chapter 9I/O System
Input/Output SystemMajor objectives:• Take an application I/O request and  send it to the physical device.• Take whatever ...
General I/O Issues• The operating system is able to improve overall  system performance if it can keep the various  device...
Disk Drive Mechanism4
Disk Structure and Organization• Moving-head disk - one head per surface• Fixed-head disk - one head per track• Data on a ...
Disk Structure• Disk drives are addressed as large 1-dimensional  arrays of logical blocks, where the logical block is  th...
Disk Track Format7
Direct Access Storage Devices• Otherwise known as DASDs     – Devices that can directly read or write to a       specific ...
Fixed-Head Drums• Developed in the early 1950s    – access times of 5 to 25 ms were considered      fast    – used a drum ...
Fixed-Head Drums•    Resemble a giant coffee can•    Covered with magnetic film•    Formatted so tracks run around it•    ...
Fixed-Head Disks• Disks resemble phonograph record albums  covered with magnetic film that has been  formatted into concen...
Movable-Head Drums• Consist of a few read/write heads that  move from track to track to cover the  entire surface of the d...
Movable-Head Disks     The read/write head floats over the       surface of the disk:     • Exist as individual units, as ...
Disk PackA typical disk pack consists of several  platters that are stacked on a common  central spindle, with a slight sp...
Architecture of M-Head Disks• Each platter has two surfaces for  recording, except the top and bottom• Each surface is for...
Optical Storage• Direct access storage• CD-ROMs contained the first optical  storage DASDs     – these were incompatible w...
Function of Optical Disc• Optical disc drive functions similar to the  magnetic disk drive• Read head is on an arm that mo...
CD Storage CapacityCD-ROM has large storage potential, more than 700 megabytes of data18
Read/Write• To read or write data, disk device must move the  arm to the appropriate track.• The time to carry this out th...
Disk Access TimeThe disk access time can be calculated as follows:Disk Access time =         Seek time + Rotational Latenc...
I/O Requests• In general, there may be many I/O  requests for a device at the same time.• These requests may come from mul...
I/O System Structure22
Stages of an I/O Request23
I/O Device Handling• A Queue of Pending Requests• A Resource Scheduler that determines the  next request to execute• A Mec...
I/O Request Queuing25
I/O Performance Optimization• I/O processing is much slower than CPU  processing. Every physical disk I/O has a  dramatic ...
Reducing Number of I/O Requests• The most efficient I/O request is one that  is never requested27
Buffering and Caching• The I/O system should make the physical I/O  requests as big as possible.• This will reduce the num...
Buffered Write29
I/O Scheduling• For most devices, a FCFS (First-Come-First-  Serve) scheduling algorithm is appropriate.  For example, one...
I/O Scheduling (Cont.)• On a typical system, there will be pending disk  I/O requests from many different processes.• The ...
Context Switching in I/O• In CPU scheduling, the context-switch time is  relatively small with respect to the service  tim...
Goal of Disk Scheduling• In any disk system with a moving  read/write head, the seek time between  cylinders takes a signi...
An Analogy• Traveling service person, a technician who  has to service requests from several clients  in a geographical ar...
Purpose of Disk Scheduling• Select a disk request from the queue of  I/O requests• Decide when to process this I/O request35
Issues in Disk Scheduling• Throughput - the number of disk requests  that are completed in some period• Fairness - some di...
Goal of Disk Scheduling?• High Throughput• FairnessThere is a trade-off between total system throughput and fairness.37
State-Dependent BehaviorThe current position of the read/write head (i.e., the state of the disk) affects the response tim...
Disk Scheduling AlgorithmsFor moving-head disk, disk scheduling algorithms are needed to minimize seek time•    FCFS sched...
Disk Scheduling Algorithms• Several algorithms exist to schedule the servicing  of disk I/O requests.• Given the following...
FCFS Scheduling• FCFS scheduling service I/O requests in  the order in which they arrive.• It is, of course, the simplest ...
FCFS Scheduling Example42
Another Example with FCFS      Illustration shows total head movement of 640 cylinders.43
SSTF• Selects the request with the minimum seek  time from the current head position.• SSTF scheduling is a form of SJF  s...
SSTF Scheduling Example45
Another Example with SSTF46
SCAN• The disk arm starts at one end of the disk, and  moves toward the other end, servicing requests  until it gets to th...
Example with SCAN Scheduling48
Another Example with SCAN49
C-SCAN• A variant of SCAN• Provides a more uniform wait time than SCAN.• The head moves from one end of the disk to the  o...
Example with C-SCAN51
C-LOOK• Variant of C-SCAN• Disk arm only travels as far as the last  request in each direction, then reverses  direction i...
Upcoming SlideShare
Loading in …5
×

disk sechduling

731 views

Published on

Published in: Education, Technology
0 Comments
1 Like
Statistics
Notes
  • Be the first to comment

No Downloads
Views
Total views
731
On SlideShare
0
From Embeds
0
Number of Embeds
2
Actions
Shares
0
Downloads
28
Comments
0
Likes
1
Embeds 0
No embeds

No notes for slide

disk sechduling

  1. 1. Chapter 9I/O System
  2. 2. Input/Output SystemMajor objectives:• Take an application I/O request and send it to the physical device.• Take whatever response comes back from the device and send it to the application.• Optimize the performance of the various I/O requests.2
  3. 3. General I/O Issues• The operating system is able to improve overall system performance if it can keep the various devices as busy as possible.• It is important for the operating system to handle device interrupts as quickly as possible. – For interactive devices (keyboard, mouse, microphone), this can make the system more responsive. – For communication devices (modem, Ethernet, etc), this can affect the effective speed of the communications. – For real-time systems, this can be the difference between the system operating correctly and 3 malfunctioning.
  4. 4. Disk Drive Mechanism4
  5. 5. Disk Structure and Organization• Moving-head disk - one head per surface• Fixed-head disk - one head per track• Data on a disk is addressed by: – Cylinder – Surface – Sector5
  6. 6. Disk Structure• Disk drives are addressed as large 1-dimensional arrays of logical blocks, where the logical block is the smallest unit of transfer.• The 1-dimensional array of logical blocks is mapped into the sectors of the disk sequentially. – Sector 0 is the first sector of the first track on the outermost cylinder. – Mapping proceeds in order through that track, then the rest of the tracks in that cylinder, and then through the rest of the cylinders from outermost to innermost.6
  7. 7. Disk Track Format7
  8. 8. Direct Access Storage Devices• Otherwise known as DASDs – Devices that can directly read or write to a specific place on a disk or drum – Also called random access storage devices• Grouped into two major categories – fixed read/write heads – movable read/write heads8
  9. 9. Fixed-Head Drums• Developed in the early 1950s – access times of 5 to 25 ms were considered fast – used a drum with a capacity of 2000 bytes • later increased to 4000 bytes • speed was 200 rpm • much faster than other drums of the time which were only 50-60 rpm• By 1970 drums increased to 1 megabyte and speed of 3000 rpm9
  10. 10. Fixed-Head Drums• Resemble a giant coffee can• Covered with magnetic film• Formatted so tracks run around it• Data recorded serially on each track by the read/write head positioned over itThese drums were quite fast, yet expensive and did store as much as other DASDs.10
  11. 11. Fixed-Head Disks• Disks resemble phonograph record albums covered with magnetic film that has been formatted into concentric circles called tracks• Data is recorded in the same manner as fixed-head drum• These were very expensive and had less storage space as compared to movable-head disks but were faster.11
  12. 12. Movable-Head Drums• Consist of a few read/write heads that move from track to track to cover the entire surface of the drum• Device that is least expensive has only one read/write head for the whole drum• Drums with several read/write heads work faster but also cost more12
  13. 13. Movable-Head Disks The read/write head floats over the surface of the disk: • Exist as individual units, as in a PC • Can also be in a disk pack, which is a stack of disks13
  14. 14. Disk PackA typical disk pack consists of several platters that are stacked on a common central spindle, with a slight space between them so the read/write heads can move between the pairs of disks.14
  15. 15. Architecture of M-Head Disks• Each platter has two surfaces for recording, except the top and bottom• Each surface is formatted with specific numbers of tracks for the data to be recorded on – number of tracks varies depending on the manufacturer – usually range from 200 to 800 tracks15
  16. 16. Optical Storage• Direct access storage• CD-ROMs contained the first optical storage DASDs – these were incompatible with most systems, as they were developed for a single system• Is a major contender for the replacement of magnetic disks because it has high- density storage and durability16
  17. 17. Function of Optical Disc• Optical disc drive functions similar to the magnetic disk drive• Read head is on an arm that moves forward and backward, track to track – disc rotates at 200-500 rpm – average seek time is 500 ms and maximum seek is 1 second – transfer rate is about 150 kilobytes/second17
  18. 18. CD Storage CapacityCD-ROM has large storage potential, more than 700 megabytes of data18
  19. 19. Read/Write• To read or write data, disk device must move the arm to the appropriate track.• The time to carry this out this is called Seek Time.• Then, the disk device must wait for the desired sector/data to rotate into position under the head (rotational latency).• Each track is recorded in units called Sectors. A sector is the smallest amount of data that can be physically read or written. 19
  20. 20. Disk Access TimeThe disk access time can be calculated as follows:Disk Access time = Seek time + Rotational Latency20
  21. 21. I/O Requests• In general, there may be many I/O requests for a device at the same time.• These requests may come from multiple processes or the same process.21
  22. 22. I/O System Structure22
  23. 23. Stages of an I/O Request23
  24. 24. I/O Device Handling• A Queue of Pending Requests• A Resource Scheduler that determines the next request to execute• A Mechanism to initiate the next request whenever a request completes.24
  25. 25. I/O Request Queuing25
  26. 26. I/O Performance Optimization• I/O processing is much slower than CPU processing. Every physical disk I/O has a dramatic impact on system performance• To improve I/O performance: – Reduce the number of I/O requests – Carry out buffering and/or caching – I/O Scheduling26
  27. 27. Reducing Number of I/O Requests• The most efficient I/O request is one that is never requested27
  28. 28. Buffering and Caching• The I/O system should make the physical I/O requests as big as possible.• This will reduce the number of physical I/O requests by the buffering factor used.• The applications logical I/O requests should copy data to/from a large memory buffer. The physical I/O requests then transfer the entire buffer.28
  29. 29. Buffered Write29
  30. 30. I/O Scheduling• For most devices, a FCFS (First-Come-First- Serve) scheduling algorithm is appropriate. For example, one wants the segments of a music file to be played in sequential order.• For some devices (disks especially), the order in which requests are processed is not inherently constrained by the device characteristics. 30
  31. 31. I/O Scheduling (Cont.)• On a typical system, there will be pending disk I/O requests from many different processes.• The correct functioning of these processes usually does not depend on the order in which the disk I/O operations actually occur.• Thus, we will want the Resource Scheduler to attempt to optimize performance for devices such as disks.31
  32. 32. Context Switching in I/O• In CPU scheduling, the context-switch time is relatively small with respect to the service time• In I/O scheduling the context-switch time is relatively large with respect to the service timeThe time to move the head between cylinders is much greater than the time it takes to read or write to a cylinder.32
  33. 33. Goal of Disk Scheduling• In any disk system with a moving read/write head, the seek time between cylinders takes a significant amount of time• This traveling head time should be minimized33
  34. 34. An Analogy• Traveling service person, a technician who has to service requests from several clients in a geographical area• Often spends more time driving than actually carrying out service34
  35. 35. Purpose of Disk Scheduling• Select a disk request from the queue of I/O requests• Decide when to process this I/O request35
  36. 36. Issues in Disk Scheduling• Throughput - the number of disk requests that are completed in some period• Fairness - some disk requests may have to wait a long time before being servedA totally fair system would ensure that the mean response time of the disk requests is the same for all processes36
  37. 37. Goal of Disk Scheduling?• High Throughput• FairnessThere is a trade-off between total system throughput and fairness.37
  38. 38. State-Dependent BehaviorThe current position of the read/write head (i.e., the state of the disk) affects the response time of the next request38
  39. 39. Disk Scheduling AlgorithmsFor moving-head disk, disk scheduling algorithms are needed to minimize seek time• FCFS scheduling: first-come-first-served• SSTF scheduling: shortest-seek-time-first• SCAN scheduling• C-SCAN scheduling: circular SCAN• LOOK scheduling• C-LOOK scheduling 39
  40. 40. Disk Scheduling Algorithms• Several algorithms exist to schedule the servicing of disk I/O requests.• Given the following disk request sequence for a disk with 100 tracks: 44, 20, 95, 4, 50, 52, 47, 61, 87, 25 Head pointer 50 (current position of R/W heads)40
  41. 41. FCFS Scheduling• FCFS scheduling service I/O requests in the order in which they arrive.• It is, of course, the simplest scheduling algorithm and actually does no scheduling.• It serves as a useful baseline to compare other scheduling algorithms.41
  42. 42. FCFS Scheduling Example42
  43. 43. Another Example with FCFS Illustration shows total head movement of 640 cylinders.43
  44. 44. SSTF• Selects the request with the minimum seek time from the current head position.• SSTF scheduling is a form of SJF scheduling; may cause starvation of some requests.• Illustration shows total head movement of 152 cylinders (or tracks).44
  45. 45. SSTF Scheduling Example45
  46. 46. Another Example with SSTF46
  47. 47. SCAN• The disk arm starts at one end of the disk, and moves toward the other end, servicing requests until it gets to the other end of the disk, where the head movement is reversed and servicing continues.• Sometimes called the elevator algorithm.• Illustration shows total head movement of 136 cylinders. 47
  48. 48. Example with SCAN Scheduling48
  49. 49. Another Example with SCAN49
  50. 50. C-SCAN• A variant of SCAN• Provides a more uniform wait time than SCAN.• The head moves from one end of the disk to the other. servicing requests as it goes. When it reaches the other end, however, it immediately returns to the beginning of the disk, without servicing any requests on the return trip.50
  51. 51. Example with C-SCAN51
  52. 52. C-LOOK• Variant of C-SCAN• Disk arm only travels as far as the last request in each direction, then reverses direction immediately, without first going all the way to the end of the disk.52

×