![Quiz
• SCAN and C-SCAN perform better for systems that
place a heavy load on the disk. [T/F]
TRUE
• If the queue has only one outstanding request all
scheduling algorithms behave as FCFS [T/F]
TRUE
http://raj-os.blogspot.in/ 31](https://image.slidesharecdn.com/diskschedulingalgorithm-52-130319070333-phpapp02/85/Disk-scheduling-algorithm-52-31-320.jpg)
![Quiz
• C-LOOK algorithm is a version of C-SCAN [T/F]
TRUE
• SCAN and C-SCAN perform better for systems that
place a heavy load on the disk. [T/F]
TRUE
http://raj-os.blogspot.in/ 32](https://image.slidesharecdn.com/diskschedulingalgorithm-52-130319070333-phpapp02/85/Disk-scheduling-algorithm-52-32-320.jpg)
Disk scheduling algorithm.52
- 1. Disk scheduling algorithms summary http://raj-os.blogspot.in/ 1
- 2. Recap In the last class, you have learnt: • LOOK and C-LOOK Scheduling http://raj-os.blogspot.in/ 2
- 3. • HOME PREVIOUS TOPIC NEXT • PREVIOUS QUESTION PAPERS FOR OS • CPP TUTORIALS http://raj-os.blogspot.in/ 3
- 4. Objectives On Completion of this period, you would be able to know • How to select a Disk-Scheduling algorithm • Factors affecting the Disk-Scheduling algorithm • Implementation of Disk-Scheduling algorithm • Summary of disk scheduling algorithm http://raj-os.blogspot.in/ 4
- 5. Selection of a Disk-Scheduling algorithm Given so many scheduling algorithms how do we choose the best one? • SSTF is common and has a natural appeal because it increases performance over FCFS http://raj-os.blogspot.in/ 5
- 6. Selection of a Disk-Scheduling Algorithm • SCAN and C-SCAN perform better for systems that place a heavy load on the disk • SCAN and C-SCAN are less likely to cause a starvation problem http://raj-os.blogspot.in/ 6
- 7. Selection of a Disk-Scheduling Algorithm • For any particular list of requests we can define an optimal order of retrieval • But the computations needed for finding a optimal schedule may not justify the savings over SSTF or SCAN http://raj-os.blogspot.in/ 7
- 8. Factors affecting Disk-Scheduling Algorithm • With any scheduling algorithm – Performance depends heavily on the number and type of requests • If the queue has only one outstanding request – All scheduling algorithms behave as FCFS http://raj-os.blogspot.in/ 8
- 9. Factors affecting Disk-Scheduling Algorithm • Requests for disk service can be influenced by the file- allocation method – A program reading contiguously allocated file • Generate several requests that are close together on the disk • Resulting in limited head movement – A program reading linked or indexed allocated file • Generate several requests that include blocks scattered on the disk • Resulting in greater head movement http://raj-os.blogspot.in/ 9
- 10. Factors affecting Disk-Scheduling Algorithm • The disk Scheduling algorithms considers only the seek distances • For modern disks the rotational latency can be nearly as large as the average seek time http://raj-os.blogspot.in/ 10
- 11. Factors affecting Disk-Scheduling Algorithm • It is difficult for the operating system to schedule for improved rotational latency – Because modern disks does not disclose physical location of logical blocks • In modern disks the disk scheduling algorithms are built in the disk drive http://raj-os.blogspot.in/ 11
- 12. Implementation of a Disk-Scheduling Algorithm • If the OS sends a batch of requests to the controller • The controller queues them and then schedule them to improve – Both the SEEK time and the Rotational latency http://raj-os.blogspot.in/ 12
- 13. Implementation of a Disk-Scheduling Algorithm • The disk-scheduling algorithm should be written as a separate module of the operating system – Allowing it to be replaced with a different algorithm if necessary • Either SSTF or LOOK is a reasonable choice for the default algorithm http://raj-os.blogspot.in/ 13
- 14. Disk Scheduling • The operating system is responsible for using hardware efficiently for the disk drives, – This means having a fast access time and disk bandwidth • Access time has two major components – Seek time is the time for the disk to move the heads to the cylinder containing the desired sector – Rotational latency is the additional time waiting for the disk to rotate the desired sector to the disk head http://raj-os.blogspot.in/ 14
- 15. Disk Scheduling • Minimize seek time • Seek time ≈ Seek distance Disk bandwidth is the total number of bytes transferred, The total time between the first request for service and the completion of the last transfer http://raj-os.blogspot.in/ 15
- 16. Disk Scheduling • Several algorithms exist to schedule the servicing of disk I/O requests • We illustrate them with a request queue (0-199) – Tracks 98, 183, 37, 122, 14, 124, 65, 67 – Head pointer 53 http://raj-os.blogspot.in/ 16
- 17. Illustration shows total head movement of 640 cylinders FCFS http://raj-os.blogspot.in/ 17
- 18. 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 236 cylinders http://raj-os.blogspot.in/ 18
- 19. SSTF SSTF http://raj-os.blogspot.in/ 19
- 20. 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 208 cylinders http://raj-os.blogspot.in/ 20
- 21. SCAN SCAN http://raj-os.blogspot.in/ 21
- 22. C-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 • Treats the cylinders as a circular list that wraps around from the last cylinder to the first one http://raj-os.blogspot.in/ 22
- 23. C-SCAN C-SCAN http://raj-os.blogspot.in/ 23
- 24. C-LOOK • Version of C-SCAN • Arm only goes 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 http://raj-os.blogspot.in/ 24
- 25. C-LOOK C-LOOK http://raj-os.blogspot.in/ 25
- 26. Selecting a Disk-Scheduling Algorithm • SSTF is common and has a natural appeal • SCAN and C-SCAN perform better for systems that place a heavy load on the disk • Performance depends on the number and types of requests http://raj-os.blogspot.in/ 26
- 27. Selecting a Disk-Scheduling Algorithm • Requests for disk service can be influenced by the file-allocation method • The disk-scheduling algorithm should be written as a separate module of the operating system, – allowing it to be replaced with a different algorithm if necessary • Either SSTF or LOOK is a reasonable choice for the default algorithm. http://raj-os.blogspot.in/ 27
- 28. Summary In this class, you have learnt: – Disk Scheduling – Various Disk Scheduling algorithms – Selection of a particular Disk Scheduling algorithm – Summary of Disk Scheduling algorithm http://raj-os.blogspot.in/ 28
- 29. Frequently Asked Questions 1. How to select a disk scheduling algorithm? 2. List the factors that effect the disk scheduling algorithm http://raj-os.blogspot.in/ 29
- 30. Frequently Asked Questions 3. Compare various scheduling algorithm http://raj-os.blogspot.in/ 30
- 31. Quiz • SCAN and C-SCAN perform better for systems that place a heavy load on the disk. [T/F] TRUE • If the queue has only one outstanding request all scheduling algorithms behave as FCFS [T/F] TRUE http://raj-os.blogspot.in/ 31
- 32. Quiz • C-LOOK algorithm is a version of C-SCAN [T/F] TRUE • SCAN and C-SCAN perform better for systems that place a heavy load on the disk. [T/F] TRUE http://raj-os.blogspot.in/ 32
- 33. Other subject materials • Web designing • Micro processors • C++ tutorials • java home http://raj-os.blogspot.in/ 33