This document discusses disk scheduling and the first-come, first-served (FCFS) disk scheduling algorithm. It begins by explaining the need for disk scheduling to improve average disk service time by scheduling disk requests. It then describes FCFS as one approach and notes that the document will cover additional disk scheduling algorithms.

1. Disk Scheduling ,
FCFS disk
scheduling algorithms
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2. HOME PREVIOUS TOPIC NEXT
PREVIOUS QUESTION PAPERS FOR OS
CPP TUTORIALS
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3. Recap
In the last class, you have learnt:
Various allocation methods
- Their advantages and disadvantages
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4. Objectives
On Completion of this period, you would be able to
know
• The Need for Disk Scheduling
• FCFS disk scheduling algorithm
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5. Need for Disk Scheduling
• Most jobs depend heavily on disk
• It is important that disk service be as fast as possible for
– Loading
– Input
– Output of files
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6. Need for Disk Scheduling
• The operating system can improve average disk
service time
– By scheduling the requests for disk access
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7. Need for Disk Scheduling
• Disk speed is composed of three parts
• First
– To access a block the system must move the head
to the appropriate track or cylinder
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8. Need for Disk Scheduling
• This head movement is called a SEEK
• The time to complete it is SEEK TIME
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9. Need for Disk Scheduling
• Once the head is at the right track
– It must wait until the desired block rotates under
read write head
• This operation is called LATENCY(ROTATIOAL)
• The delay is called LATENCY TIME
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10. Need for Disk Scheduling
• The Transfer Time
– The time required to transfer of data between the
disk and main memory
• The total time to service a disk request is the sum of
– Seek time
– Latency (rotational) time &
– Transfer time
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11. Need for Disk Scheduling
• The disk bandwidth is the ratio of
The total number of bytes transferred
The time between the first request of service and
completion of transfer
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12. Disk Scheduling
• Whenever a process needs a I/O from the disk
– It issues a system call to the OS
• The request has several pieces of information
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13. Disk Scheduling
• Whether this operation is input or output
• What the disk address for the transfer is
• What is the memory address for the transfer
• What the number of sectors to be transferred
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14. Disk Scheduling
• If the desired disk drive and controller are available
– The service can be serviced immediately
• If the drive controller is busy
– Any new requests will be placed in the queue
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15. Disk Scheduling
• Multiprogramming system with many process
• The disk queue may often have several pending
requests
• Thus when one request is completed
• The operating system chooses which pending request is
to service next
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16. Disk Scheduling
• How does operating system makes this choice
– Any of the disk scheduling algorithms can be used
• We shall discuss various disk scheduling algorithms
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17. Various Disk Scheduling
• FCFS Scheduling
• SSTF Scheduling
• SCAN Scheduling
• C-SCAN Scheduling
• LOOK Scheduling
• C-LOOK Scheduling
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18. Disk Scheduling
• All these algorithms are based on criteria i.e.
– Minimize seek time
• Seek time ≈ Seek distance
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19. Quiz
1. The operating system can improve average disk
service time by scheduling the requests for disk
access [T/F]
TRUE
2. The time required to transfer of data between the
disk and main memory is called
TRANSFER TIME
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20. Quiz
3. If the desired disk drive and controller are available
the service can be serviced immediately [T/F]
TRUE
4. The disk scheduling algorithms are based on criteria
Minimize seek time
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21. Quiz
1. The FCFS algorithm does not provide best (average)
access [T/F]
TRUE
2. The head movement in the FCFS algorithm is
minimal in all cases [T/F]
FALSE
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• Web designing
• Micro processors
• C++ tutorials
• java
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