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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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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22. Other subject materials
• Web designing
• Micro processors
• C++ tutorials
• java
home
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