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Disk Scheduling , FCFS disk scheduling algorithms
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HOME PREVIOUS TOPIC NEXTPREVIOUS QUESTION PAPERS FOR OSCPP TUTORIALS
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Recap
In the last class, you have learnt:
Various allocation methods
- Their advantages and disadvantages
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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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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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Need for Disk Scheduling
• The operating system can improve average disk
service time
– By scheduling the requests for disk access
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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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Need for Disk Scheduling
• This head movement is called a SEEK
• The time to complete it is SEEK TIME
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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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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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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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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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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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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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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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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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Various Disk Scheduling
• FCFS Scheduling
• SSTF Scheduling
• SCAN Scheduling
• C-SCAN Scheduling
• LOOK Scheduling
• C-LOOK Scheduling
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Disk Scheduling
• All these algorithms are based on criteria i.e.
– Minimize seek time
• Seek time Seek distance
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Quiz
1. The operating system can improve average disk service
time by scheduling the requests for disk access [T/F]
2. The time required to transfer of data between the disk
and main memory is called
TRUE
TRANSFER TIME
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Quiz
3. If the desired disk drive and controller are available
the service can be serviced immediately [T/F]
4. The disk scheduling algorithms are based on criteria Minimize seek time
TRUE
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Quiz
1. The FCFS algorithm does not provide best (average) access [T/F]
2. The head movement in the FCFS algorithm is minimal in all cases [T/F]
TRUE
FALSE
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