C look scheduling 51 1

1. http://raj-os.blogspot.in/ 1
C-LOOK Scheduling
•SCAN and C-SCAN algorithms move the disk arm
across the full width of the disk
•In practice neither of the algorithm is implemented
this way

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C-LOOK Scheduling
•In C-LOOK Scheduling the arm goes only as far as
final request in each direction
•Then it reverses direction immediately without going
all the way to the end of the disk

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C-LOOK Scheduling
•When the head reaches the other end,
–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

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C-LOOK Scheduling
•It is a 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

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C-LOOK Scheduling
•Consider for example an ordered disk queue with
requests involving
–Tracks 98, 183, 37, 122, 14, 124, 65, 67
–Head pointer 53
•The request queue (0-199)

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C-LOOK Scheduling
•If the disk head is initially at cylinder 53,
–it will first move from 53 to 65, (towards right)
–then to 67, 98, 122, 124, 183, 14 and 37 as shown
in the next slides

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C-LOOK Scheduling
Initial Head Position

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C-LOOK Scheduling
Head movement from 53
rd
to 65
th
track

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C-LOOK Scheduling
Head movement from 65
th
to 67
th
track

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C-LOOK Scheduling
Head movement from 98
th
track to 122

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C-LOOK Scheduling
Head movement from 98
th
track top 122

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C-LOOK Scheduling
Head movement from 122 to 124

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C-LOOK Scheduling
Head movement from 124 to 183

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C-LOOK Scheduling
Head movement 183 to 14

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C-LOOK Scheduling
Head from 14 to 37
th
track

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C-LOOK Scheduling
•Advantages
–Better than the C-SCAN algorithm in terms of head
movement