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Find the number of head movements in cylinders using SSTF scheduling.

Consider the following disk request sequence for a disk with 100 tracks.

44, 20, 95, 4, 50, 47, 61, 87, 25

Head pointer starting at 50 (current position of R/W heads).Find the number of head movements in cylinders using SSTF scheduling.

 

Answer

 Firstly we will see the current position of the R/W head which is at 50.

Next step we will look for the closest track in the service queue.

Here it is 50.

so the head moves from 50 ---> 50 

then the next closest track

so the head moves from 50 ---> 52 

& in the same way 52-->47,    47-->44,   44-->61,   61-->87,   87-->95,   95-->25,   25-->20 & at last 20-->4.

Now we will calculate the total track movement which can be-

 =(50-50) + (52-50) + (52-47) + (47-44) + (61-44) + (87-61) + (95-87) + (95-25) + (25-20) + (20-4)

= 152.

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