On a particular brand of disk drive, the time that it takes for the disk arm to

Question

On a particular brand of disk drive, the time that it takes for the disk arm to pass over a single disk track without stopping is 500 nanoseconds. However, once the head reaches the track for which it has a service request, it needs 2 milliseconds to "settle" over the required track before it can start reading or writing. Based on these timings, compare the relative times required for FCFS, SSTF, and LOOK to carry out the schedule given below.

You will need to compare SSTF to FCFS, LOOK to FCFS, and LOOK to SSTF.
the time the first request arrives in the disk request queue, the read/write head is at track 50, moving toward the outer (lower-numbered) tracks. The requested tracks are:

35, 53, 90, 67, 79, 37, 76, 47

Explanation / Answer

1)SSTF it means shortest seek time first it will move to the nearest one according to its direction so time for sstf =((50-47)+(47-37)+(37-33)+(53-33)(because*it won't be negative*)+(67-53)+(76-67)+(79-76)+(90-79))*500nano seconds + 2ms =(74*0.0005+2)ms =0.037+2 =2.037ms 2)fcfs it means first come first serve so time for fcfs=((50-35)+(53-35)(because*it won't be negative*)+(90-53)+(90-67)+(79-67)+(79-37)+(76-37)+(76-47))*500ns + 2 ms =0.1055+2 =2.1055ms 3)look like SCAN but stops moving inwards (or outwards) when no more requests in that direction exist so time for look=((50-35)+(37-35)+(47-37)+(53-47)+(67-53)+(76-67)+(79-76)+(90-79))*0.0005ms+ 2ms =70*0.0005ms +2 ms =0.035+2ms =2.035ms a)sstf vs fcfs sstf is better because time is less to access memory b)look vs fcfs look is better because time is less to access memory a)look vs sstf look is better because time is less to access memory

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