ECS 150, Operating Systems Fall Quarter 2008
Disk-Head Scheduling Algorithms
These charts give an example of the various disk-head scheduling algorithms and how they affect servicing of
requests. The disk has 200 tracks, with track 200 on the outside of the disk. The set of requests is
98 183 37 122 14 124 65 67
and the disk head starts at cylinder 53. Where direction is important (LOOK and SCAN), the disk head is moving
outward. Order of Service
This chart shows the order in which the disk requests are serviced.
algorithm request order
fcfs 98 183 37 122 14 124 65 67
pickup 65 67 98 122 124 183 37 14
sstf 65 67 37 14 98 122 124 183
scan 65 67 98 122 124 183 37 14
look 65 67 98 122 124 183 37 14
c-scan 65 67 98 122 124 183 14 37
c-look 65 67 98 122 124 183 14 37
Head Motion
This chart shows how far the disk heads move to service each request, and the mean and standard deviation of the
head motion.
algorithm number of cylinders moved total mean stdev
fcfs 45 85 146 85 108 110 59 2 640 80.00 44.47
pickup 12 2 31 24 2 59 146 23 299 37.38 47.57
sstf 12 2 30 23 84 24 2 59 236 29.50 28.62
scan 12 2 31 24 2 59 180 23 333 41.63 58.84
look 12 2 31 24 2 59 146 23 299 37.38 47.57
c-scan 12 2 31 24 2 59 231 23 384 48.00 76.18
c-look 12 2 31 24 2 59 169 23 322 40.25 55.16
Waiting Motion
This chart shows the (cumulative) number of cylinders the disk heads must move before servicing each request,
and the mean and standard deviation.
algorithm cumulative number of cylinders moved total mean stdev
fcfs 45 130 276 361 469 579 638 640 3138 392.25 228.78
pickup 12 14 45 69 71 130 276 299 916 114.50 113.24
sstf 12 14 44 67 151 175 177 236 876 109.50 85.60
scan 12 14 45 69 71 130 310 333 984 123.00 128.19
look 12 14 45 69 71 130 276 299 916 114.50 113.24
c-scan 12 14 45 69 71 130 361 384 1086 135.75 150.91
c-look 12 14 45 69 71 130 299 322 962 120.25 123.33
Version of November 3, 2008 at 2:43pm Page 1 of 1
