Disk-Head Scheduling Algorithms

Introduction

These charts give an example of the various disk-head scheduling algorithms and how they affect servicing of requests. 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

algorithmrequest order
fcfs9818337122141246567
pickup6567981221241833714
sstf6567371498122124183
scan3714656798122124183
look3714656798122124183
c-scan6567981221241831437
c-look6567981221241831437

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.

algorithmtotal number of cylinders movedtotalavgstdev
fcfs45851468510811059264080.0044.47
pickup12231242591462329937.3847.57
sstf1223023842425923629.5028.62
scan1623792312425923629.5026.97
look1623512312425920826.0020.72
c-scan12231242592312338448.0076.18
c-look12231242591692332240.2555.16

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.

algorithmcumulative number of cylinders movedtotalavgstdev
fcfs451302763614695796386403138392.25228.78
pickup1214456971130276299916114.5113.24
sstf12144467151175177236876109.50 85.60
scan16391181201511751772361032129.0073.12
look16399092123147149208864108.0062.37
c-scan12144569711303613841086135.75150.91
c-look1214456971130299322962120.25123.33

You can also obtain a PDF version of this. Version of May 14, 2008 at 8:35 AM