FCFS Scheduling

• The simplest form of disk scheduling is the first-come, first-served (FCFS) algorithm. This algorithm is intrinsically fair, but it generally does not provide the fastest service.
• Consider, for example, a disk queue with requests for I/O to blocks on cylinders in that order;

  98,183,37,122,14,124,65,67,
  

  • If the disk head is initially at cylinder 53,
  • It will first move from 53 to 98,
  • then to 183, 37, 122, 14, 124, 65, and finally to 67,
  • for a total head movement of 640 cylinders.
  • This schedule is diagrammed in Fig. 12.8.

  Figure 12.8: FCFS disk scheduling.

  
• The wild swing from 122 to 14 and then back to 124 illustrates the problem with this schedule.
• If the requests for cylinders 37 and 14 could be serviced together, before or after the requests at 122 and 124, the total head movement could be decreased substantially, and performance could be thereby improved.