Computer Science 330
Operating Systems

Fall 2025, Siena University

Home     Syllabus     Schedule

Agenda

• Announcements
  • The next readings/activities from OS zyBook
    • Chapter 19, Sections 1-3, due Monday, November 24
    • Chapter 19, Sections 4-5, Chapter 22, due Monday, December 1
  • Programming Project 5: Roger Bacon Shell with Pipes and Redirection [HTML] [PDF] parsing functionality now due, everything else in 2 weeks
• Exam 2 recap
• Hard Disk Scheduling Algorithms
  • Recall: cost of a disk access for a mechanical/magnetic storage device includes seek time and rotational latency
  • We wish to minimize seek time by minimizing the distance the read/write head has to move in order to service the incoming requests by reordering requests based on cylinder number
  • This optimization may be done by the disk, the hardware controller, or by the operating system
  • Compare algorithms by examining their performance on a given request queue
  • First-Come First-Served (FCFS)
  • Shortest Seek Time First (SSTF)/Closest Cylinder Next
  • SCAN or Elevator Algorithm
  • LOOK Algorithm
  • Circular Algorithms
  • Comparing Disk Scheduling Algorithms
    • SSTF or LOOK are often reasonable for a default algorithm
    • SCAN and C-SCAN are better for heavily loaded systems where LOOK is unlikely to save much and SSTF runs the risk of starvation
    • performance depends on the frequency and types of requests
    • we may want to consider some of this when we decide how to organize file systems
  • FreeBSD's ufs filesystem uses an elevator algorithm (see /usr/src/sys/ufs/ufs/ufs_disksubr.c)
• Error detection and correction
• Lab 8: Working Set Simulation [HTML] [PDF]