Scheduler

Process Scheduler

• First Come First Serve (FCFS)
  • automatically executes queued requests and processes in order of their arrival
• Shortest Job First (SJF)
• Shortest Time-to-Completion First
• Multi-Level Feedback Queue
  • Idea
    • If a process uses too much CPU time, it will be moved to a lower-priority queue.
    • If a process is O-bound or an interactive process, it will be moved to a higher-priority queue.
    • If a process is waiting too long in a low-priority queue and starving, it will be aged to a higher-priority queue.
  • Rules
    • Rule 1: If Priority(A) > Priority(B), A runs (B doesn’t).
    • Rule 2: If Priority(A) = Priority(B), A & B run in round-robin fashion using the time slice (quantum length) of the given queue.
    • Rule 3: When a job enters the system, it is placed at the highest priority (the topmost queue).
    • Rule 4: Once a job uses up its time allotment at a given level (regardless of how many times it has given up the CPU), its priority is reduced (i.e., it moves down one queue).
    • Rule 5: After some time period S, move all the jobs in the system to the topmost queue.
• Proportional-share Scheduling
• Completely Fair Scheduler (CFS)
  • CFS basically models an “ideal, precise multi-tasking CPU” on real hardware.

Disk Scheduler

• RAID