CS 3733 Operating Systems

Scheduling overview:

• States of a process:
  • new
  • running - using CPU
  • ready - in memory, ready for the CPU
  • waiting - waiting for an I/O device or interrupt
  • done
• State diagrams:
  • A circle represents each state.
  • An arc from state A to state B represents a transition caused by an event
  • Each arc should be labeled.
• Each state is represented by a queue or equivalent data structure
  • The job queue --- All processes in mass storage waiting for allocation to main memory.
  • The ready queue --- All processes in main memory waiting for the CPU. Usually a linked list of PCB's (process control blocks) with pointers to the first and last PCBs.
  • Several device queues --- One for each device containing all processes waiting for a particular device. These are mainly disk drives

• Types of Schedulers
  • Long-term scheduler or job scheduler decides which processes are admitted to the system.
  • Medium-term scheduler does swapping --- moving processes in and out of memory.
  • Short-term scheduler or CPU scheduler decides which process is selected from the ready queue(s).

Short-term scheduling terminology

• A process consists of a cycle of CPU bursts and I/O bursts.
• Short-term scheduler must make a decision when a process:
  • switches from running to waiting --- say for I/O
  • switches from running to ready --- because of an interrupt
  • switches from waiting to ready --- I/O has completed
• Non-preemptive scheduling: Once a process has the CPU, it keeps it until it needs I/O or is finished.
• Preemptive scheduling: a higher priority process may be selected when the OS gets in because of an interrupt or other event.
• Context switch --- Switching the CPU to another process requires saving the state of the old process and loading the saved state of the new process.
• Dispatcher --- The module that gives control of the CPU to the process selected by the CPU scheduler. It does:
  • context switching
  • switching to user mode
  • setting to program counter

• Performance criteria for CPU schedulers
  • Fairness
  • Efficiency = CPU Utilization
  • Throughput = number of processes completed per unit time.
  • Turnaround Time = Time to execute a process from submission to termination.
  • Waiting Time
  • Response Time = Time between submission of a request to the first response.
  • Response Time Variance