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CPU Scheduling

CPU Scheduling:-

CPU scheduling is a process which allows one process to use the CPU while the execution of another process is on hold. That means in waiting for the state due to unavailability of any resource like I/O etc., thereby maximizing utilization of CPU.

The job of the CPU Scheduler (the short-term scheduler) to select another process from the ready queue to run next.

  • Basic Concepts
  • Scheduling Criteria
  • Scheduling Algorithms
  • Multiple-Processor Scheduling
  • Real-Time Scheduling
  • Algorithm Evaluation

Basic Concepts


  • Maximum CPU utilization obtained with multiprogramming
  • CPU–I/O Burst Cycle – Process execution consists of a cycle of CPU execution and I/O wait.
  • CPU burst distribution

CPU Scheduler

  • Selects from among the processes in memory that are ready to execute, and allocates the CPU to one of them.
  • CPU scheduling decisions may take place when a process:
    1. Switches from running to waiting for the state.
    2. Switches from running to ready state.
    3. Switches from waiting to ready.
    4. Terminates.
  • Scheduling under 1 and 4 are not preemptive.
  • All other scheduling is


Dispatcher module gives control of the CPU to the process selected by the short-term scheduler; this involves:

✦ switching context

✦ switching to user mode

✦ jumping to the proper location in the user program to restart that program

Dispatch latency – the time it takes for the dispatcher to stop one process and start another running.

Scheduling Criteria

  • CPU utilization – keep the CPU as busy as possible
  • Throughput – # of processes that complete their execution per time unit
  • Turnaround time – the amount of time to execute a particular process
  • Waiting time – the amount of time a process has been waiting in the ready queue
  • Response time – the amount of time it takes from when a request was submitted until the first response is produced, not output (for time-sharing environment)

Optimization Criteria

  • Max CPU utilization
  • Max Throughput
  • Min turnaround time
  • Min waiting time
  • Min response time

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