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CPU Scheduling

CPU Scheduling Algorithms in Operating System (OS)

CPU scheduling algorithms are a fundamental concept in the operating system. They determine how the CPU is allocated among multiple processes to ensure efficient system performance. A good scheduling algorithm improves CPU utilization, reduces waiting time, and ensures fairness among processes.

What is CPU scheduling?

CPU scheduling is the process by which the operating system decides which process in the ready queue will be executed next by the CPU. Since multiple processes may be in memory at the same time, scheduling is essential for multitasking systems.

Objectives of CPU Scheduling

  • Maximize CPU utilization
  • Increase system throughput
  • Minimize waiting time
  • Minimize turnaround time
  • Improve response time
  • Ensure fairness

Types of Scheduling

1. Preemptive Scheduling

In preemptive scheduling, the operating system can interrupt a running process and assign the CPU to another process.

  • Better response time
  • More context switching

2. Non-Preemptive Scheduling

In non-preemptive scheduling, once a process starts execution, it runs until completion or enters a waiting state.

  • Simple implementation
  • Poor response time

CPU Scheduling Algorithms

1. First Come First Serve (FCFS)

FCFS is the simplest CPU scheduling algorithm, where processes are executed in the order of their arrival.

Characteristics:
  • Non-preemptive
  • Simple and easy to implement
  • No starvation
Disadvantages:
  • Convoy effect
  • Poor average waiting time

2. Shortest Job First (SJF)

SJF selects the process with the smallest CPU burst time for execution.

Types:
  • Non-preemptive SJF
  • Preemptive SJF (Shortest Remaining Time First)
Advantages:
  • Minimum average waiting time
Disadvantages:
  • Burst time prediction required
  • Starvation of long processes

3. Shortest Remaining Time First (SRTF)

SRTF is the preemptive version of SJF. The CPU is allocated to the process with the smallest remaining execution time.

  • Excellent response time
  • High context switching overhead

4. Priority Scheduling

In priority scheduling, each process is assigned a priority. The process with the highest priority is executed first.

Types:
  • Preemptive Priority Scheduling
  • Non-preemptive Priority Scheduling
Problems:
  • Starvation
  • Priority inversion

Solution: Aging technique

5. Round Robin (RR)

Round robin scheduling assigns a fixed time quantum to each process and executes them in a cyclic order.

  • Preemptive
  • Fair scheduling
  • Used in time-sharing systems
Disadvantage:
  • Performance depends on time quantum size

6. Multilevel Queue Scheduling

Processes are divided into multiple queues based on their type, such as system processes, interactive processes, and batch processes.

  • Each queue has its own scheduling algorithm
  • Rigid structure
  • Starvation may occur

7. Multilevel Feedback Queue (MLFQ)

MLFQ allows processes to move between queues based on their behavior and CPU usage.

  • Dynamic priority
  • Prevents starvation
  • Used in modern operating systems

8. Real-Time Scheduling Algorithms

Rate Monotonic Scheduling (RMS)

A fixed-priority scheduling algorithm where tasks with shorter periods are given higher priority.

Earliest Deadline First (EDF)

A dynamic scheduling algorithm where the task with the earliest deadline is executed first.

Comparison of CPU Scheduling Algorithms

Algorithm Preemptive Starvation Response Time
FCFS No No Poor
SJF Yes / No Yes Good
Priority Yes / No Yes Good
Round Robin Yes No Very Good
MLFQ Yes No Excellent

Conclusion

CPU scheduling algorithms play a crucial role in determining system efficiency and performance. Different algorithms are suitable for batch systems, time-sharing systems, and real-time systems. Understanding these algorithms is essential for students and professionals studying operating systems.

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