CPU Scheduling in Operating System

 

CPU Scheduling is the process of determining which task should be executed by the CPU and when. In a computer system, there can be multiple processes that are ready to run, but only one can run at a time. The CPU scheduling algorithms help to decide which process should be executed by the CPU and when, thus making efficient use of the CPU time.

The importance of CPU scheduling can be understood from the following points:

1. Improved system performance: CPU scheduling algorithms help to improve the performance of the system by making efficient use of the CPU time. By executing the most important process first, the system can provide a better response time and overall performance.

2. Fair distribution of resources: In a multi-user system, CPU scheduling algorithms ensure that each user gets a fair share of the CPU time. This helps to prevent a single user from monopolizing the CPU, thus improving the overall performance of the system.

3. Better utilization of resources: CPU scheduling algorithms help to better utilize the resources of the system. By allocating the CPU time to the most important processes, the system can ensure that the CPU is not idle, leading to better utilization of resources.

4. Increased number of processes: In a system with CPU scheduling, multiple processes can run simultaneously, leading to an increase in the number of processes that can run. This helps to improve the overall performance of the system.

Based on the scheduling algorithms, CPU scheduling can be classified into five types:

1. First-Come-First-Served (FCFS) Scheduling: In FCFS scheduling, the process that arrives first is executed first. This scheduling algorithm is simple, but it may lead to longer wait times for processes, as the CPU has to wait for the previous process to complete before executing the next process.

2. Shortest-Job-First (SJF) Scheduling: In SJF scheduling, the process with the shortest execution time is executed first. This scheduling algorithm leads to shorter wait times for processes and is efficient in terms of CPU utilization.

3. Priority Scheduling: In priority scheduling, processes are assigned a priority, and the process with the highest priority is executed first. If two processes have the same priority, then the process that arrives first is executed first. This scheduling algorithm ensures that the most important processes are executed first, leading to better system performance.

4. Round Robin Scheduling: In Round Robin scheduling, each process is given a small time slice, called a quantum. The CPU is assigned to each process for a specified quantum, and if the process is not completed within that time, the CPU is assigned to the next process. This scheduling algorithm ensures that each process gets a fair share of the CPU time, leading to better system performance.

5. Multi-Level Queue Scheduling: In Multi-Level Queue scheduling, processes are separated into different queues based on their priority. Each queue has a different scheduling algorithm, and the CPU is assigned to the process in the highest priority queue. This scheduling algorithm helps to ensure that the most important processes are executed first, leading to better system performance.

Examples:

1. First-Come-First-Served (FCFS) Scheduling: Consider a scenario where three processes, P1, P2, and P3, arrive in the order P1, P2, P3. In this scenario, the CPU scheduling would be as follows:

• P1 is executed first
• P2 is executed next
• P3 is executed last

2. Shortest-Job-First (SJF) Scheduling: Consider a scenario where three processes, P1, P2, and P3, arrive with execution times of 5 units, 3 units, and 4 units respectively. In this scenario, the CPU scheduling would be as follows:
   • P2 is executed first since it has the shortest execution time
   • P3 is executed next since it has the second shortest execution time
   • P1 is executed last since it has the longest execution time
   3. Priority Scheduling: Consider a scenario where three processes, P1, P2, and P3, arrive with priorities 1, 2, and 3 respectively. In this scenario, the CPU scheduling would be as follows:
   • P1 is executed first since it has the highest priority
   • P2 is executed next since it has the second highest priority
   • P3 is executed last since it has the lowest priority
   4. Round Robin Scheduling: Consider a scenario where three processes, P1, P2, and P3, arrive in the order P1, P2, P3 and each process is given a quantum of 2 units. In this scenario, the CPU scheduling would be as follows:
   • P1 is executed first for 2 units
   • P2 is executed next for 2 units
   • P3 is executed next for 2 units
   • P1 is executed next for the remaining 1 unit
   5. Multi-Level Queue Scheduling: Consider a scenario where three processes, P1, P2, and P3, arrive with priorities 1, 2, and 3 respectively and the system uses the Round Robin scheduling algorithm for the high priority queue and the FCFS scheduling algorithm for the low priority queue. In this scenario, the CPU scheduling would be as follows:
   • P1 is executed first since it has the highest priority
   • P2 is executed next since it has the second highest priority
   • P3 is executed last since it has the lowest priority

   In conclusion, CPU scheduling is an important aspect of computer systems that helps to improve system performance, ensure fair distribution of resources, and better utilize the available resources. Different CPU scheduling algorithms have been designed to meet specific requirements, and the choice of the scheduling algorithm depends on the requirements of the system. Whether it's ensuring that the most important processes are executed first or ensuring that each process gets a fair share of the CPU time, CPU scheduling algorithms play a critical role in the functioning of modern computer systems.