CPU scheduling in operating system
In operating systems, CPU scheduling is the process of deciding which process gets to use the CPU at a given time. It is a crucial component of any operating system because it ensures that the CPU is used efficiently and that all processes have a fair share of the CPU's resources. In this article, we will discuss the various CPU scheduling algorithms used in operating systems.
Before we discuss CPU scheduling, it is essential to understand the concept of a process. A process is a program that is currently running on the computer. A process can be either in a running state or a waiting state. When a process is in a running state, it is using the CPU. When a process is in a waiting state, it is waiting for some input/output operation to complete.
The goal of process scheduling is to ensure that all processes have a fair share of the CPU's resources. If the CPU is not shared fairly, some processes may never get a chance to run, while others may hog the CPU's resources, causing the system to slow down.
CPU Scheduling Algorithms:
There are several CPU scheduling algorithms used in operating systems. Each algorithm has its advantages and disadvantages, and the choice of algorithm depends on the specific requirements of the system. In the following sections, we will discuss some of the most commonly used CPU scheduling algorithms.
First-Come, First-Served (FCFS) Scheduling:
The first-come, first-served (FCFS) scheduling algorithm is the simplest CPU scheduling algorithm. In this algorithm, the process that arrives first is allocated the CPU first. The CPU is not released until the process completes its execution. This algorithm is easy to implement, but it is not very efficient.
One of the main drawbacks of the FCFS algorithm is that it does not take into account the length of the processes. If a long process arrives first, it will take up the CPU for a long time, causing other processes to wait. This can lead to a phenomenon known as the "convoy effect," where a long process is followed by a sequence of short processes, but they still have to wait for the long process to complete.
Shortest Job First (SJF) Scheduling:
The shortest job first (SJF) scheduling algorithm is a non-preemptive algorithm that selects the process with the shortest burst time. The burst time is the amount of time the process requires to complete its execution. This algorithm minimizes the average waiting time for all processes and is considered optimal because it reduces the waiting time for short processes.
One of the main drawbacks of the SJF algorithm is that it requires knowledge of the burst time of each process, which is often not available. Moreover, even if the burst times are known, it is not easy to predict how long a process will take to complete its execution, as the execution time may be affected by factors such as I/O operations and other system delays.
Shortest Remaining Time First (SRTF) Scheduling:
The shortest remaining time first (SRTF) scheduling algorithm is similar to the SJF algorithm but is preemptive. In this algorithm, the process with the shortest remaining burst time is allocated the CPU. If a new process with a shorter burst time arrives, the currently running process is preempted, and the new process is allocated the CPU.
The SRTF algorithm is more efficient than the SJF algorithm because it takes into account the remaining burst time of the processes. This algorithm minimizes the waiting time for short processes and ensures that long processes do not monopolize the CPU. However, the SRTF algorithm is more complex than the SJF algorithm because it requires the scheduler to keep track of the remaining burst time of all processes and to decide when to preempt a running process.
Round Robin (RR) Scheduling:
The round robin (RR) scheduling algorithm is a preemptive algorithm that allocates the CPU to each process for a fixed time slice. The time slice is typically between 10 and 100 milliseconds. When a process completes its time slice, it is preempted, and the next process in the queue is allocated the CPU. If a process completes its execution before the time slice expires, it is removed from the queue.
The RR algorithm is fair because it ensures that all processes get a fair share of the CPU. It also ensures that no process monopolizes the CPU for a long time. However, the RR algorithm may not be optimal because it can lead to high context switching overheads. Context switching is the process of saving the current state of a process and restoring the state of another process. Context switching can be time-consuming, especially when there are many processes in the system.
The priority scheduling algorithm assigns a priority value to each process. The process with the highest priority is allocated the CPU first. If two processes have the same priority, the scheduling algorithm can use another algorithm, such as FCFS or RR, to select the process to run. The priority value can be assigned by the user or the operating system.
The priority scheduling algorithm is useful in real-time systems, where some processes have more critical deadlines than others. However, priority scheduling can lead to starvation, where low priority processes may never get a chance to run if high priority processes keep arriving.
Multi-Level Queue Scheduling:
The multi-level queue scheduling algorithm divides the processes into different queues based on their characteristics, such as their priority, CPU burst time, or I/O requirements. Each queue can have its scheduling algorithm. For example, the high-priority queue can use the SJF algorithm, while the low-priority queue can use the RR algorithm. The processes move between the queues based on their behavior. For example, a process may start in the high-priority queue but move to the low-priority queue if it performs too many I/O operations.
The multi-level queue scheduling algorithm is useful in complex systems where different processes have different requirements. However, the multi-level queue scheduling algorithm can be complex to implement and manage.
In conclusion, CPU scheduling is a crucial component of any operating system. The choice of CPU scheduling algorithm depends on the specific requirements of the system, such as the nature of the processes, the performance goals, and the hardware capabilities. Each CPU scheduling algorithm has its advantages and disadvantages, and the operating system designer must choose the algorithm that best meets the system's requirements.
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