OS Process Scheduling

Process scheduling is an essential aspect of operating systems, and it involves the efficient allocation of computing resources to various processes. The operating system's scheduler is responsible for determining which processes should run, for how long they should run, and when they should be interrupted to allow other processes to execute. In this article, we will discuss the process scheduling algorithms used in modern operating systems and how they work.

Introduction to Process Scheduling

An operating system is responsible for managing system resources such as CPU, memory, and input/output devices to provide a platform for executing user applications. When multiple processes are running simultaneously on the same CPU, the operating system must allocate CPU time to each process in a fair and efficient manner. This is where the process scheduling comes in. Process scheduling is the mechanism used by operating systems to allocate CPU time to processes.

The scheduler's primary goal is to maximize system throughput, minimize the response time for individual processes, and ensure fair distribution of CPU resources. In order to achieve this, the scheduler must make a series of decisions based on the current state of the system and the characteristics of the processes it is managing.

Process States

Before we discuss the process scheduling algorithms, let's briefly talk about the different states a process can be in. A process can be in one of the following states:

1. Running: The process is currently executing on a CPU.

2. Ready: The process is waiting to be assigned a CPU for execution.

3. Blocked: The process is waiting for an event such as input/output to complete before it can continue execution.

4. Suspended: The process is temporarily removed from memory.

5. Terminated: The process has completed execution.

Process scheduling algorithms

Now, let's take a closer look at the different process scheduling algorithms used in modern operating systems.

1. First-Come-First-Serve (FCFS)

FCFS is the simplest scheduling algorithm, and it works by assigning CPU time to processes in the order they arrive. The first process that arrives is given CPU time until it completes, and then the second process is given CPU time, and so on. The problem with this algorithm is that it does not take into account the length of time each process will take to complete, and as a result, long-running processes can cause other processes to wait for an extended period of time.

2. Shortest-Job-First (SJF)

The SJF algorithm schedules processes based on their expected CPU burst time. It works by selecting the process with the shortest expected CPU burst time and assigns it to the CPU. If two processes have the same expected CPU burst time, the one that arrived first is given priority. The SJF algorithm is more efficient than the FCFS algorithm, as it minimizes the average waiting time for all processes. However, the algorithm requires knowledge of the expected CPU burst time for each process, which is not always available.

3. Priority Scheduling

Priority scheduling assigns priority levels to processes, and processes with higher priority levels are given CPU time before processes with lower priority levels. The priority level can be static or dynamic. Static priority is assigned by the system administrator or the user, while dynamic priority is adjusted based on the process's behavior. The main drawback of this algorithm is that high-priority processes can starve low-priority processes.

4. Round-Robin Scheduling (RR)

RR is a preemptive scheduling algorithm that works by assigning a fixed time slice (quantum) to each process in a round-robin fashion. If a process does not complete execution within the time slice, it is preempted and added to the end of the ready queue. The next process in the queue is then assigned the CPU time slice. The main advantage of this algorithm is that it ensures fair distribution of CPU time among processes. However, if the time slice is too short, the overhead of context switching can become significant, and if it is too long, the algorithm can resemble FCFS.

5. Priority Round-Robin Scheduling (PRR)

PRR combines the priority scheduling and round-robin scheduling algorithms. It assigns priorities to processes, and processes with higher priorities are given CPU time before processes with lower priorities. Within each priority level, the RR algorithm is used to allocate CPU time to processes. This algorithm ensures that high-priority processes are given CPU time before low-priority processes, while still maintaining fair distribution of CPU time among processes at the same priority level.

6. Multilevel Feedback Queue (MLFQ)

The MLFQ algorithm uses multiple queues with different priorities to schedule processes. Processes are initially placed in the highest priority queue, and if they use all their CPU time slice, they are demoted to a lower priority queue. If a process waits too long in a lower priority queue, it is promoted to a higher priority queue. This algorithm provides a balance between SJF and RR by giving priority to short jobs and allowing long jobs to run in lower priority queues. The main disadvantage of this algorithm is that it can be complex to implement and tune.

7. Guaranteed Scheduling

Guaranteed scheduling is a real-time scheduling algorithm that ensures that a process is given a guaranteed amount of CPU time within a specific period. This algorithm is used in real-time systems where timely execution of a process is critical, such as in aerospace or medical applications. The algorithm ensures that a process is given a certain percentage of CPU time within a given time period, and if it is unable to complete within that time, it is preempted.

Conclusion

In conclusion, process scheduling is a crucial component of modern operating systems. The scheduler's primary goal is to allocate CPU time to processes in a fair and efficient manner while maximizing system throughput and minimizing the response time for individual processes. The different scheduling algorithms have their advantages and disadvantages, and the choice of algorithm depends on the system's specific requirements. Understanding the process scheduling algorithms is essential for system administrators and developers to design and optimize systems that provide efficient resource allocation and improved performance.