Process Scheduling and Dispatching

In the realm of operating systems, process scheduling and dispatching play a crucial role in efficiently utilizing computer resources. Process scheduling refers to the method by which an operating system selects processes from a ready queue and allocates the CPU to a selected process. On the other hand, dispatching involves transferring control of the CPU from the current executing process to the newly selected process. Let's delve deeper into these concepts and understand their significance in achieving efficient task management.

The Need for Process Scheduling

In a multitasking system, where multiple processes compete for the CPU, it becomes necessary to implement process scheduling. Without it, only one process would execute at a time, leading to poor resource utilization and slower execution times for other processes waiting in the system. Process scheduling allows for concurrent execution, making the system more responsive and maximizing the utilization of available resources.

Scheduling Criteria

Various scheduling algorithms exist, each with its own set of criteria. These criteria are used by the operating system to evaluate and compare processes, aiding in the selection of the most suitable process for execution. Some common scheduling criteria include:

  1. CPU Burst Time: The time required by a process to execute on the CPU. Processes with shorter burst times are often prioritized to ensure quicker execution and faster response times.

  2. Priority: Each process is assigned a priority value, usually based on factors such as importance, urgency, or system requirements. The scheduler may favor processes with higher priority to meet specific system goals.

  3. I/O Operations: Processes that currently require I/O operations may be temporarily suspended, allowing CPU time to be allocated to other waiting processes. This ensures that CPU cycles are not wasted while waiting for I/O operations to complete.

  4. Deadlines: Real-time systems often have strict deadlines to meet. Scheduling algorithms for such systems prioritize processes to ensure that critical tasks are completed within their allotted time frames.

Scheduling Algorithms

Now, let's explore some commonly used scheduling algorithms:

  1. First-Come, First-Served (FCFS): In this simple algorithm, the CPU is assigned to the process that arrives first. It follows a non-preemptive approach, meaning the current process retains control until completion, even if a higher-priority process becomes available. FCFS can result in poor average waiting times, especially if long processes arrive before short ones.

  2. Shortest Job Next (SJN): Also known as Shortest Job First (SJF), this algorithm selects the process with the smallest burst time as the next in line. SJN minimizes the average waiting time and provides optimal performance. Nonetheless, it requires prior knowledge of the expected burst times, which is often not feasible or practical, thus limiting its usage.

  3. Round Robin (RR): RR is a preemptive algorithm that assigns a fixed time slice or quantum to each process in a cyclic manner. When a process's time quantum expires, it is placed at the back of the ready queue, and the CPU is given to the next process in line. RR ensures fair CPU allocation and prevents starvation but may suffer from high context-switching overhead.

  4. Priority Scheduling: In this algorithm, each process has an assigned priority, and the CPU is allocated to the highest-priority process. It can be preemptive or non-preemptive, depending on the system's requirements. However, it may lead to starvation of low-priority processes if not carefully implemented.

The Role of Dispatching

Once a process is selected for execution, the operating system initiates the dispatching phase. Dispatching involves several tasks, such as saving the context of the current process (registers, program counter, etc.) and loading the saved context of the selected process. This context switch allows the new process to resume its execution seamlessly from where it left off. Dispatching also involves updating process control blocks, allocating resources, and ensuring appropriate synchronization between processes.


Process scheduling and dispatching are fundamental components of modern operating systems, enabling efficient utilization of CPU time and system resources. By employing various scheduling algorithms and criteria, operating systems can prioritize tasks, meet deadlines, and provide both responsiveness and fairness to processes. Achieving an appropriate balance between process prioritization and system efficiency is a continuous challenge for designers of operating systems. Nonetheless, these concepts remain integral to improving overall system performance and user experience.

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