Briefly define FCFS scheduling

Answer 1

FCFS is "First come, first served" Scheduling: Processes are given time on the CPU in the order that they arrive.

eg:

Process | Arrival Time (ns) | Burst Time (ns)

P1 0 20

P2 0 10

P3 0 5

Scheduling Diagram for FCFS:

| P1 | P2 | P3 |

0ns 20ns 30ns 35ns

Answer 2

First-Come, First-Serve (FCFS) scheduling is one of the simplest scheduling algorithms in the realm of operating systems, and its implementation offers a straightforward approach to process management. In FCFS scheduling, processes are executed in the order they arrive in the ready queue, essentially adhering to a first-in, first-out (FIFO) strategy. The fundamental principle behind FCFS is straightforward: the first process to request the CPU gets executed first, and subsequent processes have to wait until the CPU is available again.

The mechanism of FCFS scheduling is simple to understand and implement. When a process enters the system, it gets placed in the ready queue. The CPU scheduler picks the process at the head of the queue for execution and allocates the CPU to this process. The process continues to execute until it either finishes or gets blocked for I/O or some other reason, at which point the CPU is allocated to the next process in the ready queue. This cycle continues until all processes are executed.

FCFS is non-preemptive, meaning once the CPU is allocated to a process, it retains control of the CPU until it completes its execution or gets blocked for some reason. This characteristic simplifies the scheduler design, making FCFS an attractive choice for simple systems or batch processing environments where jobs are more or less independent of each other.

However, FCFS scheduling has its set of drawbacks. It can lead to the "convoy effect," where short processes have to wait for a long process to complete, leading to suboptimal CPU utilization. The average waiting time under FCFS scheduling can be high if long processes arrive at the queue early. Moreover, FCFS does not prioritize processes based on their importance or urgency, which can be a significant limitation in scenarios where certain processes require immediate attention.

Despite its limitations, the simplicity and ease of implementation of FCFS scheduling make it a viable choice in specific scenarios, especially in systems with minimal process management requirements or in batch processing environments. It serves as a foundation for understanding more complex scheduling algorithms and provides a clear illustration of how process scheduling works at a basic level.

In a nutshell, FCFS scheduling is a fundamental, easy-to-implement scheduling algorithm that executes processes based on their arrival order, adhering to a non-preemptive, first-in, first-out strategy. While it may not be the most efficient or versatile scheduling algorithm, its simplicity makes it a useful stepping stone in the study of operating system scheduling algorithms, paving the way for understanding more advanced scheduling strategies.