Scheduling Algorithms - First Come First Served (FCFS)

Summary

This lecture introduces the first-come, first-served (FCFS) CPU scheduling algorithm, the simplest of its kind. FCFS operates on a queue basis, where processes are executed in the order they arrive, managed by a FIFO (first-in, first-out) queue. The algorithm is non-preemptive, meaning once a process starts, it holds the CPU until completion or I/O request. While easy to understand and implement, FCFS can lead to long average waiting times, especially with varying process burst times. The lecture also highlights its inefficiency in time-sharing systems, where equal CPU access is crucial, and concludes by comparing the waiting times of processes in different arrival orders.

Takeaways

• 📝 The script discusses the first come, first served (FCFS) CPU scheduling algorithm.
• 🧠 The FCFS algorithm is the simplest CPU scheduling method, where processes are allocated CPU time in the order they arrive.
• 📉 FCFS is implemented using a FIFO (First-In, First-Out) queue, ensuring a straightforward management system.
• 👥 The algorithm is non-preemptive, meaning once a process starts executing, it holds the CPU until it completes or waits for I/O.
• ⏱ The average waiting time for processes can vary greatly under FCFS, depending on the order of arrival and burst times.
• 📉 An example given in the script illustrates that changing the order of process arrival can significantly affect the average waiting time.
• 🚫 FCFS is not suitable for time-sharing systems where regular CPU access is required for each user or process.
• 🔍 The script emphasizes that the FCFS algorithm can lead to inefficiency, especially when large processes delay smaller ones in the queue.
• 🤔 The disadvantages of FCFS include potential for process starvation, where smaller processes are kept waiting by longer ones.
• 🔑 The script serves as an introduction to CPU scheduling algorithms, with more advanced algorithms to be discussed in subsequent lectures.
• 👋 The presenter concludes by hoping the explanation of FCFS was clear and invites viewers to the next lecture for further exploration of CPU scheduling.

Q & A

What is the First Come First Serve (FCFS) scheduling algorithm?

The FCFS scheduling algorithm is the simplest CPU scheduling algorithm where processes are allocated the CPU in the order they arrive, ensuring that the first process to request the CPU is the first to be served.

How does the FCFS algorithm relate to the concept of a queue?

The FCFS algorithm is implemented using a FIFO (First-In First-Out) queue, where the process that arrives first is the first to be executed, similar to how people wait in a queue for service.

What is the main disadvantage of the FCFS scheduling algorithm in terms of waiting time?

The main disadvantage of the FCFS algorithm is that it can result in a long average waiting time for processes, especially if there are processes with significantly longer burst times.

How does the order of process arrival affect the average waiting time in the FCFS algorithm?

The order of process arrival greatly affects the average waiting time in the FCFS algorithm. If shorter processes arrive first, the average waiting time is reduced, whereas if longer processes arrive first, it can significantly increase the waiting time for subsequent processes.

What is a Gantt chart in the context of the FCFS scheduling algorithm?

A Gantt chart in the context of the FCFS scheduling algorithm is a graphical representation that shows the time each process spends waiting and the time it spends executing on the CPU.

Is the FCFS scheduling algorithm pre-emptive or non-pre-emptive?

The FCFS scheduling algorithm is non-pre-emptive, meaning once a process has started executing, it cannot be interrupted by another process until it completes its execution or waits for I/O.

Why is the FCFS algorithm not suitable for time-sharing systems?

The FCFS algorithm is not suitable for time-sharing systems because it does not guarantee that each user or process gets a share of the CPU at regular intervals, potentially leading to some processes waiting indefinitely if a long-running process is ahead in the queue.

What is the significance of the burst time in the context of the FCFS scheduling algorithm?

In the context of the FCFS scheduling algorithm, the burst time is the duration a process needs to hold the CPU to complete its execution. It significantly impacts the waiting time of other processes in the queue.

How is the process control block (PCB) used in the FCFS scheduling algorithm?

In the FCFS scheduling algorithm, when a process enters the ready queue, its PCB is linked to the tail of the queue. When the CPU is free, it is allocated to the process at the head of the queue, and the running process's PCB is then removed from the queue.

What is the impact of having a large process with a long burst time in the FCFS queue?

Having a large process with a long burst time in the FCFS queue can cause a bottleneck, as it delays the execution of all other processes waiting in the queue, potentially causing them to wait for an extended period.