Question
Compare and contrast two common CPU scheduling algorithms: First-Come, First-
Served (FCFS) and Shortest-Job-First (SJF). For each algorithm, explain its basic
principle, provide one advantage, and one disadvantage
Served (FCFS) and Shortest-Job-First (SJF). For each algorithm, explain its basic
principle, provide one advantage, and one disadvantage
Answers
Answered by
GPT-4o mini
Certainly! Let's examine the two common CPU scheduling algorithms: First-Come, First-Served (FCFS) and Shortest-Job-First (SJF).
### First-Come, First-Served (FCFS)
**Basic Principle:**
FCFS is the simplest CPU scheduling algorithm. In this algorithm, processes are attended to in the order they arrive in the ready queue. The first process that arrives is the first to be executed, followed by the next, and so on. It's a non-preemptive scheduling algorithm, meaning once a process starts executing, it runs to completion.
**Advantage:**
- **Simplicity:** FCFS is straightforward and easy to implement. The logic is easy to understand, and it doesn't require complicated data structures or bookkeeping.
**Disadvantage:**
- **Convoy Effect:** FCFS can lead to the "convoy effect," where shorter processes wait for longer processes to complete. This can lead to inefficiencies as the average wait time can be quite high, especially if a long process is followed by several shorter ones.
---
### Shortest-Job-First (SJF)
**Basic Principle:**
SJF is a scheduling algorithm that prioritizes processes according to their burst time (the time they require to complete). The process with the shortest burst time is executed first. Like FCFS, SJF can be implemented in both preemptive and non-preemptive forms; in the preemptive version, if a new process arrives with a shorter burst time than the currently running process, it can preempt the current process.
**Advantage:**
- **Minimizes Average Waiting Time:** SJF minimizes the average waiting time for processes, making it more efficient than FCFS. By executing shorter jobs first, it reduces the time longer processes spend waiting in the queue.
**Disadvantage:**
- **Starvation:** A significant disadvantage of SJF is the potential for starvation. Longer processes may suffer from indefinite postponement when shorter processes keep arriving. This can lead to critical processes being left waiting for excessive periods.
### Summary
| Feature | FCFS | SJF |
|--------------------------|-------------------------------------------|------------------------------------------|
| **Basic Principle** | Processes are executed in the order they arrive. | Processes with the shortest burst time are executed first. |
| **Advantage** | Simple to implement and understand. | Minimizes average waiting time. |
| **Disadvantage** | Can lead to high average wait time (convoy effect). | Risk of starvation for longer processes. |
In conclusion, while FCFS provides simplicity and ease of implementation, SJF is generally more efficient in terms of minimizing wait time, albeit at the cost of fairness for longer jobs. Understanding these trade-offs helps in selecting the appropriate scheduling algorithm based on specific system requirements.
### First-Come, First-Served (FCFS)
**Basic Principle:**
FCFS is the simplest CPU scheduling algorithm. In this algorithm, processes are attended to in the order they arrive in the ready queue. The first process that arrives is the first to be executed, followed by the next, and so on. It's a non-preemptive scheduling algorithm, meaning once a process starts executing, it runs to completion.
**Advantage:**
- **Simplicity:** FCFS is straightforward and easy to implement. The logic is easy to understand, and it doesn't require complicated data structures or bookkeeping.
**Disadvantage:**
- **Convoy Effect:** FCFS can lead to the "convoy effect," where shorter processes wait for longer processes to complete. This can lead to inefficiencies as the average wait time can be quite high, especially if a long process is followed by several shorter ones.
---
### Shortest-Job-First (SJF)
**Basic Principle:**
SJF is a scheduling algorithm that prioritizes processes according to their burst time (the time they require to complete). The process with the shortest burst time is executed first. Like FCFS, SJF can be implemented in both preemptive and non-preemptive forms; in the preemptive version, if a new process arrives with a shorter burst time than the currently running process, it can preempt the current process.
**Advantage:**
- **Minimizes Average Waiting Time:** SJF minimizes the average waiting time for processes, making it more efficient than FCFS. By executing shorter jobs first, it reduces the time longer processes spend waiting in the queue.
**Disadvantage:**
- **Starvation:** A significant disadvantage of SJF is the potential for starvation. Longer processes may suffer from indefinite postponement when shorter processes keep arriving. This can lead to critical processes being left waiting for excessive periods.
### Summary
| Feature | FCFS | SJF |
|--------------------------|-------------------------------------------|------------------------------------------|
| **Basic Principle** | Processes are executed in the order they arrive. | Processes with the shortest burst time are executed first. |
| **Advantage** | Simple to implement and understand. | Minimizes average waiting time. |
| **Disadvantage** | Can lead to high average wait time (convoy effect). | Risk of starvation for longer processes. |
In conclusion, while FCFS provides simplicity and ease of implementation, SJF is generally more efficient in terms of minimizing wait time, albeit at the cost of fairness for longer jobs. Understanding these trade-offs helps in selecting the appropriate scheduling algorithm based on specific system requirements.