Difference Between FCFS and SJF CPU Scheduling Algorithms
Last Updated :
05 Sep, 2024
CPU scheduling is a key part of how an operating system works. It decides which task (or process) the CPU should work on at any given time. This is important because a CPU can only handle one task at a time, but there are usually many tasks that need to be processed. In this article, we are going to discuss FCFS and SJF CPU scheduling in detail.
First Come First Served CPU Scheduling
FCFS stands for First Come First Served (FCFS). First Come First Served (FCFS) is the simplest type of algorithm. It is a non-preemptive algorithm i.e. the process cannot be interrupted once it starts executing. The FCFS is implemented with the help of a FIFO queue. The processes are put into the ready queue in the order of their arrival time.
The process that arrives first becomes the head of the queue while the others that arrive after are added to the rear of the queue. In the First Come First Served (FCFS) algorithm, the process that arrives first is sent first for execution by the CPU when the CPU is free. The main disadvantage of this algorithm is that the average waiting time is often quite long. It also leads to the convoy effect. This results in lower device or CPU utilization and lower efficiency.
Advantages
- FCFS is essy to understand and easy to implement. The job that arrives first is processed first.
- There's no risk of process starvation.
- Each process is treated equally, based on arrival time, which can be seen as fair from a certain perspective.
Disadvantages
- Processes with longer burst times can cause subsequent processes to wait for a long time.
- A slow process can hold up the entire queue.
- Critical or short processes have to wait for longer processes that arrive earlier.
Shortest Job First CPU Scheduling
SJF stands for Shortest Job First (SJF). Shortest Job First (SJF) Scheduling Algorithm is based upon the burst time of the process. The processes are put into the ready queue based on their burst times. In this algorithm, the process with the least burst time is processed first.
The burst time of only those processes is compared that are present or have arrived until that time. It is also non-preemptive in nature. Its preemptive version is called Shortest Remaining Time First (SRTF) algorithm. The major advantage of this algorithm is that it gives the minimum waiting time for a given set of processes and thus reduces the average waiting time. The disadvantage of this algorithm is that long processes may never be processed by the system and may remain in the queue for very long time leading to starvation of processes.
Note: If two processes have same burst time then the tie is broken using FCFS, i.e., the process that arrived first is processed first.
Advantages
- SJF optimize the average waiting time because shorter processes are prioritized
- By executing shorter jobs first, SJF can improve overall throughput
- Since shorter jobs are executed first, making the CPU more efficient.
Disadvantages
- Predicting the exact burst time for a process can be hard.
- Longer processes may starve.
- If job lengths are not predictable or change frequently, SJF may not be practical.
Difference between FCFS and SJF scheduling algorithm
The difference between First Come First Served (FCFS) and Shortest Job First (SJF) scheduling algorithm are as follows:
| First Come First Served (FCFS) | Shortest Job First (SJF) |
|---|
| First Come First Served (FCFS) executes the processes in the order in which they arrive i.e. the process that arrives first is executed first. | Shortest Job First (SJF) executes the processes based upon their burst time i.e. in ascending order of their burst times. |
| FCFS is non preemptive in nature. | SJF is also non-preemptive but its preemptive version is also there called Shortest Remaining Time First (SRTF) algorithm. |
| FCFS results in quite long waiting time for the processes and thus increases average waiting time. | The average waiting time for given set of processes is minimum. |
| FCFS leads to the convoy effect. | It does not lead to the convoy effect. |
| FCFS algorithm is the easiest to implement in any system. | The real difficulty with SJF is knowing the length of the next CPU request or burst. |
| A process may have to wait for quite long to get executed depending on the burst time of the processes that have arrived first. | A long process may never get executed and the system may keep executing the short processes. |
| FCFS lead to lower device and CPU utilization thereby decreasing the efficiency of the system. | SJF leads to higher effectiveness of the system due to lower average waiting time. |
| FCFS results in minimal overhead. | In case of SJF, elapsed time should be recorded, results in more overhead on the processor. |
| FCFS does not suffers from starvation | SJF suffers from starvation. |
Conclusion
FCFS is the simplest CPU scheduling algorithm, but it can used to inefficient CPU usage and longer waiting times. SJF is optimization CPU scheduling algorithm, is more complex to implement and can lead to the longer processes. FCFS processes jobs according to their order of arrival. It is poor in performance and easy to implement.
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