What is First-Come-First-Served Order Scheduling?
Martin Pahulje - June 22, 2021
Order scheduling is a complex juggling act. On top of processing documents and products, delivery dates must be optimized with customer requests in mind. Orders are placed in a constant flow, so it can be challenging to keep processes paced with new and ongoing orders. Each company handles order scheduling differently, all done in the attempt of processing orders as quickly, accurately, and efficiently as possible. Simultaneously, an organization’s systems must be able to balance an inflow of orders with the processing capacity of their equipment, ensuring that machines are not overwhelmed or left idle. To face the ongoing challenge of managing orders, companies have tested and tried several order scheduling methods to find the most effective one. Perhaps the simplest and thus most effective method is first-come-first-serve (FCFS) order scheduling.
FCFS is an operating system scheduling algorithm that automatically executes queued requests and processes in order of their arrival. In this type of algorithm, the processes that request the central processing unit (CPU) first, get the CPU allocation first. Following this protocol, FCFS is the easiest CPU scheduling algorithm to utilize. As a process enters the ready queue, its process control block (PCB) is linked with the tail of the queue. Once the CPU becomes available, it is then assigned to the process at the beginning of the queue. Essentially, orders are processed in the sequence that they are placed in, simplifying the order scheduling method as much as possible.
Characteristics of FCFS
FCFS is a highly simplified method of order scheduling, supporting non-preemptive and pre-emptive scheduling algorithms. Processes are always executed on a first-come, first-serve basis, making the method extremely easy to implement and use. But, while this keeps operations straightforward, it is not necessarily superior to other methods in performance, and the general wait time tends to be higher than that of alternative approaches. An example of FCFS in daily life would be making a purchase at a grocery store. In this scheduling algorithm, customers are served according to queue manner. The first person in line at the checkout counter is the first person that will be tended to by the cashier, and the next customer to be served will be the next person in line. This order will continue until the last patron in line is served. Using this same algorithm, the CPU process works similarly.
How Does it Work?
We have explained the general working of FCFS so far, but we’ll provide you with an example that you are more likely to encounter from a business perspective. Imagine you have four processes, along with their arrival and burst time.
- Process: 1, Arrival Time: 0, Burst Time: 5
- Process: 2, Arrival Time: 3, Burst Time: 24
- Process: 3, Arrival Time: 5, Burst Time: 3
- Process: 4, Arrival Time: 8, Burst Time: 20
Arrival time is not the real-time of the system but rather the relative time. This is the time at which the process arrived in the ready queue. The lower the arrival time, the earlier the process arrives. In this example, the processes arrive in their respective process order. Burst time is the time required by the process to complete its execution. In FCFS, arrival time dictates the order that processes will be completed; burst time has no effect. There are two types of burst time in FCFS; CPU burst and I/O burst. CPU burst time is the time required by the process to utilize CPU cycles, while I/O burst time is the time required by the process to utilize I/O devices. In this example, all burst times are CPU bursts and are recorded in milliseconds.
Process 1 arrives at 0 milliseconds, so it does not have to wait and is allotted the CPU first, requiring five milliseconds to execute. Meanwhile, Process 2 has arrived in the ready queue but must wait until Process 1 is completely executed because of the non-preemptive nature of FCFS. Once Process 1 is complete, and Process 2 is performing its execution, both Process 3 and 4 arrive in the ready queue. Process 3 begins execution at the 29thmillisecond and requires three milliseconds to execute, and Process 4 begins once three is complete. The simplicity of FCFS is evident through this example, but there is more to consider in order planning and scheduling than simplicity. For you to understand if FCFS is the right fit for your organization, we will discuss the advantages and disadvantages of implementing this method.
Advantages and Disadvantages of FCFS
FCFS tends to be a popular method of order scheduling because it is so straightforward. As a result, it is the simplest form of a CPU scheduling algorithm. Not only does this make fulfilling orders easier, but it is nearly effortless to program into preexisting scheduling systems. This significantly reduces implementation time and cost, which is beneficial for organizations looking for a quick solution to order scheduling. Additionally, the overarching idea behind "first-come-first-served" is viewed to be very fair as it is a processing manner that we are accustomed to in our everyday lives. Customers understand the ideology behind FCFS and view it as appropriate, making them more understanding of waiting in a queue.
While simplicity and ease of use are significant advantages of implementing FCFS, there are some disadvantages that you should be aware of. One of the disadvantages of this method is the simplicity that it proudly offers. As is typical, the simpler a system is, the less effective it is, and this can be the case for FCFS. Furthermore, FCFS is a non-preemptive CPU scheduling algorithm. This means that after a process has been allocated to the CPU, it will never release the CPU until it finished executing. In our current business world, flexibility is highly valued, so this is a significant downfall. Should a customer wish to cancel or adjust an order, they will be unable to with FCFS. This could also bring challenges should a significant error arise, and a series of incorrect orders will be placed without a demand for them. In addition to this, the average waiting time for FCFS is relatively high compared to that of other methods. Should a time-consuming order be placed at the front of the queue, short processes in the back will have to wait for the long process in the front to be finished. This can significantly delay operations should a length and unexpected order be placed, which can be frustrating for both customers and the organization. Finally, FCFS is not an ideal technique for time-sharing systems, which can pose a challenge for organizations requiring advanced multitasking. There are appreciable advantages and disadvantages to FCFS that must be considered before deciding to implement FCFS in your organization. Overall, the process is a simple one that can easily be integrated into your organization to organize the order scheduling process, which is crucial to achieving quick and efficient operations.
Click below to download our guide on order slotting and scheduling.