FCFS vs SSTF Disk scheduling Algorithm

Introduction

Disk scheduling algorithms are critical in computer operating systems for managing and optimizing the use of a computer's hard drive. Disk access time can significantly impact the performance of the entire system, and a poorly implemented disk scheduling algorithm can result in long wait times and reduced overall efficiency. First-Come-First-Serve (FCFS) and Shortest Seek Time First (SSTF) are two common disk scheduling algorithms used in modern operating systems. FCFS processes requests in the order they are received, while SSTF prioritizes processing requests with the shortest seek time. Understanding the differences, advantages, and disadvantages of these algorithms can help system administrators and developers make informed decisions when selecting a disk scheduling algorithm for a particular system.

FCFS Disk scheduling algorithm

A. Definition of FCFS

First-Come, First-Served (FCFS) is a disk scheduling algorithm that processes disk I/O requests in the order in which they are received. The algorithm follows a queue-like structure where requests are stored in a queue and are serviced in a sequential manner as they arrive.

B. How FCFS works

When a disk I/O request is generated, it is added to the end of the request queue. The requests are serviced in the order they appear in the queue, which means that the disk head moves in a sequential manner from the outermost to the innermost track, processing each request as it comes. Once a request is completed, the next request in the queue is serviced.

C. Advantages of FCFS

FCFS is a simple and easy-to-implement algorithm. It is fair to all requests as they are processed in the order they are received, and there is no starvation of requests. The algorithm is also useful when the system has a low workload, as it ensures that all requests are processed efficiently.

D. Disadvantages of FCFS

FCFS is not an optimal algorithm in all scenarios. The main disadvantage of FCFS is its lack of consideration for seek time, as it processes requests in the order they are received, which can lead to long seek times and increased disk access times. This can result in lower system performance, especially in high workload scenarios. FCFS also does not prioritize urgent requests, which can be problematic in real-time systems or systems with time-critical operations.

Real-world examples of FCFS

FCFS is a common disk scheduling algorithm that has been used in a variety of systems. Some real-world examples of FCFS include −

• Single-user systems − In a single-user system, there is only one user making requests, and the workload is relatively low. FCFS is an ideal algorithm for such systems as it ensures all requests are processed in the order they are received, without any priority or starvation concerns.

• Batch processing systems − FCFS can be used in batch processing systems to process multiple jobs in the order they are received. It ensures that all jobs are processed efficiently and in a fair manner.

• Small-scale embedded systems − In small-scale embedded systems, FCFS can be used as a simple and efficient algorithm for processing I/O requests.

Overall, FCFS is a widely used algorithm that can be used in various systems where there is a low to moderate workload, and there are no priority or time-critical operations.

SSTF Disk Scheduling Algorithm

A. Definition of SSTF

Shortest Seek Time First (SSTF) is a disk scheduling algorithm that prioritizes processing I/O requests that are closest to the current position of the disk head. The algorithm selects the I/O request with the shortest seek time to the next request, in order to reduce disk head movement and improve disk access times.

B. How SSTF works

When an I/O request is generated, the SSTF algorithm selects the request that is closest to the current position of the disk head. The algorithm processes the request with the shortest seek time first and then selects the next closest request. This process continues until all requests are processed.

C. Advantages of SSTF

SSTF is a more efficient algorithm than FCFS, as it prioritizes processing requests with the shortest seek time. This reduces the amount of time the disk head spends moving across the disk, resulting in faster access times and improved system performance. SSTF also provides better response times for time-critical requests and can be useful in real-time systems.

D. Disadvantages of SSTF

The main disadvantage of SSTF is the possibility of starvation for requests that are located further away from the disk head. The algorithm prioritizes requests that are closest to the disk head, which can lead to other requests being ignored for extended periods. This can result in lower system performance, especially in high workload scenarios. Additionally, SSTF can suffer from the "elevator problem," where the disk head continuously moves up and down the disk, processing requests in a nonoptimal order.

E. Real-world examples of SSTF

SSTF is widely used in modern operating systems and can be found in a variety of systems, including desktops, laptops, servers, and embedded systems. It is used in situations where the workload is moderate to high, and there are time-critical requests that need to be prioritized. Some examples of systems that use SSTF include real-time systems, database servers, and video streaming services.

Comparison between FCFS and SSTF Disk Scheduling Algorithms

A. Differences between FCFS and SSTF

Aspect	FCFS	SSTF
Processing	Processes requests in the order they are received	Prioritizes requests with the shortest seek time
Disk head moves	Sequentially across the disk	Non-sequentially, to the closest request
Starvation	No starvation of requests	Possibility of starvation for requests further away
Priority	No prioritization of urgent requests	Prioritizes time-critical requests and urgent requests

B. Similarities between FCFS and SSTF

• Both algorithms are used for disk scheduling.

• Both algorithms can be implemented in various operating systems.

• Both algorithms can process I/O requests for both sequential and random access.

C. Comparison of performance metrics

Performance metric	FCFS	SSTF
Disk access time	Higher due to long seek times	Lower due to processing requests with the shortest seek time first
Response time	Lower for low workload scenarios and similar for high workload	Lower for high workload scenarios and time-critical requests
Throughput	Lower for high workload scenarios and long seek times	Higher for high workload scenarios and short seek times
Starvation	No starvation of requests	Possibility of starvation for requests
Performance metric	FCFS	SSTF

Overall, SSTF outperforms FCFS in terms of disk access time, response time for high workload scenarios, and throughput. However, SSTF has the possibility of request starvation for requests located further away from the disk head. FCFS is a simpler algorithm and can be useful in low workload scenarios or systems where all requests are of equal importance.

Conclusion

In conclusion, disk scheduling algorithms are essential for efficient disk access and system performance. FCFS and SSTF are two common algorithms that operate differently and have their respective advantages and disadvantages. FCFS is a simple algorithm that processes I/O requests in the order they are received and can be useful in low workload scenarios or systems where all requests are of equal importance. On the other hand, SSTF prioritizes processing requests with the shortest seek time and is more efficient, resulting in faster access times and improved system performance. However, it has the possibility of request starvation for requests located further away from the disk head. Ultimately, the choice of algorithm depends on the workload, system requirements, and performance metrics.