Question

Mark the answers true or false as follows: A. True B. False The first-come, first-served diskscheduling algorithm moves the heads the minimum amount it can to satisfy a pending request.

Short answer

B. False

Step-by-step solution

Understanding the First-Come, First-Served Algorithm

The First-Come, First-Served (FCFS) disk scheduling algorithm handles requests in the order they arrive in the queue without reordering them based on their location on the disk. It treats each request as a priority solely based on its arrival time.

Analyzing Head Movement in FCFS

Since FCFS processes requests in the order they arrive, the algorithm does not consider the position of the disk head at the time of each request. This can result in excessive head movement because there is no prioritization based on the current position of the head relative to the requested data.

Determining Minimum Head Movement

While the FCFS algorithm is simple and fair, it does not guarantee the minimum head movement. Other algorithms, such as Shortest Seek Time First (SSTF), are specifically designed to minimize head movement by choosing the request closest to the current head position.

Evaluating the Statement

The statement claims that the FCFS algorithm minimizes head movement. Given that FCFS handles requests based solely on arrival order without consideration of current head position, it definitely does not minimize head movement compared to other algorithms.

Key concepts

First-Come First-Served (FCFS)

The First-Come, First-Served (FCFS) disk scheduling algorithm operates under a simple and straightforward principle. It processes disk requests in the exact sequence that they arrive. This means if a request arrives first, it will be handled first, allowing no reordering or prioritization based on the location of the disk sectors being requested. This makes FCFS akin to a "queue" system, where the first request in is the first request out.

The FCFS approach is often valued for its simplicity and fairness, ensuring every request gets handled in due time. However, this fairness does not translate to efficiency in terms of disk operations. The algorithm can lead to large amounts of head movement across the disk because the head might have to travel back and forth to satisfy requests in their arrival order, without optimizing the path. So, while all queued requests are completed, they are often not done so in the most timely or efficient manner.

• Simple to implement
• Fair in terms of request handling order
• Can lead to significant delays due to excessive head movement

Shortest Seek Time First (SSTF)

Shortest Seek Time First (SSTF) is an alternative to the FCFS algorithm and offers a more efficiency-oriented approach to disk scheduling. This method selects the disk request that is closest to the current position of the head. By doing so, it reduces the overall seek time, which translates to less waiting for the subsequent requests.

In SSTF, when the disk head finishes a task, it checks the queue for the nearest request and proceeds to that one next—much like choosing the closest stop while on a road trip, minimizing the distance traveled. This often results in less head movement and quicker response times compared to FCFS.

However, SSTF is not without its shortcomings. It can lead to a problem known as "starvation," where some requests might get neglected if there are always closer ones coming in. To counter this, more advanced algorithms are sometimes used. Despite this, SSTF remains a popular choice for many systems where rapid responsiveness is essential.

• Reduces overall seek time by choosing the closest request
• Improves efficiency over FCFS in most scenarios
• Risk of starvation for some requests

Head Movement

Head movement in disk scheduling refers to the physical motion of the read/write head across the surface of the disk, which is necessary to access and process data. The amount of head movement significantly impacts the overall efficiency and speed of a disk scheduling algorithm.

The main goal of efficient disk scheduling is to minimize the head movement because it directly affects seek time—the time required for the disk arm to move to the correct track. Fewer movements mean faster disk access and improved overall system performance.

Algorithms designed for disk scheduling, like SSTF, aim at minimizing head movement by optimizing which requests to handle next, based on the current position of the head. On the other hand, algorithms like FCFS might exhaustively move the head around without a path optimization, leading to higher overhead.

Understanding and managing head movement is crucial as it directly correlates with resource usage and processing time of IO operations on the disk. Efficient algorithms can greatly reduce latency, especially in systems with high disk access demands.

• Significantly impacts seek time and system performance
• Efficient algorithms aim to minimize unnecessary movement
• Critical for optimizing IO operations and resource usage