Question

Use the following list of cylinder requests in Exercises 54-56. They are listed in the order in which they were received. \(40,12,22,66,67,33,80\) List the order in which these requests are handled if the FCFS algorithm is used. Assume that the disk is positioned at cylinder 50 .

Short answer

Requests are handled in the order: 40, 12, 22, 66, 67, 33, 80.

Step-by-step solution

Understand the FCFS Algorithm

FCFS stands for First-Come, First-Served, which means requests are processed in the exact order they arrive. This is a simple scheduling algorithm where the first request in the queue is processed first.

Identify Initial Disk Position

The disk is initially positioned at cylinder 50 according to the exercise statement. This will be our starting point for handling requests.

Process the Requests Sequentially

List the disk requests in the order they were received: 40, 12, 22, 66, 67, 33, 80. According to FCFS, these will be processed in the exact same sequence, regardless of the current position or the distance to each request.

List the Order of Handling Requests

Since we are required to list the order as per FCFS and the requests are received in the order 40, 12, 22, 66, 67, 33, 80, the handling order will be exactly as received: 40, 12, 22, 66, 67, 33, 80.

Key concepts

Understanding Cylinder Requests

Cylinder requests are a key part of disk scheduling. Each request corresponds to a position on the disk that needs data access. Imagine the surface of a disk as a collection of concentric circles called cylinders.
Each cylinder holds a track of data, so when the computer wants to read or write data, it requests access to a specific cylinder.
In our exercise, the given cylinder requests are

• 40
• 12
• 22
• 66
• 67
• 33
• 80

The request numbers represent the cylinder positions where the data is stored. How these requests are managed can impact the performance and efficiency of the disk access operations.

Disk Scheduling Algorithms Explained

Disk scheduling algorithms dictate how an operating system processes cylinder requests. They determine the order in which requests are fulfilled, aiming to reduce the seek time - the time it takes for the disk arm to move to the correct cylinder.
There are several algorithms, each with its own advantages and disadvantages:

• FCFS (First-Come, First-Served): Processes requests in the order they arrive. It's simple and fair but can be inefficient for large queues.
• SSTF (Shortest Seek Time First): Chooses the request closest to the current arm position, minimizing seek time. It can cause starvation for requests far away from the current position.
• SCAN: Moves the arm in one direction fulfilling requests until it reaches the end, then reverses direction. Balances efficiency and fairness.
• LOOK: Similar to SCAN but only goes as far as the furthest request before reversing directions.

These algorithms balance different needs for efficiency, fairness, and implementation complexity, showing there is no one-size-fits-all solution.

The Role of Initial Disk Position

In any disk scheduling task, knowing the initial disk position is vital. This refers to the starting point of the disk's read/write head on the cylinder.
In the given exercise, we begin at cylinder 50.
This initial position serves as the reference point for determining the sequence in processing the requests, especially important in algorithms like FCFS.
Why does the initial position matter? It affects:

• The total seek time, i.e., how far the arm needs to move to fulfill requests.
• The order of processing in other algorithms that don't strictly follow arrival order, such as SSTF.

By starting our scheduling from the initial position, we can understand how efficiently and quickly we can handle all incoming requests.

Sequencing with Request Queue Order

Request queue order is foundational in FCFS disk scheduling since requests are attended in the exact order they arrive. This straightforward approach ensures every request is processed without prioritization.
In our exercise, the request queue order is:

• 40
• 12
• 22
• 66
• 67
• 33
• 80

Even though the current disk head is at cylinder 50, with FCFS, we don't reorganize based on convenience or distance.
This method is easy to implement and fair since each request gets a turn based on arrival time. However, this might not be the most efficient option for minimizing seek time across all requests, especially when dealing with large and varied queues.
It highlights why understanding the request queue order is crucial when choosing the appropriate scheduling algorithm for performance optimization.