I/O scheduling and buffering in Operating Systems - Practice Problems & Coding Challenges

Challenge - 5 Problems

🧠 Conceptual

intermediate

Understanding I/O Scheduling Algorithms

Which I/O scheduling algorithm is designed to reduce the average seek time by servicing requests in the order of their position on the disk, moving the disk arm in one direction only until it reaches the end?

A First-Come, First-Served (FCFS)

B SCAN (Elevator Algorithm)

C Shortest Seek Time First (SSTF)

D Circular SCAN (C-SCAN)

💡 Hint

Think of an elevator moving up and down, servicing requests in one direction before reversing.

The SCAN algorithm moves the disk arm in one direction, servicing all requests until it reaches the end, then reverses direction. This reduces seek time compared to FCFS by avoiding unnecessary back-and-forth movement, though SSTF may have lower average seek time but can cause starvation.

📋 Factual

intermediate

Buffering Purpose in I/O Operations

What is the primary purpose of buffering in I/O operations?

A To increase CPU speed during processing

B To permanently save data on disk

C To temporarily store data to handle speed differences between devices

D To encrypt data before transmission

💡 Hint

Consider why data might need a temporary holding place when moving between fast and slow devices.

Buffering temporarily holds data to accommodate differences in speed between the CPU and I/O devices, preventing data loss or CPU waiting.

🔍 Analysis

advanced

Analyzing Effects of Buffering Strategies

Consider a system using double buffering for I/O. What is the main advantage of double buffering compared to single buffering?

A It allows simultaneous reading and writing, reducing idle time

B It halves the memory usage for buffers

C It eliminates the need for I/O scheduling

D It increases the CPU clock speed

💡 Hint

Think about how two buffers can be used to overlap operations.

Double buffering uses two buffers so one can be filled while the other is being emptied, allowing reading and writing to happen at the same time and reducing waiting.

❓ Comparison

advanced

Comparing I/O Scheduling Algorithms

Which of the following statements correctly compares SSTF and C-SCAN scheduling algorithms?

A SSTF always provides fair access to all requests, while C-SCAN can starve some requests.

B Both SSTF and C-SCAN ignore the physical location of requests on the disk.

C SSTF moves the disk arm in a fixed direction, while C-SCAN selects the closest request next.

D C-SCAN moves the disk arm in one direction only, providing more uniform wait times than SSTF.

💡 Hint

Consider how each algorithm moves the disk arm and how that affects wait times.

SSTF selects the closest request next, which can cause starvation for far requests. C-SCAN moves the arm in one direction only, servicing requests in order and then jumping back, providing more uniform wait times.

❓ Reasoning

expert

Impact of Buffer Size on System Performance

A system uses buffering for I/O operations. If the buffer size is increased significantly, which of the following is the most likely effect?

A I/O throughput improves up to a point, but excessive buffer size may increase latency.

B The system will always perform better with larger buffers, with no downsides.

C CPU utilization decreases because larger buffers slow down processing.

D Increasing buffer size causes immediate data loss due to overflow.

💡 Hint

Think about the trade-off between throughput and delay when changing buffer size.

Larger buffers can hold more data, improving throughput by reducing I/O wait times. However, very large buffers can cause delays before data is processed, increasing latency.

Practice

1. What is the main purpose of I/O scheduling in an operating system?

easy

A. To encrypt data before sending it to a device

B. To permanently store data on the hard drive

C. To increase the size of the computer's memory

D. To organize the order of input/output requests for better performance

Solution

1. Step 1: Understand the role of I/O scheduling

   I/O scheduling arranges how input/output requests are processed to improve speed and efficiency.

2. Step 2: Compare options to the definition

   Only To organize the order of input/output requests for better performance correctly describes organizing the order of I/O requests for better performance.

3. Final Answer:

   To organize the order of input/output requests for better performance -> Option D

4. Quick Check:

   I/O scheduling = Organizing requests [OK]

Hint: I/O scheduling = arranging request order for speed [OK]

Common Mistakes:

• Confusing scheduling with permanent storage
• Thinking it increases memory size
• Mixing scheduling with data encryption

2. Which of the following is the correct way to describe buffering?

easy

A. Buffering deletes data after use

B. Buffering permanently saves data to disk

C. Buffering temporarily stores data to handle speed differences

D. Buffering encrypts data for security

Solution

1. Step 1: Define buffering in I/O context

   Buffering temporarily holds data to balance speed differences between devices.

2. Step 2: Match definition with options

   Buffering temporarily stores data to handle speed differences correctly states buffering temporarily stores data to handle speed differences.

3. Final Answer:

   Buffering temporarily stores data to handle speed differences -> Option C

4. Quick Check:

   Buffering = Temporary data storage [OK]

Hint: Buffering = temporary data storage for speed balance [OK]

Common Mistakes:

• Thinking buffering saves data permanently
• Confusing buffering with data deletion
• Assuming buffering encrypts data

3. Consider a system using a First-Come-First-Serve (FCFS) I/O scheduler. If requests arrive in this order: R1, R2, R3, which request will be processed last?

medium

A. R3

B. R2

C. R1

D. Cannot determine without request sizes

Solution

1. Step 1: Understand FCFS scheduling

   FCFS processes requests in the order they arrive, first to last.

2. Step 2: Apply FCFS to given requests

   Requests arrive as R1, R2, then R3, so R3 is processed last.

3. Final Answer:

   R3 -> Option A

4. Quick Check:

   FCFS last request = last arrival [OK]

Hint: FCFS processes requests in arrival order [OK]

Common Mistakes:

• Assuming request size affects order in FCFS
• Thinking R1 or R2 could be last
• Confusing FCFS with priority scheduling

4. A programmer wrote this pseudocode for buffering data:

buffer = []
while data_available:
    buffer.append(read_data())
process(buffer)
buffer.clear()

What is the likely error in this code?

medium

A. Buffer is cleared after processing, which is correct

B. Data is not read into the buffer

C. Buffer is cleared before processing data

D. Buffer is never cleared, causing overflow

Solution

1. Step 1: Analyze the code flow

   Data is read into buffer, then processed, then buffer is cleared.

2. Step 2: Check if clearing buffer is correctly placed

   Clearing buffer after processing is correct to free space for new data.

3. Final Answer:

   Buffer is cleared after processing, which is correct -> Option A

4. Quick Check:

   Clearing buffer after use = correct [OK]

Hint: Clear buffer only after processing data [OK]

Common Mistakes:

• Clearing buffer before processing data
• Not reading data into buffer
• Forgetting to clear buffer causing overflow

5. In a system where a fast CPU sends data to a slow printer, which combination of I/O scheduling and buffering best improves performance?

hard

A. Use FCFS scheduling with no buffering

B. Use priority scheduling with buffering to hold data temporarily

C. Use shortest seek time scheduling without buffering

D. Use random scheduling with immediate data sending

Solution

1. Step 1: Identify the problem

   CPU is fast, printer is slow, so data must be buffered to avoid waiting.

2. Step 2: Choose scheduling and buffering strategy

   Priority scheduling can optimize request order; buffering holds data to match speeds.

3. Final Answer:

   Use priority scheduling with buffering to hold data temporarily -> Option B

4. Quick Check:

   Buffering + priority scheduling = better performance [OK]

Hint: Buffer slow device, prioritize important requests [OK]

Common Mistakes:

• Ignoring buffering for slow devices
• Using random or FCFS scheduling without buffering
• Assuming shortest seek time applies to printers