Thrashing and working set model in Operating Systems - Practice Problems & Coding Challenges

Challenge - 5 Problems

🧠 Conceptual

intermediate

Understanding Thrashing in Operating Systems

What is the primary cause of thrashing in an operating system?

A The system has too many processes competing for limited physical memory, causing excessive paging.

B The system has too much free memory and is not using it efficiently.

C The disk is malfunctioning and cannot read data properly.

D The CPU is idle because there are no processes to execute.

💡 Hint

Think about what happens when memory is overused and the system spends more time swapping pages than executing processes.

Thrashing happens when the system spends most of its time swapping pages in and out of memory because there isn't enough physical memory for all active processes. This slows down the system drastically.

📋 Factual

intermediate

Working Set Model Definition

Which of the following best describes the working set model in operating systems?

A It measures the CPU time used by a process to decide its priority.

B It counts the total number of pages in the system regardless of process activity.

C It tracks the set of pages a process has referenced in the last fixed number of page references to estimate its current memory needs.

D It records the number of processes waiting in the ready queue.

💡 Hint

Consider how the model helps decide how much memory a process needs based on recent activity.

The working set model keeps track of the pages a process has used recently (within a certain window of time or number of references) to estimate how much memory it needs to run efficiently without causing thrashing.

🔍 Analysis

advanced

Analyzing Effects of Working Set Size

If a process's working set size suddenly increases beyond the available physical memory, what is the most likely immediate effect on system performance?

A The CPU will switch to idle mode to save power.

B The system will experience thrashing, leading to a significant slowdown.

C The process will complete faster because it uses more memory.

D The system will automatically increase physical memory to accommodate the process.

💡 Hint

Think about what happens when memory demand exceeds supply suddenly.

If the working set size grows beyond available memory, the system must swap pages frequently, causing thrashing and slowing down all processes.

❓ Comparison

advanced

Comparing Local and Global Page Replacement

How does the working set model influence the choice between local and global page replacement policies?

A The working set model forces the system to use FIFO page replacement.

B The working set model requires global replacement to share pages among all processes equally.

C The working set model ignores page replacement policies entirely.

D The working set model supports local replacement to keep a process's pages in memory, reducing thrashing.

💡 Hint

Consider how keeping a process's recent pages in memory affects performance.

The working set model favors local page replacement because it tries to keep the pages a process currently needs in memory, which helps prevent thrashing by not evicting those pages to other processes.

❓ Reasoning

expert

Predicting System Behavior with Working Set Changes

Consider a system using the working set model for memory management. If multiple processes simultaneously increase their working set sizes, what is the most likely system-wide consequence?

A The system will likely enter thrashing, as total memory demand exceeds physical memory.

B The system will pause all processes until memory is freed.

C The CPU will allocate more time slices to each process to handle the increased memory needs.

D The system will automatically reduce the working set sizes to prevent thrashing.

💡 Hint

Think about what happens when many processes need more memory than the system has.

If many processes increase their working sets at once, the total memory demand can exceed physical memory, causing thrashing and severe performance degradation.

Practice

1. What is thrashing in operating systems?

easy

A. A system running multiple programs efficiently

B. A process running without any memory

C. A method to speed up CPU processing

D. Excessive paging causing slow system performance

Solution

1. Step 1: Understand the meaning of thrashing

   Thrashing happens when the system spends too much time swapping pages in and out of memory instead of executing processes.

2. Step 2: Identify the effect of thrashing

   This excessive paging slows down the computer significantly, reducing performance.

3. Final Answer:

   Excessive paging causing slow system performance -> Option D

4. Quick Check:

   Thrashing = Excessive paging slowing system [OK]

Hint: Thrashing means too much swapping slows the computer [OK]

Common Mistakes:

• Confusing thrashing with normal multitasking
• Thinking thrashing speeds up the CPU
• Believing thrashing means no memory usage

2. Which of the following best describes the working set of a process?

easy

A. Pages never accessed by the process

B. All pages in the system memory

C. Pages recently used by the process

D. Pages swapped out to disk

Solution

1. Step 1: Define working set

   The working set is the collection of pages a process has used recently and needs to run efficiently.

2. Step 2: Identify correct description

   Pages recently used by the process fit this definition, unlike all pages or swapped pages.

3. Final Answer:

   Pages recently used by the process -> Option C

4. Quick Check:

   Working set = Recently used pages [OK]

Hint: Working set means pages used recently by process [OK]

Common Mistakes:

• Thinking working set includes all system pages
• Confusing working set with swapped out pages
• Assuming working set is pages never accessed

3. Consider a system where a process's working set size suddenly grows beyond available memory. What is the most likely outcome?

medium

A. Thrashing will occur, slowing down the system

B. The process will run faster due to more pages

C. The system will ignore the extra pages

D. The process will terminate immediately

Solution

1. Step 1: Analyze working set growth impact

   If the working set grows beyond available memory, the system must swap pages frequently to keep up.

2. Step 2: Identify system behavior

   This frequent swapping causes thrashing, which slows down the system instead of speeding it up.

3. Final Answer:

   Thrashing will occur, slowing down the system -> Option A

4. Quick Check:

   Working set too big = Thrashing slows system [OK]

Hint: Too big working set causes thrashing and slowdown [OK]

Common Mistakes:

• Assuming more pages always improve speed
• Believing system ignores memory limits
• Thinking process ends immediately without warning

4. A system is experiencing thrashing. Which change would best reduce thrashing?

medium

A. Increase the working set size of all processes

B. Reduce the number of processes competing for memory

C. Decrease the CPU speed

D. Disable paging completely

Solution

1. Step 1: Understand cause of thrashing

   Thrashing happens when too many processes compete for limited memory, causing excessive paging.

2. Step 2: Identify best solution

   Reducing the number of processes competing for memory lowers paging demand and reduces thrashing.

3. Final Answer:

   Reduce the number of processes competing for memory -> Option B

4. Quick Check:

   Less competition = less thrashing [OK]

Hint: Fewer processes competing reduces thrashing [OK]

Common Mistakes:

• Increasing working set size worsens thrashing
• Decreasing CPU speed does not fix memory issues
• Disabling paging causes system failure

5. A system uses the working set model to manage memory. If a process's working set is 100 pages but only 80 pages fit in memory, what should the system do to avoid thrashing?

hard

A. Swap out some pages from the process's working set

B. Increase the process's working set size to 120 pages

C. Allow the process to run with only 80 pages and swap frequently

D. Terminate the process immediately

Solution

1. Step 1: Analyze working set vs memory size

   The process needs 100 pages to run well, but only 80 pages fit in memory, causing potential thrashing.

2. Step 2: Determine best memory management action

   Swapping out some pages from the working set or adjusting memory allocation helps keep the working set within limits and avoids thrashing.

3. Final Answer:

   Swap out some pages from the process's working set -> Option A

4. Quick Check:

   Adjust working set to fit memory to avoid thrashing [OK]

Hint: Adjust working set size to fit memory to prevent thrashing [OK]

Common Mistakes:

• Increasing working set size worsens thrashing
• Allowing frequent swaps causes thrashing
• Terminating process is last resort, not first