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.