Thrashing and Frame Allocation MCQs: 12 Solved Questions

Thrashing begins when active working sets no longer fit in available frames, so repeated page faults displace useful CPU work. Frame-allocation questions then turn on four decisions: diagnose the symptom, find the architectural minimum, compare working-set demand with capacity, and choose admission or suspension. The broader Virtual Memory and Demand Paging MCQ set connects the overlapping GATE 2004 and GATE IT 2006 questions to demand paging, effective-access-time arithmetic, and page replacement; here they are used to decide frame budgets and process admission. Attempt each question before opening the answer. The three named GATE attributions below open individual solved pages; the other questions use the Thrashing and Frame Allocation PYQ practice page.

Related reading: demand paging MCQs and page replacement MCQs.

Calibrate the model before the MCQs

Thrashing starts when processes receive too few frames for their active working sets. They repeatedly fault, perform heavy page I/O, block while pages move, and leave the CPU with little useful work. It is not merely swapping. It is a collapse in useful progress caused by excessive paging. A rising degree of multiprogramming can therefore lower CPU utilisation: more processes compete for frames, faults increase, and the scheduler finds fewer runnable processes.

Consider 12 allocatable frames. P1, P2, and P3 currently need working sets of 5, 4, and 6 frames.

1. Total demand = 5 + 4 + 6 = 15 frames.

2. Shortage = 15 - 12 = 3 frames.

3. Suspending P2 frees 4 frames, so remaining demand = 5 + 6 = 11 frames.

4. Spare capacity = 12 - 11 = 1 frame.

That one-frame margin lets P1 and P3 keep their current working sets resident. Reuse this 12-frame scenario when evaluating working-set admission and suspension.

Questions 1-2: recognise thrashing from its signature

Q1. Excessive page I/O

Thrashing

• (a) reduces page I/O

• (b) decreases the degree of multiprogramming

• (c) implies excessive page I/O

• (d) improves system performance

Previous year: GATE 1997

Answer: (c) implies excessive page I/O.

Excessive page I/O is the defining symptom because processes keep requesting pages that are not resident. Decreasing the degree of multiprogramming can help the system recover, but it is a remedy rather than the meaning of thrashing.

Q2. Paging instead of useful execution

Which of the following is a condition that occurs in computer systems when the system spends more time in swapping data between RAM and the hard drive than executing actual tasks?

• (a) Deadlock

• (b) Thrashing

• (c) Starvation

• (d) Fragmentation

• (e) Bottleneck

Previous year: IBPS 2023

Answer: (b) Thrashing.

Thrashing is excessive paging that displaces useful execution. Deadlock is circular resource waiting, starvation is indefinite denial of scheduling or resources, and fragmentation is memory wasted or scattered into inconvenient pieces.

Questions 3-4: identify causes without overclaiming

Q3. The page-fault cycle

What causes thrashing to occur?

• (a) Excessive paging activity

• (b) Insufficient disk space

• (c) Hardware failures

• (d) More than one of the above

• (e) None of the above

Previous year: BPSC PGT Tier-3 2024

Answer: (a) Excessive paging activity.

A page fault may evict a page that will be needed again soon. That access creates another fault, another transfer, and another likely eviction, so the cycle reinforces itself. Insufficient disk space and hardware failures cause other problems, but they do not define thrashing.

Q4. Paging algorithms and allocation

Thrashing

• (a) always occurs on large computers

• (b) is a natural consequence of virtual memory systems

• (c) can always be avoided by swapping

• (d) can be caused by poor paging algorithms

Previous year: Indian Space Research Organization 2008

Answer: (d) can be caused by poor paging algorithms.

Poor victim choices can remove pages that a process will reuse immediately and intensify faulting. The absolute claims using "always" are false, and swapping alone does not guarantee prevention. The FIFO, LRU, and Optimal page-replacement walkthrough explains victim selection, but keep the distinction clear: replacement chooses which page leaves, while allocation decides how many frames a process receives.

Questions 5-6: separate architectural minimums from allocation policy

Q5. Minimum frames for an instruction

The minimum number of page frames that must be allocated to a running process in a virtual memory environment is determined by

• (a) the instruction set architecture

• (b) page size

• (c) physical memory size

• (d) number of processes in memory

Previous year: GATE 2004

Answer: (a) the instruction set architecture.

Here is a hypothetical illustration. If the most demanding legal instruction needs one instruction page and two distinct operand pages resident together, the architecture imposes a minimum of 1 + 2 = 3 frames for that instruction to complete. Page size, installed physical memory, and process count affect capacity or allocation policy, not this architectural floor.

Q6. What allocation actually assigns

In the context of virtual memory management, what is frame allocation?

• (a) The process of assigning pages to frames

• (b) The process of assigning frames to processes

• (c) The process of replacing pages within frames

• (d) None of the above

Answer: (b) The process of assigning frames to processes.

Pages belong to a process, while frames are slots in physical memory. Allocation gives each process a frame budget, then replacement decides which resident page must leave when that budget is full. The Virtual Memory learn hub places both ideas in the full demand-paging sequence.

Questions 7-8: apply the working-set model

Q7. Read the recent-reference window

Working Set (t,k) at an instant of time t is

• (a) the set of K future references that the OS will make

• (b) the set of future references that the OS will make in next T unit of time

• (c) the set of K references with high frequency

• (d) the K set of pages that have been referenced in the last T time units

Previous year: Indian Space Research Organization 2016

Answer: (d) the K set of pages that have been referenced in the last T time units.

A working set looks backward over a recent window and records pages, not predicted future references. In the 12-frame example, recent references reveal working-set demands of 5, 4, and 6 frames. Their total of 15 tells the OS that all three current localities cannot fit at once.

Q8. Admission and suspension decisions

In the working-set strategy, which of the following is done by the operating system to prevent thrashing?

I. It initiates another process if there are enough extra frames.

II. It selects a process to suspend if the sum of the sizes of the working-sets exceeds the total number of available frames.

• (a) I only

• (b) II only

• (c) Neither I nor II

• (d) Both I and II

Previous year: GATE Information Technology 2006

Answer: (d) Both I and II.

With 12 frames, working sets of 5, 4, and 6 demand 15, which is 3 beyond capacity. Suspending the 4-frame process reduces resident demand to 5 + 6 = 11, so the remaining processes fit. If demand were only 9, the system would have 12 - 9 = 3 spare frames and could admit a new process only if its working set fit those 3 frames.

Questions 9-10: connect working sets to fault rate and frame pressure

Q9. The purpose of the working-set strategy

What does the Working Set Strategy aim to achieve?

• (a) Reduce the page fault rate

• (b) Increase the page fault rate

• (c) Optimize CPU utilization

• (d) Both reducing the page fault rate and optimizing CPU utilization

Answer: (d) Both reducing the page fault rate and optimizing CPU utilization.

Keeping a process's active pages resident lowers its page-fault rate first. Fewer processes then block on page I/O, leaving more runnable work for the CPU. A working-set policy protects utilisation indirectly by controlling page faults rather than trying to schedule blocked processes faster.

Q10. Too few relative to the locality

Which among the following can lead to thrashing?

• (a) Having too many frames

• (b) Having too few frames

• (c) Having an optimal number of frames

• (d) Having an equal number of pages and frames

Answer: (b) Having too few frames.

"Too few" is relative to the pages needed by the process's current locality, not a universal frame count. Three frames may hold a 3-page working set but cannot hold a 6-page working set. Compare allocated frames with the distinct pages in the current reference window before choosing an option.

Questions 11-12: choose the recovery action and read CPU utilisation correctly

Q11. Reduce multiprogramming

In a system experiencing Thrashing, which of the following actions can help to mitigate the problem?

• (a) Increase the degree of multiprogramming

• (b) Decrease the degree of multiprogramming

• (c) Replace the page replacement algorithm with FIFO

• (d) Increase the number of virtual (logical) pages allocated to each process

Answer: (b) Decrease the degree of multiprogramming.

Removing P2 from the example changes total working-set demand from 15 to 11 against 12 frames, leaving one frame spare. Increasing multiprogramming would add demand to an overloaded memory. More logical pages add no physical capacity, and FIFO cannot repair a shortage in allocated frames. Recovery requires lowering resident demand or supplying enough physical frames for the active working sets.

Q12. Low CPU utilisation can accompany busy I/O

What is thrashing in an operating system?

• (a) Optimal use of memory

• (b) High CPU utilization

• (c) Low CPU utilization due to excessive paging

• (d) Error in page replacement

Answer: (c) Low CPU utilization due to excessive paging.

Page I/O can be busy while the CPU has little useful work because faulting processes are blocked. High overall system activity therefore does not imply high CPU utilisation. Replacement choices may influence the fault rate, but thrashing is a system state, not merely a replacement error. The diagnostic pair is high paging activity with low useful CPU progress.

Score the set and choose the next step

Use your errors as a revision map, not as a predicted exam score.

• Missed Q1-Q4: revise the symptom, cause, and excessive-page-I/O chain.

• Missed Q5-Q6: separate frame allocation from page replacement and architectural minimums.

• Missed Q7-Q10: recompute working sets with the 12-frame example.

• Missed Q11-Q12: revise the feedback loop between multiprogramming, faults, blocking, and CPU utilisation.

After revising the weak section, attempt all 12 questions again without looking at the answers.

Resident working sets that do not fit cause thrashing; frame allocation, admission, and suspension restore a feasible demand. Continue with GATE Guidance by Sanchit Sir, then use the GATE CS Exam Preparation Courses and Test Series for the broader syllabus route.