In virtual memory management, what is page cannibalizing (page stealing) and why might an operating system use it?

Introduction / Context:
In a virtual memory system, physical page frames are a limited resource shared among many processes. When the system experiences memory pressure, the operating system may need to reassign frames dynamically between processes. The term page cannibalizing, also known as page stealing, describes one such strategy. Understanding this concept helps explain how the operating system responds when some processes demand more memory than others and how it attempts to avoid or recover from thrashing.

Given Data / Assumptions:

• The system uses paging to implement virtual memory.
• Physical memory is divided into frames that can be allocated to processes.
• At times, some processes may have more frames than they currently need, while others may lack enough frames to hold their working sets.
• The question asks what page cannibalizing is and why an operating system might use it.

Concept / Approach:
Page cannibalizing refers to the operating system taking page frames away from one or more processes and reallocating them to another process that needs additional memory. This often occurs when the system is under memory pressure and cannot satisfy new or growing demands simply by allocating unused frames. The operating system may monitor page fault rates or working sets to decide from which processes it can safely steal frames. By doing so, it tries to ensure that heavily used processes have enough memory to avoid excessive paging, even if this means reducing the frames for less active processes.

Step-by-Step Solution:
Step 1: Recognize that total physical memory is limited and must be shared among all running processes.Step 2: Understand that some processes may be overallocated frames relative to their current working sets, while others are suffering from high page fault rates because they lack sufficient frames.Step 3: The operating system can decide to reclaim frames from processes with low activity or large idle working sets, freeing those frames for processes that need them more urgently.Step 4: This act of taking frames from one process and giving them to another is called page cannibalizing or page stealing.Step 5: The goal is to improve overall system performance and reduce thrashing by matching frame allocations more closely to each process working set needs.

Verification / Alternative check:
Working set based memory management and page fault frequency control algorithms illustrate how operating systems adjust frame allocations over time. For example, if a process page fault frequency falls below a certain threshold, the system may decide it can spare some frames, which can then be reassigned to a process whose page fault frequency is too high. Documentation on memory managers often uses terms such as page stealing to describe these operations. These examples align with the idea of page cannibalizing as reallocating frames among processes under memory pressure.

Why Other Options Are Wrong:
Option B is incorrect because page cannibalizing does not remove virtual memory or paging; rather, it adjusts how existing frames are distributed among processes. Option C is wrong because deleting files to free disk space affects storage, not the allocation of page frames in main memory. Option D is incorrect because scheduling time slices on the CPU is a separate concern from memory management and does not involve reassigning page frames between processes.

Common Pitfalls:
Students may confuse page cannibalizing with basic page replacement within a single process. Page replacement chooses which of a process own pages to evict when it needs a new page. Page cannibalizing operates at a higher level by changing how many frames a process receives in the first place. Another pitfall is to assume that page stealing is always beneficial; overly aggressive stealing can push other processes toward thrashing. Operating systems must therefore balance fairness and performance when implementing this strategy.

Final Answer:
Page cannibalizing is the process by which the operating system takes page frames away from one or more processes and reallocates them to another process that urgently needs more memory, typically under high memory pressure, in an effort to improve overall performance.