Question

What is the "Thrashing" phenomenon in virtual memory systems?

• A. High CPU utilization due to heavy process loads.
• B. Excessive paging activity where the system spends more time swapping than executing.
• C. Frequent disk crashes due to high read/write speeds.
• D. Corruption of the page table by a malicious process.

Hint:

Thrashing is the result of a "positive feedback loop" gone wrong. The system works harder to fix a problem (low CPU utilization) but the action taken (adding more processes) actually worsens the root cause (lack of memory frames).

Answer 1

The Correct Option is B

Concept:
Thrashing is a critical state in a computer system where the performance collapses because the operating system is spending nearly all of its time performing I/O operations (swapping pages in and out) rather than executing actual instructions for the processes.

Step 1: The Cause: Over-multiprogramming.
The operating system usually tries to increase CPU utilization by increasing the "Degree of Multiprogramming" (adding more processes to the ready queue). However, if the total "Working Set" (the sum of pages required by all active processes) exceeds the total number of physical frames available in RAM, processes start stealing pages from each other.

Step 2: The Chain Reaction.
The sequence of events leads to a total system freeze:
• A process needs a page and faults.
• It replaces a page belonging to another process.
• The second process now needs its page back and faults.
• This creates a queue at the paging device (disk).
• CPU utilization drops because most processes are waiting for the disk.
• The OS sees low CPU utilization and mistakenly adds more processes, making the situation even worse.

Step 3: Detection and Solution.
Thrashing can be detected by monitoring the page-fault frequency. To stop thrashing, the operating system must decrease the degree of multiprogramming by suspending or terminating some processes until the remaining ones have enough frames to operate efficiently.