What is page fault frequency (PFF) and how does an operating system mitigate thrashing?

• A. PFF measures page faults per unit time; thrashing occurs when working set exceeds available memory; mitigations include increasing memory, reducing load, swap less frequently, or suspending processes.
• B. PFF measures page faults per I/O operation; thrashing occurs when disk is full.
• C. PFF measures CPU usage; thrashing occurs due to high CPU frequency.
• D. PFF is a type of memory page; thrashing is unrelated.

Answer: (A)

Page fault frequency is the rate at which page faults happen per unit time. When a program references data that's not currently in RAM, the system must fetch it from disk, causing a page fault. If page faults occur very often, the CPU spends most of its time waiting for disk I/O rather than doing useful work, a condition known as thrashing. Thrashing happens because the working set—the set of pages actively used by the process—doesn't fit in physical memory, so pages are constantly swapped in and out.

To mitigate thrashing, the operating system reduces memory pressure so the working set fits in RAM. Practical ways include increasing available memory, lowering the number of active processes (reducing the degree of multiprogramming), or suspending some processes to shrink the total working set. Tuning the paging behavior or page replacement policy can help too, but the core idea is to reduce the amount of memory being demanded simultaneously so page faults drop and normal execution can resume.