Thrashing

Last Updated : 11 Sep, 2025

Thrashing occurs when the operating system spends more time swapping pages between main memory and disk than executing processes. This leads to excessive page faults and a significant drop in CPU utilization.

The cycle works like this:

  • High degree of multiprogramming: Too many processes are loaded into memory.
  • Lack of frames: Each process gets fewer frames than needed.
  • Page replacement policy: Frequent replacements increase page faults.

This repeated cycle of low CPU utilization → more processes → more page faults is called Thrashing.

Locality Model

The concept of locality of reference helps explain thrashing:

  • A locality is a set of pages that a program actively uses together.
  • For example, when a function is called, instructions, local variables, and global references define a locality.
  • If the number of frames allocated to a process covers its current locality → few page faults.
  • If frames are fewer than the locality size → frequent page faults → thrashing.

Thrashing happens when active localities of multiple processes cannot fit into memory simultaneously.

Techniques to Handle Thrashing

1. Working Set Model

Based on the Locality Model: a process uses a set of pages (locality) actively at a time.

  • If enough frames are allocated to cover the current locality few page faults.
  • If frames < locality size process will thrash.
  • Working Set (WSSᵢ) = pages referenced in the last Δ references (window size).
  • Total demand: D = Σ WSSᵢ

Cases:

  1. If D > m (m = available frames) i.e Thrashing occurs.
  2. If D ≤ m i.e No thrashing.

Accuracy depends on Δ:

  • Large Δ, overlapping working sets.
  • Small Δ, locality may not be fully captured.

2. Page Fault Frequency

PFF is a technique to control thrashing by directly monitoring the page fault rate of processes.

Working:

  1. Define an upper limit and a lower limit for acceptable page fault rate.
  2. If fault rate > upper limit, give more frames to the process.
  3. If fault rate < lower limit, take away frames.
  4. If no free frames are available suspend some processes and reallocate frames.
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