Thrashing and working set model in Operating Systems - Step-by-Step Execution

Concept Flow - Thrashing and working set model

Process starts running

↓

Needs pages in memory

↓

Check if enough frames

↓

Load pages

↓

Process runs

↓

Working set tracked

↓

Adjust frames based on working set

↓

Process runs smoothly

This flow shows how a process requests memory pages, how lack of frames causes thrashing, and how the working set model helps manage frames to avoid thrashing.

Execution Sample

Operating Systems

Process requests pages
If frames < working set size
  Page faults increase
  Thrashing occurs
OS tracks working set
OS adjusts frames
Process runs smoothly

This pseudo-code shows how the system checks if the process has enough memory frames to hold its working set and adjusts to prevent thrashing.

Analysis Table

Step	Condition	Action	Result
1	Process starts running	Request pages	Pages needed in memory
2	Frames available >= working set size?	Yes	Load pages into frames
3	Process runs	Access pages	Few page faults
4	Frames available < working set size?	Yes	Page faults increase
5	Page faults high	Thrashing occurs	Process slows down
6	OS tracks working set	Calculate pages used recently	Working set size determined
7	OS adjusts frames	Allocate frames to match working set	Page faults reduce
8	Process runs	Access pages	Runs smoothly again

💡 Thrashing stops when OS allocates enough frames to cover the working set size.

State Tracker

Variable	Start	After Step 3	After Step 5	After Step 7	Final
Frames available	Enough	Enough	Insufficient	Adjusted	Enough
Page faults	0	Low	High	Reduced	Low
Working set size	Unknown	Unknown	Tracked	Determined	Stable

Key Insights - 3 Insights

Why does thrashing cause the process to slow down?

Because the process keeps causing page faults and waiting for pages to load, it spends more time swapping pages than doing useful work (see execution_table step 5).

How does the working set model help prevent thrashing?

It tracks the pages the process uses recently and ensures enough frames are allocated to hold these pages, reducing page faults (see execution_table steps 6 and 7).

What happens if the OS does not adjust frames based on the working set?

The process will continue to thrash because it does not have enough frames to hold its needed pages, causing high page faults and slow execution (see execution_table steps 4 and 5).

Visual Quiz - 3 Questions

Look at the execution_table at which step does thrashing start?

A Step 5

B Step 4

C Step 3

D Step 6

💡 Hint

Check the 'Action' and 'Result' columns where 'Thrashing occurs' is mentioned.

According to variable_tracker, what happens to page faults after OS adjusts frames?

A Page faults increase

B Page faults stay high

C Page faults reduce

D Page faults become zero

💡 Hint

Look at the 'Page faults' row after Step 7 in variable_tracker.

If the working set size is underestimated, what is likely to happen?

A Process will run smoothly

B Thrashing will continue

C Frames will be over-allocated

D Page faults will be zero

💡 Hint

Refer to key_moments about the importance of matching frames to working set size.

Concept Snapshot

Thrashing happens when a process has too few memory frames, causing many page faults and slow execution.
The working set model tracks the pages a process uses recently.
OS allocates frames to cover the working set to reduce thrashing.
Adjusting frames based on working set size helps the process run smoothly.

Full Transcript

Thrashing occurs when a process does not have enough memory frames to hold the pages it needs, causing frequent page faults and slowing down the process. The working set model helps by tracking the pages the process uses recently and ensuring enough frames are allocated to hold these pages. When the OS detects thrashing, it calculates the working set size and adjusts the number of frames allocated to the process. This reduces page faults and allows the process to run smoothly again.