Memory Management

Operating Systems
└── Memory Management
    └── Virtual Memory
        ├── Paging
        ├── Demand Paging
        ├── Page Replacement
        ├── Thrashing
        └── Working Set Model

Working Set Model

1. Principle of Locality

Programs do not access all memory randomly.

They usually access:

• the same data repeatedly (Temporal Locality) ⭐
• nearby memory locations (Spatial Locality) ⭐

This behavior is called the Principle of Locality.

2. Working Set

The working set is: The set of pages currently being actively used by a process during a time interval.

Notation:

$W(t, \Delta)$ Where:

• $t$ = current time
• $\Delta$ = working set window

It contains all pages referenced in the last $\Delta$ time units.

3. Purpose of Working Set Model

The OS keeps the working set pages in memory to:

• reduce page faults
• improve performance
• prevent thrashing

4. Thrashing

Thrashing occurs when:

• too many page faults happen
• CPU spends more time swapping pages than executing processes

Cause:

• insufficient frames allocated to processes

5. Relation Between Working Set and Thrashing

• If all working set pages fit in RAM → system works efficiently
• If working sets do not fit → thrashing occurs

Thus:

Working Set Model is used to control/prevent thrashing.

6. Important Exam Point

Concept	Related To
Working Set Model	Principle of Locality
Thrashing	Excessive Paging
Paging	Virtual Memory
Segmentation	Memory division by logical units

Important One-Liners

• Working set model is based on locality of reference.
• Working set helps prevent thrashing.
• Thrashing increases page faults and reduces CPU utilization.
• Working set is used in virtual memory management.