Thrashing and techniques to handle thrashing

 Let's break down the concepts of thrashing, the working set model, and the page fault frequency approach in simpler terms:

1. Thrashing in Virtual Memory:

Thrashing occurs when the computer system spends most of its time dealing with page faults (swapping data between main memory and secondary storage) rather than performing actual processing. This happens because the system is overloaded with too many processes, and there are not enough memory frames available to handle all the data needed by these processes efficiently.

Causes of Thrashing:

- High degree of multiprogramming: Too many processes are loaded into memory simultaneously.

- Lack of frames: There are not enough memory frames available to accommodate all the active processes.

- Page replacement policy: The method used to replace pages in memory can also contribute to thrashing.

2. Working Set Model:

The working set model is a way to prevent thrashing by managing the allocation of memory frames to processes based on their current locality. Locality refers to the tendency of a process to access a group of pages together. The working set of a process includes the pages that it has recently accessed.

The idea is to allocate enough frames to a process to hold its current locality. If the process has fewer frames than needed for its locality, it will experience frequent page faults and thrashing. If it has more frames than needed, other processes may suffer from insufficient frames.

3. Page Fault Frequency Approach:

This approach directly addresses the page fault rate to handle thrashing. The page fault rate indicates how often a process experiences page faults. If the rate is too high, it suggests that the process has too few frames allocated to it. Conversely, a very low page fault rate may indicate excessive frame allocation.

To avoid thrashing, an upper and lower limit is established for the desired page fault rate. If the rate goes above the upper limit, more frames are allocated to the process to reduce page faults. If the rate falls below the lower limit, some frames can be taken away from the process to free up resources.

In summary, thrashing is a situation where the computer system is overwhelmed with page faults, causing little actual work to be done. The working set model and the page fault frequency approach are techniques to prevent thrashing by managing memory allocation more effectively based on process locality and page fault rates. These methods help ensure that the system runs efficiently without getting stuck in a cycle of constant page swapping.