If a page is no longer being used, it will drop out of the working set Δ time units after its last reference.

Thus, the working set is an approximation of a program’s locality. The accuracy of the working set depends heavily on the choice of Δ:

If Δ is too small, it will not encompass the entire locality.

If Δ is too large, it will overlap several localities

If Δ is infinite, the working set becomes the set of all pages accessed during the entire execution of the process.

The working set size for a process i is denoted as WSSi, and the total demand for frames can be expressed as:

D = Σ WSSi D - total demand for frames

Each process is actively using the pages in its working sets. Thus the process needs WSSi frames.
If there are not enough frames to fulfill demand then thrashing will occur because some processes will not have enough frames.
The working set strategy prevents thrashing while keeping the degrees of multiprogramming as high as possible. Thus it optimizes CPU utilization.
It is difficult to keep track of the working set. The working set window is a moving window. At each memory reference, a new reference appears at one end and the oldest reference drops off at the other end.

Thread

A thread is the basic unit of CPU utilization. A thread consists of a thread ID, a program counter, a register set, and a stack. It shares its code section, data section, and other operating resources such as open files and signals with other threads belonging to the same process.

A single thread is used for a traditional (heavyweight) process. If a process has multiple threads of control, it can perform more than one task at a time.

Several software packages that are running on modern PCs are multithreaded. An application is implemented as a separate process with several threads of control.

Eg. A word processor may have a thread for displaying graphics, another thread for responding to keystrokes from the user, and a third thread for performing spelling and grammar checking.

There is a requirement for a single application to perform several similar tasks. Eg: A web server accepts client requests for web pages, images, etc. A web server may have several clients concurrently accessing it. If the webserver runs as a single-threaded process, it would be able to service only one client at a time. So, other clients may have to wait for their requests to be served.

One of the solutions is to have the server run as a simple process that accepts requests. So what happens is that when the server receives a request, it creates a separate process to service that request. This process creation is time-consuming and resource-intensive.

The solution that can be used is to use one process that contains multiple threads. This is known as a multithreaded process. In this, the webserver would create a separate thread that would listen for client requests rather than create another process.

Benefits:

Responsiveness: It