Definition:
The ability of an operating system to execute different parts of a program, called threads, simultaneously. The programmer must carefully design the program in such a way that all the threads can run at the same time without interfering with each other.

Multithreading is a specialized form of multitasking. In general, there are two types of multitasking: process-based and thread-based. A process is, in essence, a program that is executing. Thus, process-based multitasking is the feature that allows your computer to run two or more programs concurrently. For example, it is process-based multitasking that allows you to run a word processor at the same time you are using a spreadsheet or browsing the Internet. In process-based multitasking, a program is the smallest unit of code that can be dispatched by the scheduler.

A thread is a dispatchable unit of executable code. The name comes from the concept of a “thread of execution.” In a thread-based multitasking environment, all processes have at least one thread, but they can have more. This means that a single program can perform two or more tasks concurrently. For instance, a text editor can be formatting text at the same time that it is printing, as long as these two actions are being performed by two separate threads. The differences between process-based and thread-based multitasking can be summarized like this: Process-based multitasking handles the concurrent execution of programs. Thread-based multitasking deals with the concurrent execution of pieces of the same program.

Multithreading Changes the Architecture of a Program

Multithreading changes the fundamental architecture of a program. Unlike a single-threaded program that executes in a strictly linear fashion, a multithreaded program executes portions of itself concurrently. Thus, all multithreaded programs include an element of parallelism. Consequently, a major issue in multithreaded programs is managing the interaction of the threads.

As explained earlier, all processes have at least one thread of execution, which is called the main thread. The main thread is created when your program begins. In a multithreaded program, the main thread creates one or more child threads. Thus, each multithreaded process starts with one thread of execution and then creates one or more additional threads. In a properly designed program, each thread represents a single logical unit of activity.

The principal advantage of multithreading is that it enables you to write very efficient programs because it lets you utilize the idle time that is present in most programs. Most I/O devices, whether they are network ports, disk drives, or the keyboard, are much slower than the CPU. Often, a program will spend a majority of its execution time waiting to send or receive data. With the careful use of multithreading, your program can execute another task during this idle time. For example, while one part of your program is sending a file over the Internet, another part can be reading keyboard input, and still another can be buffering the next block of data to send.

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J. Jeyaseelan
Everything Possible