Subject: Operating System

Topic: PROCESS MANAGEMENT

Difficulty: Medium

A thread is a flow of execution through the process code, with its own program counter that keeps track of which instruction to execute next, system registers which hold its current working variables, and a stack which contains the execution history.

A thread is also called a lightweight process. Threads provide a way to improve application performance through parallelism. Threads represent a software approach to improving performance of operating system by reducing the overhead thread is equivalent to a classical process.

Each thread belongs to exactly one process and no thread can exist outside a process. Each thread represents a separate flow of control. Threads have been successfully used in implementing network servers and web server. They also provide a suitable foundation for parallel execution of applications on shared memory multiprocessors. The following figure shows the working of a single-threaded and a multithreaded process.

Threads are implemented in following two ways −

User Level Threads − User managed threads.

Kernel Level Threads − Operating System managed threads acting on kernel, an operating system core.

User Level Threads

In this case, the thread management kernel is not aware of the existence of threads. The thread library contains code for creating and destroying threads, for passing message and data between threads, for scheduling thread execution and for saving and restoring thread contexts. The application starts with a single thread.

Advantages

Thread switching does not require Kernel mode privileges.

User level thread can run on any operating system.

Scheduling can be application specific in the user level thread.

User level threads are fast to create and manage.

Disadvantages

In a typical operating system, most system calls are blocking.

Multithreaded application cannot take advantage of multiprocessing.

Kernel Level Threads

In this case, thread management is done by the Kernel. There is no thread management code in the application area. Kernel threads are supported directly by the operating system. Any application can be programmed to be multithreaded. All of the threads within an application are supported within a single process.

The Kernel maintains context information for the process as a whole and for individuals threads within the process. Scheduling by the Kernel is done on a thread basis. The Kernel performs thread creation, scheduling and management in Kernel space. Kernel threads are generally slower to create and manage than the user threads.

Advantages

Kernel can simultaneously schedule multiple threads from the same process on multiple processes.

If one thread in a process is blocked, the Kernel can schedule another thread of the same process.

Kernel routines themselves can be multithreaded.

Disadvantages

Kernel threads are generally slower to create and manage than the user threads.

Transfer of control from one thread to another within the same process requires a mode switch to the Kernel.