A thread is a single sequence stream within a process. Threads have same properties as of the process so they are called as light weight processes². Threads are executed one after another but gives the illusion as if they are executing in parallel¹. Each thread has different states. Each thread has:

• A program counter

• A register set

• A stack space

Threads are not independent of each other as they share the code, data, OS resources etc².

There are two types of threads in operating system:

• User-level threads: These threads are created and managed by the user without any intervention from the operating system. The user-level threads are implemented by libraries that provide the thread creation, scheduling, and management functions. The user-level threads are independent of each other and can run on different processors. The user-level threads have some advantages and disadvantages²: Advantages: The thread switching does not require any system call, so it is fast and efficient. The user can design their own scheduling algorithm for the threads. The user-level threads can run on any operating system that supports the thread library. Disadvantages: If one thread performs a blocking operation, such as I/O, the entire process will be blocked, as the operating system is unaware of the existence of other threads. The user-level threads cannot take advantage of the multiprocessing, as the operating system assigns only one processor to each process.

• Kernel-level threads: These threads are created and managed by the operating system. The kernel-level threads are supported directly by the operating system, which provides the system calls for thread creation, scheduling, and management. The kernel-level threads can run concurrently on multiple processors. The kernel-level threads have some advantages and disadvantages²: Advantages: If one thread performs a blocking operation, the operating system can switch to another thread within the same process, or to a thread in another process. The kernel-level threads can take advantage of the multiprocessing, as the operating system can schedule threads on different processors. Disadvantages: The thread switching requires a system call, which involves the mode switch from user mode to kernel mode, so it is slow and costly. The kernel-level threads have less flexibility, as the user cannot design their own scheduling algorithm for the threads.