Process

• The process is something that is currently under execution. So, an active program can be called a Process.
• The execution of a process must follow a sequential order.
• The process is not as same as the program but it is very different from it.
• A process is an 'active' entity as opposed to the program which is considered to be a 'passive' entity.
• A process can be defined as an entity that represents the basic unit of work to be implemented in the system.
• For example, search something on the Internet using a browser, starting a video player to watch a video.

Thread

• A thread is also called a Lightweight Process.
• Each thread belongs to exactly one process and no thread can exist outside a process.
• But, one process can contain multiple threads.
• Each thread represents a separate flow of control, thread is a path of execution within a process.
• A thread is a flow of execution through the process code, with its program counter that keeps track of which instruction to execute next, system registers that hold its current working variables, and a stack that contains the execution history.
• Threads are generally used to improve the application through Parallelism.
• In the operating system, there are two types of threads:
  • Kernel-level thread
  • User-level thread

Advantages and Disadvantages of Implementing Threads in Kernel Space

Advantages:

• Multiple threads of the same process can be scheduled on different processors.
• The routines of the kernel can also be multithreaded.
• If a kernel-level thread is blocked, another thread of the same process can be scheduled by the kernel.
• These threads are fully aware of all other threads.
• Hence, Scheduler may decide to give more time to a process having a large number of threads than a process having a small number of threads. -These threads are good for applications that frequently block.

Disadvantages:

• Kernel manages and schedules threads as well as processes.
• This requires a full Thread Control Block (TCB) for each thread to maintain information about threads.
• Therefore, this creates significant overhead and increases complexity.
• The implementation of threads at kernel space is more difficult and slow than implementing threads at user space.
• These threads are slow and inefficient. Like operations of threads are hundreds of times slower than that of user-level threads.
• A mode switch to kernel mode is required to transfer control from one thread to another in a process.

Advantages and Disadvantages of Implementing Threads in User Space

Advantages:

• Implementing threads at user space is easier and faster to create than threads at the kernel level.
• They can also be more easily managed.
• These threads can be run on any operating system.
• It does not need any support from the kernel-level threads.
• The overall process is quick and effective because there is no requirement for a system call.
• No need for the kernel-mode privileges for the thread to switch in user space.
• The organization is straightforward because the threads are produced, switched, and controlled without interference from the kernel.
• Do not require modification to operating systems.

Disadvantages:

• It has poor scheduling due to a thread holding a lock.
• These threads are invisible to the OS. Hence, they are not well integrated with the OS.
• The entire process is blocked if one user-level thread performs a blocking operation.
• There is a lack of coordination between threads and the kernel.
• These threads require non-blocking systems call like a multithreaded kernel. Otherwise, the entire process will be blocked in the kernel.
• One thread causes a page fault, the process blocks.
• Multithreaded applications cannot use multiprocessing to their benefit.