User-level threads apply in user-level libraries, before via systems calls, so thread switching does not require to call operating system and to cause interrupt to the kernel. Actually, the kernel knows nothing about user-level threads and handles them as if they were single-threaded processes.
Advantages of User-Level Threads:
1. The most obvious benefit of this technique is that a user-level threads package can be implemented on an Operating System that does not support threads. Some other advantages are
2. A user-level thread does not require alteration to operating systems.
3. Straightforward Representation:
Each thread is represented simply by a registers, PC, stack and a small control block, all stored in the user process address space.
4. Fast and well-organized:
Thread switching is not much more costly than a procedure call.
5. Unproblematic Management:
This basically means that making a thread, switching between threads and synchronization between threads can all be done lacking of intervention of the kernel.
Disadvantages of User-Level Threads:
1. There is a short of coordination between threads and operating system kernel. Consequently, process as whole gets one time slice irrespective of whether process has one thread or 1000 threads within. It is up to every thread to relinquish control to other threads.
2. A user-level thread needs non-blocking systems call that means a multithreaded kernel. Or else, entire process will blocked in the kernel, even if there are run able threads left in the processes. For instance, if one thread causes a page fault, the process blocks.