Thread Models

In a User-Level Thread environment, all thread management is done by application. Kernel is unaware of the existence of thread. Application uses a thread library for thread management (creation, destroy, pass data, scheduling and storing thread context). The application begins in a process with a single thread. Application spawn threads in the same process. The context of the thread consists of registers, program counter and stack pointer. The kernel schedule execution in the level of process.

Advantages of user level thread are:
(1) Thread switching completely in user mode and no context switching
(2) Differetn application can use different scheduling algorithm
(3) The threading model can run on any OS as there is no need for the kernel to support

Disadvantages of user level thread are:
(1) When ULT executes a system call, the process and all the thread will be blocked. A technique call jacketing which convert the blocking call to a non-blocking call. The jacket routine checks if the device is busy. If it is, the router will block the thread and pass control to another thread.
(2) As kernel schedule process to only 1 processor, ULT cannot take advantage of multiprocessor environment.

In a Kernel-Level Thread environment, thread management is done by kernel. Application create thread using a kernel API. The disadvantage is that transferring control from one thread to another requires switching from user to kernel mode. In a benchmark, kernel mode thread switching can be 30 times slower.