In most UNIX systems, the process is the unit of execution. In the SunOS 5.7 system, a thread is the unit of execution. A thread is a sequence of instructions executed within a program. A process consists of one or more threads. There are two types of threads: application threads, which run in user space, and kernel threads, which run in kernel space.
The kernel is multithreaded (MT). Many kernel threads can be running kernel code, and may be doing so concurrently on a multiprocessor (MP) machine. Kernel threads may also be pre-empted by other kernel threads at any time. This is a departure from the traditional UNIX model where only one process can run kernel code at any one time, and that process is not pre-emptable (though it is interruptible).
The multithreading of the kernel imposes some additional restrictions on the device drivers. For more information on multithreading considerations, see Chapter 4, Multithreadingand Appendix G, Advanced Topics.