Unix Threads Question:

User-space threads live without any support from the
kernel; they maintain all of their state in user space.
Since the kernel does not know about them, they cannot be
scheduled to run on multiple processors in parallel.
Kernel-supported threads fall into two classes.

In a "pure" kernel-supported system, the kernel is
responsible for scheduling all threads.

Systems in which the kernel cooperates with a user-level
library to do scheduling are known as two-level, or hybrid,
systems. Typically, the kernel schedules LWPs, and the user-
level library schedules threads onto LWPs.
Because of its performance problems (caused by the need to
cross the user/kernel protection boundary twice for every
thread context switch), the former class has fewer members
than does the latter (at least on Unix variants). Both
classes allow threads to be run across multiple processors
in parallel.