3. User thread - Threads can also be built in user space. This means that a library or program is responsable for scheduling and executing threads. When this is done in user space, there is still a penalty for a context switch. However, the cost is less than an operating system context switch. Sometimes, user space threads are called fibers, to suggest that they are "lighter" than kernel threads. For the remainder of this article, I will refer to user space threads as fibers, and kernel threads as threads. Fibers have an additional advantage over kernel threads: Only one thread can modify a shared resource at a time, since only one fiber can be executing at a time.