The traditional UNIX process model – Everything is a process; if it's not a process, it's a file – has a lot going for it. The very fact that it is still the model followed by operating systems after a span of nearly five decades amply validates this. Of course, nowadays, the thread is important; a thread is merely an execution path within a process. Threads share all process resources, including the user VAS, except for the stack. Every thread has its own private stack region (this makes perfect sense; if not, how could threads truly run in parallel, as it's the stack that holds execution context).
The other reason we focus on the thread and not the process is made clearer in Chapter 10, The CPU Scheduler, Part 1. For now, we shall just say this: the thread, not the process, is the kernel...
