Multithreading Models - One to Many model

Question

I've been reading the dinosaur book and have been confused by this particular model.

The books says that for the one to many model "Thread management is done by the thread library in user space, so it is efficient; but the entire process will block if a thread makes a blocking system call. Also, because only one thread can access the kernel at a time, multiple threads are unable to run in parallel on multiprocessors"

What I'm confused about is what is meant by an entire process will block if a blocking system call is made? Does this mean if I have a multi-threaded program and one of it's threads blocks then all of its threads will have to wait, effectively stalling the program?

If a program undergoing execution causes a block with this model does it mean that another separate program can't be swapped in to be executed because the kernel thread is blocking? If that answer is YES another program(process) could be swapped in than why couldn't a multi-threaded program simply execute another one of its threads while the blocking thread is forced to wait?

Answer 1

If you manage your threads in user level, it means that the swapping is done by your application, not by OS scheduler. Each thread must reach some point where he surrenders (or loses) the control to the management mechanism, but that mechanism is also user-level, so if one of the threads is in the middle of doing a system call - your thread management system (and through that all the other threads) must wait until the kernel code is done.

The OS is still active all the time, and may still preempt the entire program, so other processes will not starve, only the internal "threads" you manage yourself. These threads can't get started during that block because the mechanism responsible of starting them is also blocked by the kernel.