Question

my question:

1)In book modern operating system, it says the threads and processes can be in kernel mode or user mode, but it does not say clearly what’s the difference between them .

2)Why the switch for the kernel-mode threads and process costs more than the switch for user-mode threads and process?

3) now, I am learning Linux,I want to know how would I create threads and processes in Kernel mode and user mode respectively IN LINUX SYSTEM?

4)In book modern operating system, it says that it is possible that process would be in user- mode, but the threads which are created in the user-mode process can be in kernel mode. How would this be possible?

Answer 1

User-mode threads are scheduled in user mode by something in the process, and the process itself is the only thing handled by the kernel scheduler.

That means your process gets a certain amount of grunt from the CPU and you have to share it amongst all your user mode threads.

Simple case, you have two processes, one with a single thread and one with a hundred threads.

With a simplistic kernel scheduling policy, the thread in the single-thread process gets 50% of the CPU and each thread in the hundred-thread process gets 0.5% each.

With kernel mode threads, the kernel itself manages your threads and schedules them independently. Using the same simplistic scheduler, each thread would get just a touch under 1% of the CPU grunt (101 threads to share the 100% of CPU).

In terms of why kernel mode switching is more expensive, it probably has to do with the fact that you need to switch to kernel mode to do it. User mode threads do all their stuff in user mode (obviously) so there’s no involving the kernel in a thread switch operation.

Under Linux, you create threads (and processes) with the clone call, similar to fork but with much finer control over things.

Your final point is a little obtuse. I can’t be certain but it’s probably talking about user and kernel mode in the sense that one could be executing user code and another could be doing some system call in the kernel (which requires switching to kernel or supervisor mode).

That’s not the same as the distinction when talking about the threading support (user or kernel mode support for threading). Without having a copy of the book to hand, I couldn’t say definitively, but that’d be my best guess.