Confusion regarding the Blocking of "peer threads" when a user-level thread blocks

Question

I was reading about differences between threads and processes, and literally everywhere online, one difference is commonly written without much explanation:

If a process gets blocked, remaining processes can continue execution. If a user level thread gets blocked, all of its peer threads also get blocked.

It doesn't make any sense to me. What would be the sense of concurrency if a scheduler cannot switch between a blocked thread and a ready/runnable thread. The reason given is that since the OS doesn't differentiate between the various threads of a given parent process, it blocks all of them at once.

I find it very unconvincing, since all modern OS have thread control blocks with a thread ID, even if it is valid only within the memory space of the parent process. Like the example given in Galvin's Operating Systems book, I wouldn't want the thread which is handling my typing to be blocked if the spell checking thread cannot connect to some online dictionary, perhaps.

Either I am understanding this concept wrong, or all these websites have just copied some old thread differences over the years. Moreover, I cannot find this statement in books, like Galvin's or maybe in William Stalling's COA book where threads have been discussed.

These are resouces where I found the statements:

• https://www.geeksforgeeks.org/difference-between-process-and-thread/
• https://www.tutorialspoint.com/difference-between-process-and-thread
• https://www.guru99.com/difference-between-process-and-thread.html
• https://www.javatpoint.com/process-vs-thread

Answer 1

There is a difference between kernel-level and user-level threads. In simple words:

• Kernel-level threads: Threads that are managed by the operating system, including scheduling. They are what is executed on the processor. That's what probably most of us think of threads.
• User-level threads: Threads that are managed by the program itself. They are also called fibers or coroutines in some contexts. In contrast to kernel-level threads, they need to "yield the execution", i.e. switching from one user-level to another user-level thread is done explicitly by the program. User-level threads are mapped to kernel-level threads.

As user-level threads need to be mapped to kernel-level threads, you need to choose a suiteable mapping. You could map each user-level to a separate kernel-level thread. You could also map many user-level to one kernel-level thread. In the latter mapping, you let multiple concurrent execution paths be executed by a single thread "as we know it". If one of those paths blocks, recall that user-level threads need to yield the execution, then the executing (kernel-level) thread blocks, which causes all other assigned paths to also be effectively blocked. I think, this is what the statement refers to. FYI: In Java, user-level threads – the multithreading you do in your programs – are mapped to kernel-level threads by the JVM, i.e. the runtime system.

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