GATE CS 2014 Set 1 Q.30 (CS Q.20): Processes, Threads and System Calls

Which one of the following is FALSE?

User level threads are not scheduled by the kernel.

When a user level thread is blocked, all other threads of its process are blocked.

Context switching between user level threads is faster than context switching between kernel level threads.

Kernel level threads cannot share the code segment.

Official answer and solution

Solution

Recall the fundamental distinctions between user-level threads and kernel-level threads regarding scheduling, blocking, and resource sharing.

Let us examine the options:

(A) True: The OS kernel is unaware of user-level threads; they are managed and scheduled entirely by a user-space thread library without kernel involvement.
(B) True: In a many-to-one user-level threading model, if one user-level thread makes a blocking system call, the entire kernel process blocks, thereby blocking all peer threads.
(C) True: Context switching between user-level threads does not require trapping into kernel mode, saving registers and context switch overhead, making it significantly faster than kernel-level thread context switching.
(D) False: All threads belonging to the same process (whether user-level or kernel-level) share the process address space, including the code segment (text), data segment, and open file descriptors. Therefore, kernel-level threads of the same process certainly can share the code segment.

Hence, statement D is FALSE.

Final answer

Common trap

Assuming kernel-level threads have separate address spaces like separate processes, whereas threads within a process share the same code and data.

The answer is the one in the official GATE answer key. The solution is this site's own explanation.

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