Which of the following multithreading model is followed in Linux OS?
One User Thread mapping to One Kernel Thread
Many User Threads mapping to One Kernel Thread
One User Thread mapping to Many Kernel Threads
Many User Threads mapping to Many Kernel Threads
ANSWER
Linux follows the one-to-one model: one user thread maps to one kernel thread, option (a).
Official answer: ASource: ISRO's revised and final answer key of 16 February 2024
A multithreading model fixes how user-level threads are bound to kernel-schedulable entities. The three common models are many-to-one, one-to-one and many-to-many.
In Linux, a thread made through the standard POSIX thread library (NPTL) is created with the clone() system call. Each thread gets its own kernel task, with its own task structure and scheduling entity.
So every user thread corresponds to exactly one kernel thread. This is the one-to-one model, which Windows also uses.
Option (b) is the many-to-one model, used by old green-thread libraries. It cannot use several processors, and one blocking call blocks every thread. Linux does not use it.
Option (c) does not describe a standard model, because a single user thread is not split across several kernel threads.
Option (d) is the many-to-many model, found in older Solaris and some other older systems. Linux does not use it.
Therefore Linux follows the one-to-one model, option (a).
KEY IDEA
Linux creates every thread with clone(), and each thread is a separate kernel task that the scheduler sees. That makes the threading model one-to-one.
WATCH OUT FOR
Picking many-to-many because it sounds the most flexible, or many-to-one because user-level libraries are lightweight. Linux threads are scheduled individually by the kernel, so neither applies.
From the ISRO CS 2023 paper. Every question of this paper · The full paper (PDF, ISRO) · All 3 ISRO Processes & threads questions, with notes
