Question

how do the user-level threads map to the kernel threads ?

Answer 1

Please focus on the Bold and Underlined parts of the answer.

> What is a Thread ?

A thread is a flow of execution through the process code, with its own program counter that keeps track of which instruction to execute next, system registers which hold its current working variables, and a stack which contains the execution history.A thread is also called a Lightweight Process.

> There are two types of threads to be managed in a modern system :

User threads

Kernel threads.

> How do the user-level threads map to the kernel threads ?

There are two types of processes: user and kernel(OS). Each process can have single or multiple threads. The threads of a user process are called a user-level thread and the threads of a kernel process are called kernel-level threads. Scheduler, dispatcher etc. contains kernel-level threads. Both user-level threads and kernel-level threads can able to run on CPU.

There may be several user-processes in ready queue for execution, scheduler(kernel) decides which user process/ thread to run on CPU, dispatcher(kernel) gives control to the corresponding user-thread on CPU, the corresponding user-thread executes and terminates.

To run on a CPU, user-level threads must ultimately mapped to an associated kernel-level thread.

In general, user-level threads can be implemented using one of four models.

• Many-to-one
• One-to-one
• Many-to-many
• Two-level

All models maps user-level threads to kernel-level threads. A kernel thread is similar to a process in a non-threaded (single-threaded) system. The kernel thread is the unit of execution that is scheduled by the kernel to execute on the CPU.

Many-to-one

In the many-to-one model all user level threads execute on the same kernel thread. The process can only run one user-level thread at a time because there is only one kernel-level thread associated with the process.

One-to-one

In the one-to-one model every user-level thread execute on a separate kernel-level thread.

Many-to-many

In the many-to-many model the process is allocated m number of kernel-level threads to execute n number of user-level thread.

Please find the related images for more understanding.
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