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Introduction
In the ever-evolving world of computer science, the efficient management of concurrent operations is paramount to maximizing the performance and responsiveness of applications. Two fundamental concepts in this arena are user-level threads and kernel-level threads. While both types of threads enable multitasking by allowing multiple sequences of programmed instructions to run concurrently, they differ significantly in their implementation, performance characteristics, and use cases.
A process is the instance of a program executed by one or many threads.
On the other hand, a thread is a basic unit of CPU utilization. It consists of its thread ID, a program counter, a register, and a stack. With the help of threads, a program can be split into two or more concurrently running tasks.
A single-threaded process consists of only one thread, whereas a multi-threaded process consists of two or more threads running concurrently, sharing resources. This multithreading is achieved using time-slicing, where a single processor switches between multiple threads.
In multiprocessor or multi-core systems, threading is achieved by multiprocessing. Both time-sliced and multiprocessor threading are supported by modern operating systems today. The kernel allows programmers to manipulate threads via system call; such threads are called ‘kernel-level threads. Threads can also be implemented by users, and such threads are known as ‘user-level threads. The kernel is unaware of the existence of these threads.
A process with three threads
Check out this blog here to learn the fundamental concepts related to threads.
What are User Level Threads?
User-level threads are managed without kernel support by the run-time system and are supported above the kernel. The kernel does not know anything about the user-level threads. It treats them as if they are single-threaded processes.
Advantages:
User-level threads are fast to create and efficient.
User-level threads are easier to manage than kernel-level threads.
Each thread is represented by a PC, register, stack, and control block. All of these are stored in the user process address space.
The switching between threads takes the same time as a procedural call.
Everything can be done without the interference of the OS.
User-level threads are generic and can run on any Operating System.
Disadvantages:
OS is unaware of user-level threads, so the scheduler cannot schedule them properly.
The entire process will get blocked if one user-level thread performs a blocking operation.
Multi-processor applications in user-level threads cannot use multiprocessing to their full advantage.
What are Kernel Level Threads?
Kernel-level threads are supported and managed by the operating system. No runtime system is required in the case of this type of thread. The kernel has a thread table that keeps track of all threads in the system, rather than having a thread table in each process. In addition to that, the kernel keeps track of processes using a traditional process table. The kernel includes system calls for creating and managing threads.
Advantages:
The kernel is fully aware of kernel-level threads, so the scheduler handles the process better.
The kernel can still schedule another thread for execution if one thread is blocked.
This type of thread is suitable for applications that are frequently blocked.
Multi-processor applications in kernel-level threads can fully utilize multiprocessing to their advantage.
Disadvantages:
It is slower to create and inefficient.
The overhead associated with kernel-level threads requires a thread control block.
Kernel-level threads are not generic and are specific to the Operating System.
Kernel-level threads are not as easy to manage as user-level threads.
What is the fundamental difference between process and program?
A program is a passive entity that stores a group of instructions to be executed. A process, on the other hand, is an active entity. A program is known as a process when it is running/executing.
What is paging?
Paging is a memory management scheme for OS. It allows OS to retrieve processes from secondary memory to main memory in the form of pages. It eliminates the need for contiguous physical memory allocation.
How are user threads mapped to kernel threads?
User threads can be mapped to kernel threads in three ways: one-to-one, many-to-one, and many-to-many. One-to-one maps each user thread to a kernel thread, many-to-one maps multiple user threads to one kernel thread, and many-to-many allows multiple user threads to correspond to multiple kernel threads.
Mention a few benefits of multithreaded programming.
A few benefits of multithreaded programming are as follows,
Increased responsiveness to the user.
Resource sharing between threads allows a multithreaded application to have several threads of activity within the same address space.
Increased concurrency in a multiprocessor machine.
Lesser cost.
Lesser context switch time.
What is multitasking?
In computing, multitasking refers to the concurrent execution of multiple tasks or processes over a certain period of time.
Conclusion
In this blog, we discussed threads and their types in Operating Systems. We talked in detail about the user-level and kernel-level threads.
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