Difference between User Level thread and Kernel Level thread
Last Updated : 23 Aug, 2024
User-level threads are threads that are managed entirely by the user-level thread library, without any direct intervention from the operating system's kernel, whereas, Kernel-level threads are threads that are managed directly by the operating system's kernel. In this article, we will see the overview of the User Level thread and Kernel Level thread. and also understand the basic required terms.
User-Level Thread
The User-level Threads are implemented by the user-level software. These threads are created and managed by the thread library, which the operating system provides as an API for creating, managing, and synchronizing threads. it is faster than the kernel-level threads, it is basically represented by the program counter, stack, register, and PCB.
User-level threads are typically employed in scenarios where fine control over threading is necessary, but the overhead of kernel threads is not desired. They are also useful in systems that lack native multithreading support, allowing developers to implement threading in a portable way.
Example - User threads library includes POSIX threads, Mach C-Threads
Advantages of User-Level Threads
- Quick and easy to create: User-level threads can be created and managed more rapidly.
- Highly portable: They can be implemented across various operating systems.
- No kernel mode privileges required: Context switching can be performed without transitioning to kernel mode.
Disadvantages of User-Level Threads
- Limited use of multiprocessing: Multithreaded applications may not fully exploit multiple processors.
- Blocking issues: A blocking operation in one thread can halt the entire process.
Kernel-Level Thread
Threads are the units of execution within an operating system process. The OS kernel is responsible for generating, scheduling, and overseeing kernel-level threads since it controls them directly. The Kernel-level threads are directly handled by the OS directly whereas the thread’s management is done by the kernel.
Each kernel-level thread has its own context, including information about the thread's status, such as its name, group, and priority.
Example - The example of Kernel-level threads are Java threads, POSIX thread on Linuxs, etc.
Advantages of Kernel-Level Threads
- True parallelism: Kernel threads allow real parallel execution in multithreading.
- Execution continuity: Other threads can continue to run even if one is blocked.
- Access to system resources: Kernel threads have direct access to system-level features, including I/O operations.
Disadvantages of Kernel-Level Threads
- Management overhead: Kernel threads take more time to create and manage.
- Kernel mode switching: Requires mode switching to the kernel, adding overhead.
Difference Between User-Level Thread and Kernel-Level Thread
| Parameters | User Level Thread | Kernel Level Thread |
|---|
| Implemented by | User threads are implemented by user-level libraries. | Kernel threads are implemented by Operating System (OS). |
| Recognize | The operating System doesn't recognize user-level threads directly. | Kernel threads are recognized by Operating System. |
| Implementation | Implementation of User threads is easy. | Implementation of Kernel-Level thread is complicated. |
| Context switch time | Context switch time is less. | Context switch time is more. |
| Hardware support | No hardware support is required for context switching. | Hardware support is needed. |
| Blocking operation | If one user-level thread performs a blocking operation then the entire process will be blocked. | If one kernel thread performs a blocking operation then another thread can continue execution. |
| Multithreading | Multithreaded applications cannot take full advantage of multiprocessing. | Kernels can be multithreaded. |
| Creation and Management | User-level threads can be created and managed more quickly. | Kernel-level level threads take more time to create and manage. |
| Operating System | Any operating system can support user-level threads. | Kernel-level threads are operating system-specific. |
| Thread Management | Managed by a thread library at the user level. | The application code doesn't contain thread management code; it's an API to the kernel mode. |
| Example | POSIX threads, Mach C-Threads. | Java threads, POSIX threads on Linux. |
| Advantages | Simple and quick to create, more portable, does not require kernel mode privileges for context switching. | Allows for true parallelism, multithreading in kernel routines, and can continue execution if one thread is blocked. |
| Disadvantages | Cannot fully utilize multiprocessing, entire process blocked if one thread blocks. | Requires more time to create/manage, involves mode switching to kernel mode. |
| Memory management | In user-level threads, each thread has its own stack, but they share the same address space. | In kernel-level threads have their own stacks and their own separate address spaces, so they are better isolated from each other. |
| Fault tolerance | User-level threads are less fault-tolerant than kernel-level threads. If a user-level thread crashes, it can bring down the entire process. | Kernel-level threads can be managed independently, so if one thread crashes, it doesn't necessarily affect the others. |
| Resource utilization | Limited access to system resources, cannot directly perform I/O operations. | It can access to the system-level features like I/O operations. |
| Portability | User-level threads are more portable than kernel-level threads. | Less portable due to dependence on OS-specific kernel implementations.
|
Conclusion
Understanding the differences between user-level threads and kernel-level threads is crucial for selecting the appropriate threading model for your application. While user-level threads offer simplicity and portability, they may not fully utilize multiple processors. Kernel-level threads, with better fault tolerance and direct access to system resources, are better suited for complex, performance-intensive applications.