What is Thread in OS? Types of Threads in OS (Operating System)

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What is a Thread?

Thread is the smallest unit of execution within a process, enabling concurrent operations. Threads share the same memory space as the parent process that is allowing them to execute tasks individually. They enhance efficiency by handling multiple tasks simultaneously, making them fundamental to multitasking and parallel processing in modern computing.

What is Thread in OS?

In operating systems, a thread refers to the smallest sequence of programmed instructions that can be managed independently by the scheduler. Threads are part of a process and share its resources, including memory and file descriptors, but have their own execution context. They enable concurrent execution, allowing multiple tasks to run simultaneously within a single process.

What is Thread and its Types in OS

Threads improve system performance by reducing overhead and enabling efficient multitasking. They facilitate parallelism, allowing tasks to be divided into smaller units of work that can be executed concurrently on multi-core processors, thereby maximizing CPU utilization and overall system efficiency.

Article Hot Headlines:

In this section, we will show you all headlines about this entire article; you can check them as your choice; below shown all:

  1. What is a Thread?
  2. What is Thread in OS?
  3. Why Do We Need Thread in OS?
  4. Why Multithreading is Needed in Operating Systems?
  5. Difference Between Process and Thread
  6. Components of Threads
  7. Issues with Threading in OS
  8. Benefits of Threading in OS
  9. Types of Threads in Operating System
  • User Level Thread (ULT) in OS
  • Kernel Level Thread (KLT) in OS
  • Hybrid Threads in OS
  • Many-to-One Threads in OS
  • One-to-One Threads in OS
  • Many-to-Many Threads in OS

Let’s Get Started!!

Why Do We Need Thread in OS?

Threads are essential in operating systems for several reasons:

Concurrency: Threads enable multiple tasks to run concurrently within a single process. This allows for better resource utilization and improves the system’s responsiveness to handle multiple tasks simultaneously.

Responsiveness: Without threads, a single task might block the entire process, leading to unresponsiveness. Threads can prevent this issue by allowing other tasks to continue execution while one thread is waiting for a resource or performing a time-consuming operation.

Efficiency: Threads reduce the overhead associated with creating and managing processes since they share the same memory space. This leads to lower memory consumption and faster communication between threads, promoting system efficiency.

Scalability: By leveraging threads, applications can be designed to scale better on multi-processor systems that are taking advantage of the increasing number of cores available on advanced hardware.

Why Multithreading is Needed in Operating Systems?

Multithreading is needed in operating systems to enhance efficiency and responsiveness. By enabling multiple threads within a process, the OS can efficiently execute concurrent tasks, preventing one task from blocking the entire process.

This allows for better resource utilization and responsiveness, as other threads can continue execution while one thread waits for a resource or performs a time-consuming operation. Multithreading also facilitates parallelism on multi-core processors, maximizing CPU utilization and improving overall system performance.

Hence, multithreading is crucial in modern operating systems to optimize resource management, enhance responsiveness, and take advantage of the processing power offered by multi-core hardware.

Difference Between Process and Thread

Processes and threads are both fundamental concepts in modern operating systems, and they play crucial roles in executing programs and managing resources. Here are the main differences between processes and threads, including:





An independent program unitA part of a process that shares resources

Resource Allocation

Owns its own resourcesShares resources of the parent process


Inter-process communication (IPC)Direct communication within the process

Creation Overhead

Heavier               Lighter

Memory Space

Each process has its own memory spaceThreads share the same memory space
Context SwitchingSlower due to separate memory spaceFaster due to shared memory space


More isolated and less concurrentMore concurrent and can synchronize easily

Fault Isolation

One process’s crash doesn’t affect othersOne thread’s crash can crash the whole process


More heavyweight, less scalableLightweight, can be more scalable

Cooperation and Synchronization

Require more explicit communication and synchronizationEasier cooperation and synchronization

Components of Threads

Threads consist of several key components, like as:

Thread ID: A unique identifier assigned to each thread within a process, used for management and identification purposes.

Program Counter: A register that keeps track of the address of the currently executing instruction, enabling the thread’s execution to be resumed after interruptions.

Registers: Thread-specific registers that hold the thread’s local variables and execution state.

Stack: A separate stack for each thread that stores function calls, return addresses, and local variables.

Thread-Specific Data: Memory space dedicated to each thread, allowing them to maintain thread-local variables.

Thread Control Block (TCB): A data structure containing essential information about the thread, managed by the operating system.

Issues with Threading in OS

  • Threads competing for shared resources may lead to inefficiencies and performance bottlenecks.
  • Managing thread access to shared data requires careful synchronization to avoid data inconsistencies and potential deadlocks.
  • Concurrent threads can interfere with each other’s execution, causing unexpected behaviours.
  • Some threads might be given preferential treatment, leading to starvation of others.
  • Developing and debugging threaded applications is more intricate due to non-deterministic behaviour and potential race conditions.
  • Frequent context switching between threads introduces overhead, impacting overall system performance.
  • Efficient thread management and synchronization mechanisms are essential to overcome these issues and achieve optimal performance in threaded OS applications.

Benefits of Threading in OS

  • Enables multiple tasks to run simultaneously within a single process
  • Prevents one task from blocking the entire process, improving overall system responsiveness
  • Threads share the same memory space, reducing memory overhead and facilitating efficient resource utilization.
  • Utilizes multi-core processors, dividing tasks into smaller units for faster execution
  • Allows applications to take advantage of increasing core counts in modern hardware
  • Reduces the overhead associated with creating and managing multiple processes
  • Facilitates task partitioning, allowing for finer-grained control over operations
  • Threads within the same process can communicate easily through shared memory.
  • Different threads can handle different aspects of a task, leading to more maintainable code.
  • Threads enable the UI to remain responsive while handling background tasks.

Types of Threads in Operating System

Threads in the operating system are divided into many types that are depending on several thread models. These models define about how they are managed and scheduled along with the operating system. Here, we will cover different types of threads and its pros and cons; below shown each one, you can read them:

User Level Thread (ULT) in OS

User-level thread refers to a thread managed entirely by the user-level thread library without direct kernel involvement. The OS kernel only sees the process containing these threads, treating it as a single entity. User-level threads offer lightweight creation, scheduling, and switching, making them more efficient for certain applications.

However, they can suffer from limited parallelism due to their dependence on a single kernel-level thread. Nevertheless, user-level threads provide flexibility and faster context switching, allowing developers to design responsive and scalable applications for specific use cases, such as event-driven programs and systems with high concurrency requirements.

Advantages of User Level Thread:

  • User-level threads are faster to create, schedule, and switch compared to kernel-level threads, leading to improved performance in certain scenarios.
  • User-level thread libraries can implement custom scheduling algorithms tailored to the application’s requirements, offering better control over thread execution.
  • These threads are independent of the underlying operating system, making them highly portable across different platforms and architectures.
  • Since user-level threads are managed by the application without kernel interference, they avoid the overhead associated with kernel-level thread management.
  • Developers have more control over thread behaviour and can optimize resource allocation, making user-level threads suitable for specific application needs.

Disadvantages of User Level Thread:

  • User-level threads are dependent on a single kernel-level thread, which may not fully utilize multi-core processors, leading to suboptimal performance in highly parallel applications.
  • User-level threads often require context switches to interact with the kernel for system calls, resulting in extra overhead compared to kernel-level threads.
  • Since user-level threads are managed at the application level, they cannot take advantage of true concurrent execution across multiple processes.
  • Coordinating and synchronizing user-level threads require additional effort and may lead to potential race conditions or deadlock situations without proper care.

Kernel Level Thread (KLT) in OS

Kernel-level thread is also known as a native thread, is a thread managed directly by the operating system’s kernel. Each kernel-level thread has its own program counter, stack, and register set, enabling true parallel execution on multi-core processors.

The OS schedules these threads for execution, providing efficient handling of I/O and system calls without blocking the entire process. Kernel-level threads offer better responsiveness and performance in multi-threaded applications compared to user-level threads.

However, they come with higher overhead in creation and context switching due to direct kernel involvement. Nevertheless, kernel-level threads ensure system-level tasks and reliability in critical applications.

Advantages of Kernel Level Thread:

  • Kernel-level threads can run concurrently on multiple CPU cores, fully utilizing multi-core processors for improved performance.
  • Kernel-level threads can handle I/O operations independently, allowing other threads to continue execution without being blocked.
  • These threads offer quicker context switching, enabling faster response times to events and user interactions.
  • As managed directly by the kernel, kernel-level threads benefit from the OS’s built-in mechanisms for process management and resource allocation, ensuring stability and reliability.

Disadvantages of Kernel Level Thread:

  • Creation and management of kernel-level threads require more system resources and time compared to user-level threads, affecting scalability.
  • The number of kernel-level threads is usually constrained by the OS, potentially limiting the level of parallelism in certain scenarios.
  • Kernel-level threads accessing shared resources may lead to synchronization issues, like deadlocks or race conditions, necessitating careful programming.
  • Context switches involving kernel-level threads incur higher overhead compared to user-level threads, impacting overall system performance.
  • Kernel-level threads are tied to the specific OS, reducing portability and hindering application migration across different operating systems.

Hybrid Threads in OS

Hybrid threads are also known as two-level threads that combine the advantages of user-level threads and kernel-level threads in an operating system. They use a two-level thread model, where multiple lightweight user-level threads are managed by a smaller number of kernel-level threads.

User-level threads offer fast creation, scheduling, and context switching, while kernel-level threads provide true parallelism and efficient I/O handling. The OS kernel schedules and manages the kernel-level threads, allowing better resource allocation and synchronization.

Hybrid threads strike a balance between performance and resource utilization, making them suitable for applications that require both high concurrency and efficient system-level operations.

Advantages of Hybrid Threads:

  • Hybrid threads combine the efficiency of user-level threads with the parallelism of kernel-level threads, leading to better overall application performance.
  • The use of multiple user-level threads managed by a smaller set of kernel-level threads allows for higher concurrency, enabling better utilization of multi-core processors.
  • User-level threads provide faster context switching, resulting in improved application responsiveness to events and user interactions.
  • The smaller number of kernel-level threads reduces overhead and resource consumption compared to a fully kernel-managed thread model.
  • Hybrid threads offer a balance between control and simplicity, allowing developers to optimize thread management for specific application requirements, ensuring a more adaptable solution.

Disadvantages of Hybrid Threads:

  • Managing both user-level and kernel-level threads introduces additional complexity to the thread management system.
  • The interaction between user-level threads and kernel-level threads may cause interference, leading to unpredictable behaviour and reduced performance.
  • Hybrid thread implementations can be more challenging to port across different operating systems due to the combination of user-level and kernel-level components.
  • Debugging applications with hybrid threads can be more complex, as issues may arise from both user-level and kernel-level thread interactions.

Many-to-One Threads in OS

Many-to-one threading is a threading model where multiple user-level threads are mapped onto a single kernel-level thread. This approach is known as the “green thread” model and is managed entirely by the runtime library or the user-level thread library. The kernel remains unaware of these user-level threads.

Advantages of Hybrid Threads:

  • Many-to-one threading is lightweight as it does not involve kernel-level thread creation and management, resulting in faster thread creation and context switching.
  • Implementing many-to-one threading is relatively straightforward as it relies on user-level thread libraries, reducing the complexity of thread management.
  • This threading model is well-suited for I/O-bound applications where threads frequently block on I/O operations, as the overhead of kernel involvement is minimized.
  • Since many-to-one threads are managed at the user-level, they can be easily ported across different operating systems without significant modifications.
  • The lack of kernel thread management overhead makes many-to-one threading efficient for applications with a large number of lightweight threads, which helps conserve system resources.

Disadvantages of Hybrid Threads:

  • Many-to-one threading restricts parallelism since all user-level threads are mapped to a single kernel-level thread, preventing true concurrent execution on multi-core processors.
  • If one user-level thread blocks due to a system call or any other blocking operation, it halts the entire process, causing potential inefficiencies in resource utilization.
  • CPU-bound applications that heavily rely on computation may not benefit from many-to-one threading, as they cannot efficiently leverage multiple cores for parallel processing.
  • Exploiting concurrency and fully utilizing available hardware resources is challenging with many-to-one threading due to the limitation of executing only one thread at a time, even on multi-core systems.

One-to-One Threads in OS

One-to-One threading is a threading model in many types of operating systems where each user-level thread corresponds to a dedicated kernel-level thread. This approach provides true parallelism, allowing multiple threads to execute simultaneously on multi-core processors.

Each thread can be individually scheduled and managed by the kernel, leading to efficient utilization of system resources. One-to-One threading overcomes the limitations of many-to-one threading, such as blocking issues and limited parallelism.

However, it may incur higher overhead due to the creation and management of kernel-level threads. This model is commonly going to use in modern operating systems to improve the performance and responsiveness for multi-threaded applications.

Advantages of One-to-One Threads:

  • One-to-One threading enables true parallel execution, as each user-level thread is mapped to a separate kernel-level thread, taking full advantage of multi-core processors.
  • Since each thread has its kernel-level counterpart, blocking operations by one thread do not affect the entire process, ensuring better responsiveness and resource utilization.
  • With individual kernel-level threads, the operating system can optimize thread scheduling and resource allocation, leading to improved performance and reduced contention for shared resources.

Disadvantages of One-to-One Threads:

  • Creating and managing individual kernel-level threads for each user-level thread can incur higher resource overhead, especially for applications with a large number of threads.
  • Frequent context switching between kernel-level threads can be expensive, impacting the overall performance of the system.
  • The separate kernel-level thread for each user-level thread requires additional memory space, which can become a concern when dealing with a massive number of threads.

Many-to-Many Threads in OS

Many-to-Many threading is a threading model in operating systems that provides a flexible approach to thread management. In this model, multiple user-level threads are mapped to a smaller or equal number of kernel-level threads.

The user-level threads are managed by a thread library, while the kernel handles the kernel-level threads. This allows for better load balancing and efficient use of system resources.

Many-to-Many threading overcomes the limitations of other models, such as many-to-one’s blocking issues and one-to-one’s higher resource overhead.

Advantages of Many-to-Many Threads:

  • Many-to-Many threading allows for better scalability as multiple user-level threads can be mapped to a smaller pool of kernel-level threads, efficiently utilizing available system resources.
  • The thread library can dynamically distribute user-level threads among available kernel-level threads, achieving better load balancing and optimizing overall system performance.
  • Blocking operations by one user-level thread do not affect other threads since they are managed independently by the thread library and the kernel.
  • Many-to-Many threading strikes a balance between one-to-one and many-to-one models, reducing both the resource overhead of individual kernel-level threads and the limitations on the total number of threads, leading to more efficient resource utilization.

Disadvantages of Many-to-Many Threads:

  • The additional layer of mapping user-level threads to kernel-level threads can introduce overhead, impacting the overall performance of the system.
  • Coordinating the scheduling and synchronization of user-level threads with the kernel-level threads can lead to potential synchronization challenges and inefficiencies.
  • Applications using the many-to-many model often rely heavily on the underlying thread library, which may limit portability across different operating systems.
  • Some operating systems may not provide native support for many-to-many threading, requiring custom thread library implementations, which can lead to compatibility and maintenance issues.

Summing Up

Now, we can hope that you have been completely educated about what is thread in OS; as well as different types of threads in OS (Operating System) with ease. If this article is helpful for you, then please share it along with your friends, family members or relatives over social media platforms like as Facebook, Instagram, Linked In, Twitter, and more.

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