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A kernel thread, sometimes called a LWP (Lightweight Process) is created and scheduled by the kernel. Kernel threads are often more expensive to create than user threads and the system calls to directly create kernel threads are very platform specific.

A user thread is normally created by a threading library and scheduling is managed by the threading library itself (Which runs in user mode). All user threads belong to process that created them. The advantage of user threads is that they are portable.

The major difference can be seen when using multiprocessor systems, user threads completely managed by the threading library can't be ran in parallel on the different CPUs, although this means they will run fine on uniprocessor systems. Since kernel threads use the kernel scheduler, different kernel threads can run on different CPUs.

Many systems implement threading differently,

A many-to-one threading model maps many user processes directly to one kernel thread, the kernel thread can be thought of as the main process.

A one-to-one threading model maps each user thread directly to one kernel thread, this model allows parallel processing on the multiprocessor systems. Each kernel thread can be thought of as a VP (Virtual Process) which is managed by the scheduler.

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User-Level ThreadsUser-level threads implement in user-level libraries, rather than via systems calls, so thread switching does not need to call operating system and to cause interrupt to the kernel. In fact, the kernel knows nothing about user-level threads and manages them as if they were single-threaded processes.

Advantages:

The most obvious advantage of this technique is that a user-level threads package can be implemented on an Operating System that does not support threads. Some other advantages are

  • User-level threads does not require modification to operating systems.
  • Simple Representation:

    Each thread is represented simply by a PC, registers, stack and a small control block, all stored in the user process address space.

  • Simple Management:

    This simply means that creating a thread, switching between threads and synchronization between threads can all be done without intervention of the kernel.

  • Fast and Efficient:

    Thread switching is not much more expensive than a procedure call.

Disadvantages:

  • There is a lack of coordination between threads and operating system kernel. Therefore, process as whole gets one time slice irrespect of whether process has one thread or 1000 threads within. It is up to each thread to relinquish control to other threads.
  • User-level threads requires non-blocking systems call i.e., a multithreaded kernel. Otherwise, entire process will blocked in the kernel, even if there are runable threads left in the processes. For example, if one thread causes a page fault, the process blocks.
Kernel-Level ThreadsIn this method, the kernel knows about and manages the threads. No runtime system is needed in this case. Instead of thread table in each process, the kernel has a thread table that keeps track of all threads in the system. In addition, the kernel also maintains the traditional process table to keep track of processes. Operating Systems kernel provides system call to create and manage threads.

Advantages:

  • Because kernel has full knowledge of all threads, Scheduler may decide to give more time to a process having large number of threads than process having small number of threads.
  • Kernel-level threads are especially good for applications that frequently block.

Disadvantages:

  • The kernel-level threads are slow and inefficient. For instance, threads operations are hundreds of times slower than that of user-level threads.
  • Since kernel must manage and schedule threads as well as processes. It require a full thread control block (TCB) for each thread to maintain information about threads. As a result there is significant overhead and increased in kernel complexity.
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Q: What is the difference between Kernel and User level thread?
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Describe the action taken by a kernel to context switch between kernel level threads?

Context switching between kernel threads typically requires saving the value of the CPU registers from the thread being switched out and restoring the CPU registers of the new thread being scheduled.


What is kernel thread?

KERNEL In computing, the kernel is the central component of most computer operating systems; it is a bridge between applications and the actual data processing done at the hardware level. The kernel's responsibilities include managing the system's resources (the communication between hardware and software components).


What are the advantages and disadvantages of user level threads and kernel level threads?

- USER LEVEL THREADS Aadvantages: · User-level threads can be implemented on operating system that does not support threads. · Implementing user-level threads does not require modification of operating system where everything is managed by the thread library · Simple representation which the thread is represented by a the thread ID, program counter, register, stack , all stored in user process address space · Simple management where creating new threads, switching threads and synchronization between threads can be done without intervention of the kernel · Fast and efficient where switching thread is much more inexpensive compared to a system call - Disadvantages: · There is a lack of coordination between threads and operating system kernel. A process gets one time slice no matter it has 1 thread or 10000 threads within it. It is up to the thread itself to give up the control to other threads · If one thread made a blocking system call, the entire process can be blocked in the kernel, even if other threads in the same process are in the ready state KERNEL LEVEL THREAD: - Advantages: · Because kernel has the full knowledge of all the threads, scheduler may decide to allocate more time to a process having large number of threads than process having small number of thread, where the kernel threads come useful for intense application - Disadvantages: · Kernel level threads are slow and inefficient, since kernel must manage and schedule all the threads as well as the processes. It requires a full TCB for each thread to maintain information about threads, which results in increasing of overheads and kernel complexity


What is kernel and shell and what is the difference between these two?

A kernel is the heart of the operating system and acts as a middle ground between hardware and software. Some (if not all) device drivers are kernel-level drivers as kernelmodules. A shell is a command terminal that allows the user to interact with the user.


Is there one stack per process when user level threads are used?

In both kernel and User level they have one stack per thread


What are kernel level threads?

KERNEL In computing, the kernel is the central component of most computer operating systems; it is a bridge between applications and the actual data processing done at the hardware level. The kernel's responsibilities include managing the system's resources (the communication between hardware and software components).


What are user level threads?

•No special support needed from the kernel (use any Unix) •Thread creation and context switch are fast (no syscall) •Defines its own thread model and scheduling policies


What are the circumstances of user level thread better than kernel level thread?

There are two distinct models of thread controls, and they are user-level threads and kernel-level threads. The thread function library to implement user-level threads usually runs on top of the system in user mode. Thus, these threads within a process are invisible to the operating system. User-level threads have extremely low overhead, and can achieve high performance in computation. However, using the blocking system calls like read(), the entire process would block. Also, the scheduling control by the thread runtime system may cause some threads to gain exclusive access to the CPU and prevent other threads from obtaining the CPU. Finally, access to multiple processors is not guaranteed since the operating system is not aware of existence of these types of threads. On the other hand, kernel-level threads will guarantee multiple processor access but the computing performance is lower than user-level threads due to load on the system. The synchronization and sharing resources among threads are still less expensive than multiple-process model, but more expensive than user-level threads. Thus, user-level thread is better than kernel level thread.


What is the Linux kernel?

The Linux kernel is the central component of the GNU/Linux operating system. The kernel is the lowest level of interaction between the hardware and the operating system. Individual applications are at a higher level. The kernel along with supporting applications make up the operating system.


Is user level threads are transparent to kernel level threads?

user level theads are not tranparent to kernel level threads.


What takes longer a context switch with user level threads or one with kernel level threads?

Kernel level threads take a longer time to context switch since OS will have to save and reload each and every TCB (Thread Control block) where as in user level no kernel intervention threads simply context switch more efficiently. But there are disadvantages such as since OS sees the user leve threads as a whole process it will not give a large portion of CPU time for execution if a thread is blocked the whole process goes to the waiting state please correct my answer if im wrong godlovesu49@hotmail.com thanks regards yo


When a user-level thread executes a system call not only is that thread blocked but also all the threads within the process are blocked Why is that so?

A system call is actually a transition from user mode to kernel space to carry out a required operation.. For exp: Write() function resides in kernel space and can be accessed by a only corresponding wrapper function which was implemented in user space.. In case of Windows OS, Win32 API is the implementation to such as wrapper functions that make the actual system calls from user mode.. A kernel thread is created, scheduled and destroyed by the kernel.. Whereas a library thread (user thread) is created, scheduled and destroyed by a user level library like special threading libraries.. The kernel does know of nothing about library threads since they live in a process's boundaries and bound to its life cycle tightly.. Actually, a process is not the primary execution unit in all operating systems.. When a process is made by kernel, a new kernel thread is created and attached to that process in question.. So the library threads created in user mode by a user mode library must share the time slices given to the kernel thread by the scheduler arbitrarily during the lifetime of a process.. So a process actually has one kernel thread and all other library threads have to share the kernel thread's cycles.. Hence when a library thread makes a blocking call, all the threads within the process are blocked because as I said, actually process has only one kernel thread assigned to it and others try to make use of it.. So to prevent other threads from blocking, either you should use library threads that make use of kernel threads or you could just use the CreateThread() Win32 API system function for kernel threads but the synchronization mechanism must be provided by the programmer using events, signals, mutex, semaphore etc.. Sun, BSD Unix flavours, Windows etc follow the same threading architecture in their systems as POSIX standard.. However, a thread is a process in Linux.. That's why Linux is so powerful in server systems.. So the control is left to programmers to create a POSIX way of treading model by Clone(2) system call(s).. So address space and data etc can be shared by lightweight processes easily.. When a Linux kernel thread (child process) is crashed, it won't affect the rest of the threads belong to parent process.. This is just the opposite in other operating systems that a crashing thread will destroy all of the threads in the process.. NPTL is a great threading library that was implemented by using Linux clone(2) system call.. Linux also has another type of kernel thread only lives in kernel space that can be utilized by kernel code like modules.. User threads can't be run in parallel on the different CPUs because of this. However, they are portable.. Kernel threads can be scheduled by kernel to be run on a SMP system.. Hope this helps.. hsaq19@ TH Algan