THREADS MANAGEMENT IN LINUX OPERATING SYSTEM Thread of execution is the smallest sequence of programmed instructions that can be managed independently by an operating system scheduler. The scheduler itself is a light-weight process. The implementation of threads and processes differs from one operating system to another, but in most cases, a thread is contained inside a process. Multiple threads can exist within the same process and share resources such as memory, while different processes do not share these resources. In particular, the threads of a process share its code and its context . On a single processor, multithreading is generally implemented by time-division multiplexing (as in multitasking): the processor switches between different threads. This context switching generally happens frequently enough that the user perceives the threads or tasks as running at the same time. On a multiprocessor or multi-core system, threads can be truly concurrent, with every processor or core executing a separate thread simultaneously. Many modern operating systems directly support both time-sliced and multiprocessor threading with a process scheduler. The kernel of an operating system allows programmers to manipulate threads via the system call interface. Some implementations are called a kernel thread, whereas a lightweight process (LWP) is a specific type of kernel thread that shares the same state and information.

Threads are created like normal tasks, with the exception that the clone() system call is passed flags corresponding to specific resources to be shared: clone(CLONE_VM | CLONE_FS | CLONE_FILES | CLONE_SIGHAND, 0);
The previous code results in behavior identical to a normal fork(), except that the address space, filesystem resources, file descriptors, and signal handlers are shared. In other words, the new task and its parent are what are popularly called threads.

In contrast, a normal fork() can be implemented as clone(SIGCHLD, 0);
And vfork() is implemented as clone(CLONE_VFORK | CLONE_VM | SIGCHLD, 0);
The flags provided to clone() help specify the behavior of the new process and detail what resources the parent and child will share

Kernel Threads
A kernel thread is the lightest unit of kernel scheduling. At least one kernel thread exists within each process. If multiple kernel threads can exist within a process, then they share the same memory and file resources. Kernel threads are preemptively multitasked if the operating system's process scheduler is preemptive. Kernel threads do not own resources except for a stack, a copy of the registers including the program counter, and thread-local storage The kernel can assign one thread to each logical core in a system , and can swap out threads that get blocked. However, kernel threads take much longer than user threads to be swapped.

User Threads

Threads are sometimes implemented in userspace libraries, thus called user threads. The kernel is unaware of them, so they are managed and scheduled in userspace. Some implementations base their user threads on top of several kernel threads, to benefit from multi-processor machines (M:N model). In this article the term "thread" (without kernel or user qualifier) defaults to referring to kernel threads. User threads as implemented by virtual machines are also called green threads. User threads are generally fast to create and manage, but cannot take advantage of multithreading or multiprocessing and get blocked if all of their associated kernel threads get blocked even if there are some user threads that are ready to run.

PROCESS MANAGEMENT IN LINUX OPERATING SYSTEM
A process is an instance of a program running in a computer. It is close in meaning to task , a term used in some operating systems. In UNIX and some other operating systems, a process is started when a program is initiated (either by a user entering a shell command or by another program). Like a task, a process is a running program with which a particular set of data is associated so that the process can be kept track of. An application that is being shared by multiple users will generally have one process at some stage of execution for each user.
A process can initiate a subprocess, which is a called a child process (and the initiating process is sometimes referred to as its parent ). A child process is a replica of the parent process and shares some of its resources, but cannot exist if the parent is terminated.
Threads of execution, often shortened to threads, are the objects of activity within the process. Each thread includes a unique program counter, process stack, and set of processor registers. The kernel schedules individual threads, not processes. In traditional Linux systems, each process consists of one thread. In modern systems, however, multithreaded programs—those that consist of more than one thread—are common. As you will see later, Linux has a unique implementation of threads: It does not differentiate between threads and processes. To Linux, a thread is just a special kind of process.
A process begins its life when, not surprisingly, it is created. In Linux, this occurs by means of thefork() system call, which creates a new process by duplicating an existing one. The process that calls fork() is the parent, whereas the new process is the child. The parent resumes execution and the child starts execution at the same place, where the call returns. The fork() system call returns from the kernel twice: once in the parent process and again in the newborn child.
Often, immediately after a fork it is desirable to execute a new, different, program. The exec*()family of function calls is used to create a new address space and load a new program into it. In modern Linux kernels, fork() is actually implemented via the clone() system call, which is discussed in a following section.
Finally, a program exits via the exit() system call. This function terminates the process and frees all its resources. A parent process can inquire about the status of a terminated child via thewait4() system call, which enables a process to wait for the termination of a specific process.