Creating or destroying a process is relatively expensive, as resources must be acquired or released. Processes are isolated by process isolation, and do not share address spaces or file resources except through explicit methods such as inheriting file handles or shared memory segments, or mapping the same file in a shared way – see interprocess communication. Resources include memory (for both code and data), file handles, sockets, device handles, windows, and a process control block. Processes own resources allocated by the operating system. The term ' light-weight process' variously refers to user threads or to kernel mechanisms for scheduling user threads onto kernel threads.Ī process is a 'heavyweight' unit of kernel scheduling, as creating, destroying, and switching processes is relatively expensive. User threads may be executed by kernel threads in various ways (one-to-one, many-to-one, many-to-many). Cooperatively scheduled user threads are known as fibers different processes may schedule user threads differently. If these do not share data, as in Erlang, they are usually analogously called processes, while if they share data they are usually called (user) threads, particularly if preemptively scheduled. At the user level a process such as a runtime system can itself schedule multiple threads of execution. Kernel scheduling is typically uniformly done preemptively or, less commonly, cooperatively. In particular, the threads of a process share its executable code and the values of its dynamically allocated variables and non- thread-local global variables at any given time.Īt the kernel level, a process contains one or more kernel threads, which share the process's resources, such as memory and file handles – a process is a unit of resources, while a thread is a unit of scheduling and execution. The multiple threads of a given process may be executed concurrently (via multithreading capabilities), sharing resources such as memory, while different processes do not share these resources. In many cases, a thread is a component of a process. In Modern Operating Systems, Tanenbaum shows that many distinct models of process organization are possible. The implementation of threads and processes differs between operating systems. In computer science, a thread of execution is the smallest sequence of programmed instructions that can be managed independently by a scheduler, which is typically a part of the operating system. Scheduling, Preemption, Context Switching Smallest sequence of programmed instructions that can be managed independently by a scheduler A process with two threads of execution, running on one processor Program vs.

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