Thread (computer science)

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In computer science, a thread of execution is the smallest unit of processing that can be scheduled by an operating system. It generally results from a fork of a computer program into two or more concurrently running tasks. 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 the latter's instructions (its code) and its context (the values that its variables reference at any given moment). To give an analogy, multiple threads in a process are like multiple cooks reading off the same cook book and following its instructions, not necessarily from the same page.

On a single processor, multithreading generally occurs 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, the threads or tasks will actually run at the same time, with each processor or core running a particular thread or task.

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.

Programs can have user-space threads when threading with timers, signals, or other methods to interrupt their own execution, performing a sort of ad-hoc time-slicing.

• 1 Threads compared with processes
• 2 Multithreading: Advantages/Uses
• 3 Processes, kernel threads, user threads, and fibers
  • 3.1 Thread and fiber issues
    • 3.1.1 Concurrency and data structures
    • 3.1.2 I/O and scheduling
• 4 Models
  • 4.1 1:1 (Kernel-level threading)
  • 4.2 N:1 (User-level threading)
  • 4.3 N:M (Hybrid threading)
• 5 Implementations
  • 5.1 Kernel-level implementation examples
  • 5.2 User-level implementation examples
  • 5.3 Hybrid implementation examples
  • 5.4 Fiber implementation examples
• 6 Programming Language Support
• 7 See also
• 8 References
• 9 External links

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