Fiber (computer science)

In computer science, a fiber is a particularly lightweight thread of execution.

Like threads, fibers share address space; where a distinction exists, it is that fibers use co-operative multitasking while threads use pre-emptive multitasking. Threads often depend on the kernel's thread scheduler to preempt a busy thread and resume another thread; fibers yield themselves to run another fiber while executing. The article on threads contains more on the distinction between threads and fibers.

Fibers and coroutines

Fibers describe essentially the same concept as coroutines. The distinction, if there is any, is that coroutines are a language-level construct, a form of control flow, while fibers are a systems-level construct, viewed as threads that happen not to run concurrently. Priority is contentious; fibers may be viewed as an implementation of coroutines [http://www.flounder.com/fibers.htm A Fiber Class] ] , or as a substrate on which to implement coroutines. [http://msdn.microsoft.com/msdnmag/issues/03/09/CoroutinesinNET/default.aspx Implementing Coroutines for .NET by Wrapping the Unmanaged Fiber API] , Ajai Shankar, MSDN Magazine]

Advantages and disadvantages

Because fibers multitask cooperatively, thread safety is less of an issue than with preemptively scheduled threads, and synchronization constructs including spinlocks and atomic operations are unnecessary when writing fibered code, as they are implicitly synchronized. However, many libraries yield a fiber implicitly as a method of conducting non-blocking I/O; as such, some caution and documentation reading is advised. A disadvantage is that fibers cannot utilise multiprocessor machines without also using preemptive threads; however, an N:M threading model with no more preemptive threads than CPU cores can be more efficient than either pure fibers or pure preemptive threading.

Operating system support

Less support from the operating system is needed for fibers than for threads. They can be implemented in modern Unix systems using the library functions getcontext, setcontext and swapcontext in ucontext.h, as in GNU Portable Threads.

On Microsoft Windows, fibers are created using the ConvertThreadToFiber and CreateFiber calls; a fiber that is currently suspended may be resumed in any thread. Fiber-local storage, analogous to thread-local storage, may be used to create unique copies of variables. [http://msdn2.microsoft.com/en-us/library/ms682661.aspx Fibers] , MSDN Library]

See also

*getcontext/setcontext library routines

References

External links

* [http://www.gnu.org/software/pth/ GNU Portable threads]
* [http://freshmeat.net/projects/libpcl/ Portable Coroutines]