8 Multi-threaded applications

SWI-Prolog multithreading is based on standard C-language multithreading support. It is not like ParLog or other parallel implementations of the Prolog language. Prolog threads have their own stacks and only share the Prolog heap: predicates, records, flags and other global non-backtrackable data. SWI-Prolog thread support is designed with the following goals in mind.

• Multi-threaded server applications
  Todays computing services often focus on (internet) server applications. Such applications often have need for communication between services and/or fast non-blocking service to multiple concurrent clients. The shared heap provides fast communication and thread creation is relatively cheap.^{73On an dual AMD-Athlon 1600, SWI-Prolog 5.1.0 creates and joins 4,957 threads per second elapsed time.}

• Interactive applications
  Interactive applications often need to perform extensive computation. If such computations are executed in a new thread, the main thread can process events and allow the user to cancel the ongoing computation. User interfaces can also use multiple threads, each thread dealing with input from a distinct group of windows. See also section 8.8.

• Natural integration with foreign code
  Each Prolog thread runs in a native thread of the operating system, automatically making them cooperate with MT-safe foreign-code. In addition, any foreign thread can create its own Prolog engine for dealing with calling Prolog from C-code.

SWI-Prolog multi-threading is based on the POSIX thread standard Butenhof, 1997 used on most popular systems except for MS-Windows. On Windows it uses the pthread-win32 emulation of POSIX threads mixed with the Windows native API for smoother and faster operation.