thread_pool.pl -- Resource bounded thread management

The module library(thread_pool) manages threads in pools. A pool defines properties of its member threads and the maximum number of threads that can coexist in the pool. The call thread_create_in_pool/4 allocates a thread in the pool, just like thread_create/3. If the pool is fully allocated it can be asked to wait or raise an error.

The library has been designed to deal with server applications that receive a variety of requests, such as HTTP servers. Simply starting a thread for each request is a bit too simple minded for such servers:

• Creating many CPU intensive threads often leads to a slow-down rather than a speedup.
• Creating many memory intensive threads may exhaust resources
• Tasks that require little CPU and memory but take long waiting for external resources can run many threads.

Using this library, one can define a pool for each set of tasks with comparable characteristics and create threads in this pool. Unlike the worker-pool model, threads are not started immediately. Depending on the design, both approaches can be attractive.

The library is implemented by means of a manager thread with the fixed thread id __thread_pool_manager. All state is maintained in this manager thread, which receives and processes requests to create and destroy pools, create threads in a pool and handle messages from terminated threads. Thread pools are not saved in a saved state and must therefore be recreated using the initialization/1 directive or otherwise during startup of the application.

See also

- http_handler/3 and http_spawn/2.

 thread_pool_create(+Pool, +Size, +Options) is det

Create a pool of threads. A pool of threads is a declaration for creating threads with shared properties (stack sizes) and a limited number of threads. Threads are created using thread_create_in_pool/4. If all threads in the pool are in use, the behaviour depends on the wait option of thread_create_in_pool/4 and the backlog option described below. Options are passed to thread_create/3, except for

backlog(+MaxBackLog)

Maximum number of requests that can be suspended. Default is infinite. Otherwise it must be a non-negative integer. Using backlog(0) will never delay thread creation for this pool.

The pooling mechanism does not interact with the detached state of a thread. Threads can be created both detached and normal and must be joined using thread_join/2 if they are not detached.

Undocumented predicates

The following predicates are exported, but not or incorrectly documented.

 thread_create_in_pool(Arg1, Arg2, Arg3, Arg4)

 thread_pool_destroy(Arg1)

 thread_pool_property(Arg1, Arg2)

 current_thread_pool(Arg1)