/* Part of SWI-Prolog Author: Jan Wielemaker E-mail: J.Wielemaker@vu.nl WWW: http://www.swi-prolog.org Copyright (c) 2008-2016, University of Amsterdam VU University Amsterdam All rights reserved. Redistribution and use in source and binary forms, with or without modification, are permitted provided that the following conditions are met: 1. Redistributions of source code must retain the above copyright notice, this list of conditions and the following disclaimer. 2. Redistributions in binary form must reproduce the above copyright notice, this list of conditions and the following disclaimer in the documentation and/or other materials provided with the distribution. THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE. */ :- module(thread_pool, [ thread_pool_create/3, % +Pool, +Size, +Options thread_pool_destroy/1, % +Pool thread_create_in_pool/4, % +Pool, :Goal, -Id, +Options current_thread_pool/1, % ?Pool thread_pool_property/2 % ?Pool, ?Property ]). :- use_module(library(error)). :- use_module(library(lists)). :- use_module(library(option)). :- use_module(library(rbtrees)). :- use_module(library(debug)). /** 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 http_handler/3 and http_spawn/2. */ :- meta_predicate thread_create_in_pool(+, 0, -, :). :- predicate_options(thread_create_in_pool/4, 4, [ wait(boolean), pass_to(system:thread_create/3, 3) ]). :- multifile create_pool/1. %! 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. thread_pool_create(Name, Size, Options) :- must_be(list, Options), pool_manager(Manager), thread_self(Me), thread_send_message(Manager, create_pool(Name, Size, Options, Me)), wait_reply. %! thread_pool_destroy(+Name) is det. % % Destroy the thread pool named Name. % % @error existence_error(thread_pool, Name). thread_pool_destroy(Name) :- pool_manager(Manager), thread_self(Me), thread_send_message(Manager, destroy_pool(Name, Me)), wait_reply. %! current_thread_pool(?Name) is nondet. % % True if Name refers to a defined thread pool. current_thread_pool(Name) :- pool_manager(Manager), thread_self(Me), thread_send_message(Manager, current_pools(Me)), wait_reply(Pools), ( atom(Name) -> memberchk(Name, Pools) ; member(Name, Pools) ). %! thread_pool_property(?Name, ?Property) is nondet. % % True if Property is a property of thread pool Name. Defined % properties are: % % * options(Options) % Thread creation options for this pool % * free(Size) % Number of free slots on this pool % * size(Size) % Total number of slots on this pool % * members(ListOfIDs) % ListOfIDs is the list or threads running in this pool % * running(Running) % Number of running threads in this pool % * backlog(Size) % Number of delayed thread creations on this pool thread_pool_property(Name, Property) :- current_thread_pool(Name), pool_manager(Manager), thread_self(Me), thread_send_message(Manager, pool_properties(Me, Name, Property)), wait_reply(Props), ( nonvar(Property) -> memberchk(Property, Props) ; member(Property, Props) ). %! thread_create_in_pool(+Pool, :Goal, -Id, +Options) is det. % % Create a thread in Pool. Options overrule default thread % creation options associated to the pool. In addition, the % following option is defined: % % * wait(+Boolean) % If =true= (default) and the pool is full, wait until a % member of the pool completes. If =false=, throw a % resource_error. % % @error resource_error(threads_in_pool(Pool)) is raised if wait % is =false= or the backlog limit has been reached. % @error existence_error(thread_pool, Pool) if Pool does not % exist. thread_create_in_pool(Pool, Goal, Id, QOptions) :- meta_options(is_meta, QOptions, Options), catch(thread_create_in_pool_(Pool, Goal, Id, Options), Error, true), ( var(Error) -> true ; Error = error(existence_error(thread_pool, Pool), _), create_pool_lazily(Pool) -> thread_create_in_pool_(Pool, Goal, Id, Options) ; throw(Error) ). thread_create_in_pool_(Pool, Goal, Id, Options) :- select_option(wait(Wait), Options, ThreadOptions, true), pool_manager(Manager), thread_self(Me), thread_send_message(Manager, create(Pool, Goal, Me, Wait, Id, ThreadOptions)), wait_reply(Id). is_meta(at_exit). %! create_pool_lazily(+Pool) is semidet. % % Call the hook create_pool/1 to create the pool lazily. create_pool_lazily(Pool) :- with_mutex(Pool, ( mutex_destroy(Pool), create_pool_sync(Pool) )). create_pool_sync(Pool) :- current_thread_pool(Pool), !. create_pool_sync(Pool) :- create_pool(Pool). /******************************* * START MANAGER * *******************************/ %! pool_manager(-ThreadID) is det. % % ThreadID is the thread (alias) identifier of the manager. Starts % the manager if it is not running. pool_manager(TID) :- TID = '__thread_pool_manager', ( thread_running(TID) -> true ; with_mutex('__thread_pool', create_pool_manager(TID)) ). thread_running(Thread) :- catch(thread_property(Thread, status(Status)), E, true), ( var(E) -> ( Status == running -> true ; thread_join(Thread, _), print_message(warning, thread_pool(manager_died(Status))), fail ) ; E = error(existence_error(thread, Thread), _) -> fail ; throw(E) ). create_pool_manager(Thread) :- thread_running(Thread), !. create_pool_manager(Thread) :- thread_create(pool_manager_main, _, [ alias(Thread), inherit_from(main) ]). pool_manager_main :- rb_new(State0), manage_thread_pool(State0). /******************************* * MANAGER LOGIC * *******************************/ %! manage_thread_pool(+State) manage_thread_pool(State0) :- thread_get_message(Message), ( update_thread_pool(Message, State0, State) -> debug(thread_pool(state), 'Message ~p --> ~p', [Message, State]), manage_thread_pool(State) ; format(user_error, 'Update failed: ~p~n', [Message]) ). update_thread_pool(create_pool(Name, Size, Options, For), State0, State) :- !, ( rb_insert_new(State0, Name, tpool(Options, Size, Size, WP, WP, []), State) -> thread_send_message(For, thread_pool(true)) ; reply_error(For, permission_error(create, thread_pool, Name)), State = State0 ). update_thread_pool(destroy_pool(Name, For), State0, State) :- !, ( rb_delete(State0, Name, State) -> thread_send_message(For, thread_pool(true)) ; reply_error(For, existence_error(thread_pool, Name)), State = State0 ). update_thread_pool(current_pools(For), State, State) :- !, rb_keys(State, Keys), debug(thread_pool(current), 'Reply to ~w: ~p', [For, Keys]), reply(For, Keys). update_thread_pool(pool_properties(For, Name, P), State, State) :- !, ( rb_lookup(Name, Pool, State) -> findall(P, pool_property(P, Pool), List), reply(For, List) ; reply_error(For, existence_error(thread_pool, Name)) ). update_thread_pool(Message, State0, State) :- arg(1, Message, Name), ( rb_lookup(Name, Pool0, State0) -> update_pool(Message, Pool0, Pool), rb_update(State0, Name, Pool, State) ; State = State0, ( Message = create(Name, _, For, _, _, _) -> reply_error(For, existence_error(thread_pool, Name)) ; true ) ). pool_property(options(Options), tpool(Options, _Free, _Size, _WP, _WPT, _Members)). pool_property(backlog(Size), tpool(_, _Free, _Size, WP, WPT, _Members)) :- diff_list_length(WP, WPT, Size). pool_property(free(Free), tpool(_, Free, _Size, _, _, _)). pool_property(size(Size), tpool(_, _Free, Size, _, _, _)). pool_property(running(Count), tpool(_, Free, Size, _, _, _)) :- Count is Size - Free. pool_property(members(IDList), tpool(_, _, _, _, _, IDList)). diff_list_length(List, Tail, Size) :- '$skip_list'(Length, List, Rest), ( Rest == Tail -> Size = Length ; type_error(difference_list, List/Tail) ). %! update_pool(+Message, +Pool0, -Pool) is det. % % Deal with create requests and completion messages on a given % pool. There are two messages: % % * create(PoolName, Goal, ForThread, Wait, Id, Options) % Create a new thread on behalf of ForThread. There are % two cases: % * Free slots: create the thread % * No free slots: error or add to waiting % * exitted(PoolName, Thread) % A thread completed. If there is a request waiting, % create a new one. update_pool(create(Name, Goal, For, _, Id, MyOptions), tpool(Options, Free0, Size, WP, WPT, Members0), tpool(Options, Free, Size, WP, WPT, Members)) :- succ(Free, Free0), !, merge_options(MyOptions, Options, ThreadOptions), select_option(at_exit(AtExit), ThreadOptions, ThreadOptions1, true), catch(thread_create(Goal, Id, [ at_exit(worker_exitted(Name, Id, AtExit)) | ThreadOptions1 ]), E, true), ( var(E) -> Members = [Id|Members0], reply(For, Id) ; reply_error(For, E), Members = Members0 ). update_pool(Create, tpool(Options, 0, Size, WP, WPT0, Members), tpool(Options, 0, Size, WP, WPT, Members)) :- Create = create(Name, _Goal, For, Wait, _, _Options), !, option(backlog(BackLog), Options, infinite), ( can_delay(Wait, BackLog, WP, WPT0) -> WPT0 = [Create|WPT], debug(thread_pool, 'Delaying ~p', [Create]) ; WPT = WPT0, reply_error(For, resource_error(threads_in_pool(Name))) ). update_pool(exitted(_Name, Id), tpool(Options, Free0, Size, WP0, WPT, Members0), Pool) :- succ(Free0, Free), delete(Members0, Id, Members1), Pool1 = tpool(Options, Free, Size, WP, WPT, Members1), ( WP0 == WPT -> WP = WP0, Pool = Pool1 ; WP0 = [Waiting|WP], debug(thread_pool, 'Start delayed ~p', [Waiting]), update_pool(Waiting, Pool1, Pool) ). can_delay(true, infinite, _, _) :- !. can_delay(true, BackLog, WP, WPT) :- diff_list_length(WP, WPT, Size), BackLog > Size. %! worker_exitted(+PoolName, +WorkerId, :AtExit) % % It is possible that '__thread_pool_manager' no longer exists % while closing down the process because the manager was killed % before the worker. % % @tbd Find a way to discover that we are terminating Prolog. :- public worker_exitted/3. worker_exitted(Name, Id, AtExit) :- catch(thread_send_message('__thread_pool_manager', exitted(Name, Id)), _, true), call(AtExit). /******************************* * UTIL * *******************************/ reply(To, Term) :- thread_send_message(To, thread_pool(true(Term))). reply_error(To, Error) :- thread_send_message(To, thread_pool(error(Error, _))). wait_reply :- thread_get_message(thread_pool(Result)), ( Result == true -> true ; Result == fail -> fail ; throw(Result) ). wait_reply(Value) :- thread_get_message(thread_pool(Reply)), ( Reply = true(Value0) -> Value = Value0 ; Reply == fail -> fail ; throw(Reply) ). /******************************* * HOOKS * *******************************/ %! create_pool(+PoolName) is semidet. % % Hook to create a thread pool lazily. The hook is called if % thread_create_in_pool/4 discovers that the thread pool does not % exist. If the hook succeeds, thread_create_in_pool/4 retries % creating the thread. For example, we can use the following % declaration to create threads in the pool =media=, which holds a % maximum of 20 threads. % % == % :- multifile thread_pool:create_pool/1. % % thread_pool:create_pool(media) :- % thread_pool_create(media, 20, []). % == /******************************* * MESSAGES * *******************************/ :- multifile prolog:message/3. prolog:message(thread_pool(Message)) --> message(Message). message(manager_died(Status)) --> [ 'Thread-pool: manager died on status ~p; restarting'-[Status] ].