thread_pool.h

// thread_pool.h

/*    Copyright 2009 10gen Inc.
 *
 *    Licensed under the Apache License, Version 2.0 (the "License");
 *    you may not use this file except in compliance with the License.
 *    You may obtain a copy of the License at
 *
 *    http://www.apache.org/licenses/LICENSE-2.0
 *
 *    Unless required by applicable law or agreed to in writing, software
 *    distributed under the License is distributed on an "AS IS" BASIS,
 *    WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 *    See the License for the specific language governing permissions and
 *    limitations under the License.
 */

#include <boost/function.hpp>
#include <boost/bind.hpp>
#undef assert
#define assert xassert

namespace mongo {

namespace threadpool {
    class Worker;

    typedef boost::function<void(void)> Task; //nullary function or functor

    // exported to the mongo namespace
    class ThreadPool : boost::noncopyable{
        public:
        explicit ThreadPool(int nThreads=8);

        // blocks until all tasks are complete (tasks_remaining() == 0)
        // You should not call schedule while in the destructor
        ~ThreadPool();

        // blocks until all tasks are complete (tasks_remaining() == 0)
        // does not prevent new tasks from being scheduled so could wait forever.
        // Also, new tasks could be scheduled after this returns.
        void join();


        // task will be copied a few times so make sure it's relatively cheap
        void schedule(Task task);

        // Helpers that wrap schedule and boost::bind.
        // Functor and args will be copied a few times so make sure it's relatively cheap
        template<typename F, typename A>
        void schedule(F f, A a){ schedule(boost::bind(f,a)); }
        template<typename F, typename A, typename B>
        void schedule(F f, A a, B b){ schedule(boost::bind(f,a,b)); }
        template<typename F, typename A, typename B, typename C>
        void schedule(F f, A a, B b, C c){ schedule(boost::bind(f,a,b,c)); }
        template<typename F, typename A, typename B, typename C, typename D>
        void schedule(F f, A a, B b, C c, D d){ schedule(boost::bind(f,a,b,c,d)); }
        template<typename F, typename A, typename B, typename C, typename D, typename E>
        void schedule(F f, A a, B b, C c, D d, E e){ schedule(boost::bind(f,a,b,c,d,e)); }


        int tasks_remaining() { return _tasksRemaining; }

        private:
        mongo::mutex _mutex;
        boost::condition _condition;

        list<Worker*> _freeWorkers; //used as LIFO stack (always front)
        list<Task> _tasks; //used as FIFO queue (push_back, pop_front)
        int _tasksRemaining; // in queue + currently processing
        int _nThreads; // only used for sanity checking. could be removed in the future.
        
        // should only be called by a worker from the worker's thread
        void task_done(Worker* worker);
        friend class Worker;
    };

} //namespace threadpool

using threadpool::ThreadPool;

} //namespace mongo