/* This is an example illustrating the use of the thread_pool object from the dlib C++ Library. This is a very simple example. It creates a thread pool with 3 threads and then sends a few simple tasks to the pool. */ #include "dlib/threads.h" #include "dlib/misc_api.h" // for dlib::sleep #include "dlib/logger.h" using namespace dlib; // We will be using the dlib logger object to print out messages in this example // because its output is timestamped and labeled with the thread that the log // message came from. So this will make it easier to see what is going on in // this example. Here we make an instance of the logger. See the logger // documentation and examples for detailed information regarding its use. logger dlog("main"); // Here we make an instance of the thread pool object thread_pool tp(3); // ---------------------------------------------------------------------------------------- class test { /* The thread_pool accepts "tasks" from the user and schedules them for execution in one of its threads when one becomes available. Each task is just a request to call a member function on a particular object (or if you use futures you may make tasks that call global functions). So here we create a class called test with a few member functions which we will have the thread pool call as tasks. */ public: void task_0() { dlog << LINFO << "task_0 start"; // Here we ask the thread pool to call this->subtask() three different times // with different arguments. Note that calls to add_task() will return // immediately if there is an available thread to hand the task off to. However, // if there isn't a thread ready then add_task blocks until there is such a thread. // Also note that since task_0() is executed within the thread pool (see main() below) // calls to add_task() will execute the requested task within the calling thread // in cases where the thread pool is full. This means it is safe to have // tasks running in the thread pool spawn sub tasks which is what we are doing here. tp.add_task(*this,&test::subtask,1); // schedule call to this->subtask(1) tp.add_task(*this,&test::subtask,2); // schedule call to this->subtask(2) tp.add_task(*this,&test::subtask,3); // schedule call to this->subtask(3) // wait_for_all_tasks() is a function that blocks until all tasks // submitted to the thread pool by the thread calling wait_for_all_tasks() // finish. So this call blocks until the 3 tasks above are done. tp.wait_for_all_tasks(); dlog << LINFO << "task_0 end" ; } void subtask(long a) { dlib::sleep(200); dlog << LINFO << "subtask end " << a; } void task_1(long a, long b) { dlog << LINFO << "task_1 start: " << a << ", " << b; dlib::sleep(700); dlog << LINFO << "task_1 end: " << a << ", " << b; } }; // ---------------------------------------------------------------------------------------- void add ( long a, long b, long& result ) { dlib::sleep(400); result = a + b; } // ---------------------------------------------------------------------------------------- int main() { // tell the logger to print out everything dlog.set_level(LALL); test a; dlog << LINFO << "schedule a few tasks"; // schedule a call to a.task_1(10,11) tp.add_task(a, &test::task_1, 10, 11); // schedule the thread pool to call a.task_0(). uint64 id = tp.add_task(a, &test::task_0); // schedule a call to a.task_1(12,13) tp.add_task(a, &test::task_1, 12, 13); dlog << LINFO << "wait for a.task_0() to finish"; // now wait for our a.task_0() task to finish. To do this we use the id // returned by add_task to reference the task we want to wait for. tp.wait_for_task(id); dlog << LINFO << "a.task_0() finished, now start another task_1() call"; // schedule a call to a.task_1(14,15) tp.add_task(a, &test::task_1, 14, 15); dlog << LINFO << "wait for all tasks to finish"; // here we wait for all tasks which were requested by the main thread // to complete. tp.wait_for_all_tasks(); dlog << LINFO << "all tasks finished"; // The thread pool also allows you to use futures to pass arbitrary objects into the tasks. // For example: future n1, n2, result; n1 = 3; n2 = 4; // add a task that is supposed to go call add(n1, n2, result); tp.add_task(add, n1, n2, result); // This line will wait for the task in the thread pool to finish and when it does // result will return the integer it contains. In this case r will be assigned a value of 7. long r = result; // print out the result dlog << LINFO << "result = " << r; // We can also use futures with member functions like so: tp.add_task(a, &test::task_1, n1, n2); // and we can still wait for tasks like so: tp.wait_for_all_tasks(); dlog << LINFO << "all tasks using futures finished"; /* A possible run of this program might produce the following output (the first column is the time the log message occurred and the value in [] is the thread id for the thread that generated the log message): 0 INFO [0] main: schedule a few tasks 0 INFO [1] main: task_1 start: 10, 11 0 INFO [2] main: task_0 start 200 INFO [2] main: subtask end 2 200 INFO [3] main: subtask end 1 200 INFO [3] main: task_1 start: 12, 13 201 INFO [0] main: wait for a.task_0() to finish 400 INFO [2] main: subtask end 3 400 INFO [2] main: task_0 end 400 INFO [0] main: a.task_0() finished, now start another task_1() call 401 INFO [2] main: task_1 start: 14, 15 401 INFO [0] main: wait for all tasks to finish 700 INFO [1] main: task_1 end: 10, 11 901 INFO [3] main: task_1 end: 12, 13 1101 INFO [2] main: task_1 end: 14, 15 1101 INFO [0] main: all tasks finished 1503 INFO [0] main: result = 7 1503 INFO [3] main: task_1 start: 3, 4 2203 INFO [3] main: task_1 end: 3, 4 2203 INFO [0] main: all tasks using futures finished */ }