// The contents of this file are in the public domain. See LICENSE_FOR_EXAMPLE_PROGRAMS.txt
/*
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<long> 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
*/
}