boost::fiber scheduling - when and how

问题

According to the documentation

the currently-running fiber retains control until it invokes some operation that passes control to the manager

I can think about only one operation - boost::this_fiber::yield which may cause control switch from fiber to fiber. However, when I run something like

bf::fiber([](){std::cout << "Bang!" << std::endl;}).detach();
bf::fiber([](){std::cout << "Bung!" << std::endl;}).detach();

I get output like

Bang!Bung!
\n
\n

Which means control was passed between << operators from one fiber to another. How it could happen? Why? What is the general definition of controll passing from fiber to fiber in the context of boost::fiber library?

EDIT001: Cant get away without code:

#include <boost/fiber/fiber.hpp>
#include <boost/fiber/mutex.hpp>
#include <boost/fiber/barrier.hpp>
#include <boost/fiber/algo/algorithm.hpp>
#include <boost/fiber/algo/work_stealing.hpp>

namespace bf = boost::fibers;

class GreenExecutor
{
    std::thread worker;
    bf::condition_variable_any cv;
    bf::mutex mtx;
    bf::barrier barrier;
public:
    GreenExecutor() : barrier {2}
    {   
        worker = std::thread([this] {
            bf::use_scheduling_algorithm<bf::algo::work_stealing>(2);
            // wait till all threads joining the work stealing have been registered
            barrier.wait();
            mtx.lock();
            // suspend main-fiber from the worker thread
            cv.wait(mtx);
            mtx.unlock();
        });
        bf::use_scheduling_algorithm<bf::algo::work_stealing>(2);
        // wait till all threads have been registered the scheduling algorithm
        barrier.wait();
    }

    template<typename T>
    void PostWork(T&& functor)
    {
        bf::fiber {std::move(functor)}.detach();
    }

    ~GreenExecutor()
    {
        cv.notify_all();
        worker.join();
    }
};

int main()
{
    GreenExecutor executor;
    std::this_thread::sleep_for(std::chrono::seconds(1));
    int i = 0;
    for (auto j = 0ul; j < 10; ++j) {
        executor.PostWork([idx {++i}]() {
            auto res = pow(sqrt(sin(cos(tan(idx)))), M_1_PI);
            std::cout << idx << " - " << res << std::endl;
        });
    }
    while (true) {
        boost::this_fiber::yield();
    }
    return 0;
}

Output

2 - 1 - -nan
0.503334 3 - 4 - 0.861055
0.971884 5 - 6 - 0.968536
-nan 7 - 8 - 0.921959
0.9580699
- 10 - 0.948075
0.961811

回答1:

Ok, there were a couple of things I missed, first, my conclusion was based on misunderstanding of how stuff works in boost::fiber
The line in the constructor mentioned in the question bf::use_scheduling_algorithm<bf::algo::work_stealing>(2);
was installing the scheduler in the thread where the GreenExecutor instance was created (in the main thread) so, when launching two worker fibers I was actually initiating two threads which are going to process submitted fibers which in turn would process these fibers asynchronously thus mixing the std::cout output. No magic, everything works as expected, the boost::fiber::yield still is the only option to pass control from one fiber to another

来源：https://stackoverflow.com/questions/51017582/boostfiber-scheduling-when-and-how

标签

c++

boost

fibers