Coroutines: A Scheduler for Tasks by Dian-Lun Lin

The last post “A Concise Introduction to Coroutines by Dian-Lun Lin” provide the theory. Today, Dian-Lun presents his single-threaded scheduler for C++ coroutines.

This post assumes you are familiar with the previous post “A Concise Introduction to Coroutines by Dian-Lun Lin“.

A Single-threaded Scheduler for C++ Coroutines

In this section, I implement a single-threaded scheduler to schedule coroutines. Let’s begin with the interface:

Task TaskA(Scheduler& sch) {
  std::cout << "Hello from TaskA\n";
  co_await sch.suspend();
  std::cout << "Executing the TaskA\n";
  co_await sch.suspend();
  std::cout << "TaskA is finished\n";
}

Task TaskB(Scheduler& sch) {
  std::cout << "Hello from TaskB\n";
  co_await sch.suspend();
  std::cout << "Executing the TaskB\n";
  co_await sch.suspend();
  std::cout << "TaskB is finished\n";
}


int main() {

  Scheduler sch;

  sch.emplace(TaskA(sch).get_handle());
  sch.emplace(TaskB(sch).get_handle());

  std::cout << "Start scheduling...\n";

  sch.schedule();

Both TaskA and TaskB are coroutines. I construct a scheduler in the main function and place the two tasks (coroutine handles) into the scheduler. I then call schedule to schedule the two tasks. A task is a coroutine object that is defined as follows:

struct Task {

  struct promise_type {
    std::suspend_always initial_suspend() noexcept { return {}; }
    std::suspend_always final_suspend() noexcept { return {}; }

    Task get_return_object() { 
        return std::coroutine_handle<promise_type>::from_promise(*this); 
    }
    void return_void() {}
    void unhandled_exception() {}
  };

  Task(std::coroutine_handle<promise_type> handle): handle{handle} {}

  auto get_handle() { return handle; }

  std::coroutine_handle<promise_type> handle;
};

Note that I return std::suspend_always in both initial_suspend and final_suspend functions. This is because I want to hand the entire coroutine execution over to the scheduler. Coroutines are executed only after I call schedule. The scheduler is defined as follows:

class Scheduler {

  //std::queue<std::coroutine_handle<>> _tasks;
  std::stack<std::coroutine_handle<>> _tasks;

  public: 

    void emplace(std::coroutine_handle<> task) {
      _tasks.push(task);
    }

    void schedule() {
      while(!_tasks.empty()) {
        //auto task = _tasks.front();
        auto task = _tasks.top();
        _tasks.pop();
        task.resume();

        if(!task.done()) { 
          _tasks.push(task);
        }
        else {
          task.destroy();
        }
      }
    }

    auto suspend() {
      return std::suspend_always{};
    }
};

In the scheduler, I store tasks into a stack. I implement emplace method to allow users to push a task into the stack. In schedule method, I keep popping a task from the stack. After resuming a task, I check if that task is done. If not, I push the task back to the stack for later scheduling. Otherwise, I destroy the finished task. After executing the program, the results are the following:

The scheduler stores tasks using a stack (last in, first out). Interestingly, if I replace the stack with the queue (first in, first out), the execution results become:

For completeness, here are both programs:

// stackScheduler.cpp

#include <coroutine>
#include <iostream>
#include <stack>


struct Task {

  struct promise_type {
    std::suspend_always initial_suspend() noexcept { return {}; }
    std::suspend_always final_suspend() noexcept { return {}; }

    Task get_return_object() { 
        return std::coroutine_handle<promise_type>::from_promise(*this); 
    }
    void return_void() {}
    void unhandled_exception() {}
  };

  Task(std::coroutine_handle<promise_type> handle): handle{handle} {}

  auto get_handle() { return handle; }

  std::coroutine_handle<promise_type> handle;
};

class Scheduler {

  std::stack<std::coroutine_handle<>> _tasks;

  public: 

    void emplace(std::coroutine_handle<> task) {
      _tasks.push(task);
    }

    void schedule() {
      while(!_tasks.empty()) {
        auto task = _tasks.top();
        _tasks.pop();
        task.resume();

        if(!task.done()) { 
          _tasks.push(task);
        }
        else {
          task.destroy();
        }
      }
    }

    auto suspend() {
      return std::suspend_always{};
    }
};


Task TaskA(Scheduler& sch) {
  std::cout << "Hello from TaskA\n";
  co_await sch.suspend();
  std::cout << "Executing the TaskA\n";
  co_await sch.suspend();
  std::cout << "TaskA is finished\n";
}

Task TaskB(Scheduler& sch) {
  std::cout << "Hello from TaskB\n";
  co_await sch.suspend();
  std::cout << "Executing the TaskB\n";
  co_await sch.suspend();
  std::cout << "TaskB is finished\n";
}


int main() {

  std::cout << '\n';

  Scheduler sch;

  sch.emplace(TaskA(sch).get_handle());
  sch.emplace(TaskB(sch).get_handle());

  std::cout << "Start scheduling...\n";

  sch.schedule();

  std::cout << '\n';

}

// queueScheduler.cpp

#include <coroutine>
#include <iostream>
#include <queue>


struct Task {

  struct promise_type {
    std::suspend_always initial_suspend() noexcept { return {}; }
    std::suspend_always final_suspend() noexcept { return {}; }

    Task get_return_object() { 
        return std::coroutine_handle<promise_type>::from_promise(*this); 
    }
    void return_void() {}
    void unhandled_exception() {}
  };

  Task(std::coroutine_handle<promise_type> handle): handle{handle} {}

  auto get_handle() { return handle; }

  std::coroutine_handle<promise_type> handle;
};

class Scheduler {

  std::queue<std::coroutine_handle<>> _tasks;

  public: 

    void emplace(std::coroutine_handle<> task) {
      _tasks.push(task);
    }

    void schedule() {
      while(!_tasks.empty()) {
        auto task = _tasks.front();
        _tasks.pop();
        task.resume();

        if(!task.done()) { 
          _tasks.push(task);
        }
        else {
          task.destroy();
        }
      }
    }

    auto suspend() {
      return std::suspend_always{};
    }
};


Task TaskA(Scheduler& sch) {
  std::cout << "Hello from TaskA\n";
  co_await sch.suspend();
  std::cout << "Executing the TaskA\n";
  co_await sch.suspend();
  std::cout << "TaskA is finished\n";
}

Task TaskB(Scheduler& sch) {
  std::cout << "Hello from TaskB\n";
  co_await sch.suspend();
  std::cout << "Executing the TaskB\n";
  co_await sch.suspend();
  std::cout << "TaskB is finished\n";
}


int main() {

  std::cout << '\n';

  Scheduler sch;

  sch.emplace(TaskA(sch).get_handle());
  sch.emplace(TaskB(sch).get_handle());

  std::cout << "Start scheduling...\n";

  sch.schedule();

  std::cout << '\n';

}

What’s Next?

This blog post from Dian-Lun Lin showed a straightforward scheduler for coroutines. I use Dian-Lun’s scheduler in my next post for further experiments.

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