A Priority Scheduler for Coroutines

In this post, I will extend the straightforward scheduler from Dian-Lun with priorities.

This is the third post in my mini-series about schedulers for C++ coroutines. The first two posts were guest posts by Dian-Lun Lin:

Dian-Lun’s schedulers were based on the container adaptor std::stack and std::queue. The std::stack schedules its tasks according to its strategy last-in first-out, but the std::queue applies first-in first-out.

The following code snippet shows the queue-based scheduler:

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{};
    }
};

Extending this scheduler with priorities is pretty straightforward.

A Priority-Queue based Scheduler

std::priority_queue is besides std::stack, and std::queue the third container adaptor in C++98.

The std::priority_queue is a similar to a std::queue. The main difference to the std::queue is that their greatest element is always at the top of the priority queue. std::priority_queue uses by default the comparison operator std::less. The lookup time into a std::priority_queue is constant, but the insertion and extraction are logarithmic.

Let me exchange the std::queue in the previous scheduler with a std::priority_queue:

// priority_queueScheduler.cpp

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


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 {
                                                            // (1)
  std::priority_queue<std::pair<int, std::coroutine_handle<>>> _prioTasks;

  public: 

    void emplace(int prio, std::coroutine_handle<> task) {   // (2)
      _prioTasks.push(std::make_pair(prio, task));
    }

    void schedule() {
      while(!_prioTasks.empty()) {                           // (3)
        auto [prio, task] = _prioTasks.top();
        _prioTasks.pop();
        task.resume();

        if(!task.done()) { 
          _prioTasks.push(std::make_pair(prio, task));        // (4)
        }
        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 scheduler1;

  scheduler1.emplace(0, TaskA(scheduler1).get_handle());       // (5)   
  scheduler1.emplace(1, TaskB(scheduler1).get_handle());

  scheduler1.schedule();

  std::cout << '\n';

  Scheduler scheduler2;

  scheduler2.emplace(1, TaskA(scheduler2).get_handle());      // (6)
  scheduler2.emplace(0, TaskB(scheduler2).get_handle());

  scheduler2.schedule();

  std::cout << '\n';

}

First, the std::priority_queue uses a pair (priority, handle) (line 1). Now, this pair is placed on the _prioTask (line 2). When the scheduler runs, it checks if the _prioTask is empty (line 3). If not, the first task is accessed, removed, and resumed. When the task is not done, it is pushed back onto the _prioTasks (line 4).

Using a std::priority_queue<std::pair<int, std::coroutine_handle<>>> has the nice side-effect, that tasks with higher priority run first. It makes no difference, in which order the tasks are placed on the scheduler (lines 5 and 6); the task with priority 1 runs first.

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Let me simplify the coroutine, before I improve its priority handling in my next post.

The Simplified Coroutine

Here are the previous coroutines TaskA and TaskB:

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";
}

First, instead of calling co_await on the scheduler, I replace it with the direct call of the predefined awaitable std::suspend_always. This allows me to remove the suspend member function of the scheduler. Second, the coroutine gets the name of its task:

Task createTask(const std::string& name) {
  std::cout << name << " start\n";
  co_await std::suspend_always();
  std::cout << name << " execute\n";
  co_await std::suspend_always();
  std::cout << name << " finish\n";
}

Finally, here is the simplified program with the corresponding output.

// priority_queueSchedulerSimplified.cpp

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


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::priority_queue<std::pair<int, std::coroutine_handle<>>> _prioTasks;

  public: 

    void emplace(int prio, std::coroutine_handle<> task) {
      _prioTasks.push(std::make_pair(prio, task));
    }

    void schedule() {
      while(!_prioTasks.empty()) {
        auto [prio, task] = _prioTasks.top();
        _prioTasks.pop();
        task.resume();

        if(!task.done()) { 
          _prioTasks.push(std::make_pair(prio, task));
        }
        else {
          task.destroy();
        }
      }
    }

};


Task createTask(const std::string& name) {
  std::cout << name << " start\n";
  co_await std::suspend_always();
  std::cout << name << " execute\n";
  co_await std::suspend_always();
  std::cout << name << " finish\n";
}


int main() {

  std::cout << '\n';

  Scheduler scheduler1;

  scheduler1.emplace(0, createTask("TaskA").get_handle());
  scheduler1.emplace(1, createTask("  TaskB").get_handle());

  scheduler1.schedule();

  std::cout << '\n';

  Scheduler scheduler2;

  scheduler2.emplace(1, createTask("TaskA").get_handle());
  scheduler2.emplace(0, createTask("  TaskB").get_handle());

  scheduler2.schedule();

  std::cout << '\n';

}

What’s Next?

In my next post, I will improve the priority handling of the tasks.

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