tf::Runtime class | Taskflow QuickStart

# tf::Runtime class

class to include a runtime object in a task

A runtime object allows users to interact with the scheduling runtime inside a task, such as scheduling an active task, spawning a subflow, and so on.

    tf::Task A, B, C, D;
    std::tie(A, B, C, D) = taskflow.emplace(
      [] () { return 0; },
      [&C] (tf::Runtime& rt) {  // C must be captured by reference
        std::cout << "B\n";
        rt.schedule(C);
      },
      [] () { std::cout << "C\n"; },
      [] () { std::cout << "D\n"; }
    );
    A.precede(B, C, D);
    executor.run(taskflow).wait();

A runtime object is associated with the worker and the executor that runs the task.

## Public functions

auto executor() -> Executor&
  obtains the running executor
auto worker() -> Worker&
  acquire a reference to the underlying worker
void schedule(Task task)
  schedules an active task immediately to the worker's queue
template<typename F>
auto async(F&& f) -> auto
  runs the given callable asynchronously
template<typename P, typename F>
auto async(P&& params, F&& f) -> auto
  runs the given callable asynchronously
template<typename F>
void silent_async(F&& f)
  runs the given function asynchronously without returning any future object
template<typename P, typename F>
void silent_async(P&& params, F&& f)
  runs the given function asynchronously without returning any future object
template<typename T>
void corun(T&& target)
  co-runs the given target and waits until it completes
void corun_all()
  corun all asynchronous tasks spawned by this runtime with other workers

## Function documentation

### Executor& tf::Runtime::executor()

obtains the running executor

The running executor of a runtime task is the executor that runs the parent taskflow of that runtime task.

    tf::Executor executor;
    tf::Taskflow taskflow;
    taskflow.emplace([&](tf::Runtime& rt){
      assert(&(rt.executor()) == &executor);
    });
    executor.run(taskflow).wait();

### void tf::Runtime::schedule(Task task)

schedules an active task immediately to the worker's queue

Parameters
task | the given active task to schedule immediately

This member function immediately schedules an active task to the task queue of the associated worker in the runtime task. An active task is a task in a running taskflow. The task may or may not be running, and scheduling that task will immediately put the task into the task queue of the worker that is running the runtime task. Consider the following example:

    tf::Task A, B, C, D;
    std::tie(A, B, C, D) = taskflow.emplace(
      [] () { return 0; },
      [&C] (tf::Runtime& rt) {  // C must be captured by reference
        std::cout << "B\n";
        rt.schedule(C);
      },
      [] () { std::cout << "C\n"; },
      [] () { std::cout << "D\n"; }
    );
    A.precede(B, C, D);
    executor.run(taskflow).wait();

The executor will first run the condition task `A` which returns `0` to inform the scheduler to go to the runtime task `B`. During the execution of `B`, it directly schedules task `C` without going through the normal taskflow graph scheduling process. At this moment, task `C` is active because its parent taskflow is running. When the taskflow finishes, we will see both `B` and `C` in the output.

###
template<typename F>
auto tf::Runtime::async(F&& f)

runs the given callable asynchronously

Template parameters
F | callable type
Parameters
f | callable object

The method creates an asynchronous task to launch the given function on the given arguments. The difference to tf::Executor::async is that the created asynchronous task pertains to the runtime object. Applications can explicitly issue tf::Runtime::corun_all to wait for all spawned asynchronous tasks to finish. For example:

    std::atomic<int> counter(0);
    taskflow.emplace([&](tf::Runtime& rt){
      auto fu1 = rt.async([&](){ counter++; });
      auto fu2 = rt.async([&](){ counter++; });
      fu1.get();
      fu2.get();
      assert(counter == 2);

      // spawn 100 asynchronous tasks from the worker of the runtime
      for(int i=0; i<100; i++) {
        rt.async([&](){ counter++; });
      }

      // wait for the 100 asynchronous tasks to finish
      rt.corun_all();
      assert(counter == 102);
    });

This method is thread-safe and can be called by multiple workers that hold the reference to the runtime. For example, the code below spawns 100 tasks from the worker of a runtime, and each of the 100 tasks spawns another task that will be run by another worker.

    std::atomic<int> counter(0);
    taskflow.emplace([&](tf::Runtime& rt){
      // worker of the runtime spawns 100 tasks each spawning another task
      // that will be run by another worker
      for(int i=0; i<100; i++) {
        rt.async([&](){
          counter++;
          rt.async([](){ counter++; });
        });
      }

      // wait for the 200 asynchronous tasks to finish
      rt.corun_all();
      assert(counter == 200);
    });

###
template<typename P, typename F>
auto tf::Runtime::async(P&& params, F&& f)

runs the given callable asynchronously

Template parameters
P | task parameters type
F | callable type
Parameters
params | task parameters
f | callable

    taskflow.emplace([&](tf::Runtime& rt){
      auto future = rt.async("my task", [](){});
      future.get();
    });

###
template<typename F>
void tf::Runtime::silent_async(F&& f)

runs the given function asynchronously without returning any future object

Template parameters
F | callable type
Parameters
f | callable

This member function is more efficient than tf::Runtime::async and is encouraged to use when there is no data returned.

    std::atomic<int> counter(0);
    taskflow.emplace([&](tf::Runtime& rt){
      for(int i=0; i<100; i++) {
        rt.silent_async([&](){ counter++; });
      }
      rt.corun_all();
      assert(counter == 100);
    });

This member function is thread-safe.

###
template<typename P, typename F>
void tf::Runtime::silent_async(P&& params, F&& f)

runs the given function asynchronously without returning any future object

Template parameters
F | callable type
Parameters
params | task parameters
f | callable

    taskflow.emplace([&](tf::Runtime& rt){
      rt.silent_async("my task", [](){});
      rt.corun_all();
    });

###
template<typename T>
void tf::Runtime::corun(T&& target)

co-runs the given target and waits until it completes

A corunnable target must have `tf::Graph& T::graph()` defined.

co-run a taskflow and wait until all tasks complete

    tf::Taskflow taskflow1, taskflow2;
    taskflow1.emplace([](){ std::cout << "running taskflow1\n"; });
    taskflow2.emplace([&](tf::Runtime& rt){
      std::cout << "running taskflow2\n";
      rt.corun(taskflow1);
    });
    executor.run(taskflow2).wait();

Although tf::Runtime::corun blocks until the operation completes, the caller thread (worker) is not blocked (e.g., sleeping or holding any lock). Instead, the caller thread joins the work-stealing loop of the executor and returns when all tasks in the target completes.

#### Attention

The method is not thread-safe as it modifies the anchor state of the node for exception handling.

### void tf::Runtime::corun_all()

corun all asynchronous tasks spawned by this runtime with other workers

Coruns all asynchronous tasks (tf::Runtime::async, tf::Runtime::silent_async) with other workers until all those asynchronous tasks finish.

    std::atomic<size_t> counter{0};
    taskflow.emplace([&](tf::Runtime& rt){
      // spawn 100 async tasks and wait
      for(int i=0; i<100; i++) {
        rt.silent_async([&](){ counter++; });
      }
      rt.corun_all();
      assert(counter == 100);

      // spawn another 100 async tasks and wait
      for(int i=0; i<100; i++) {
        rt.silent_async([&](){ counter++; });
      }
      rt.corun_all();
      assert(counter == 200);
    });

#### Attention

The method is not thread-safe as it modifies the anchor state of the node for exception handling.

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