| Taskflow: A General-purpose Task-parallel Programming System

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Executor

After you create a task dependency graph, you need to submit it to threads for execution. In this chapter, we will show you how to execute a task dependency graph.

# Create an Executor

To execute a taskflow, you need to create an executor of type tf::Executor. An executor is a thread-safe object that manages a set of worker threads and executes tasks through an efficient work-stealing algorithm. Issuing a call to run a taskflow creates a topology, a data structure to keep track of the execution status of a running graph. tf::Executor takes an unsigned integer to construct with `N` worker threads. The default value is std::thread::hardware_concurrency.

tf::Executor executor1; // create an executor with the number of workers
// equal to std::thread::hardware_concurrency
tf::Executor executor2(4); // create an executor of 4 worker threads
tf::Executor
class to create an executor
Definition executor.hpp:62

Note
  Creating a tf::Executor has non-negligible overhead. Unless your application requires multiple executors, we recommend creating a single tf::Executor and reusing it to run multiple taskflows.

# Understand Work Stealing in Executor

Taskflow designs a highly efficient work-stealing algorithm to schedule and run tasks in an executor. Work-stealing is a dynamic scheduling algorithm widely used in parallel computing to distribute and balance workload among multiple threads or cores. Specifically, within an executor, each worker maintains its own local queue of tasks. When a worker finishes its own tasks, instead of becoming idle or going sleep, it (thief) tries to steal a task from the queue another worker (victim). The figure below illustrates the idea of work-stealing:

The key advantage of work-stealing lies in its decentralized nature and efficiency. Most of the time, worker threads work on their local queues without contention. Stealing only occurs when a worker becomes idle, minimizing overhead associated with synchronization and task distribution. This decentralized strategy effectively balances the workload, ensuring that idle workers are put to work and that the overall computation progresses efficiently. That being said, the internal scheduling mechanisms in tf::Executor are not trivial, and it's not easy to explain every detail in just a few sentences. If you're interested in learning more about the technical details, please refer to our paper published in 2022 IEEE Transactions on Parallel and Distributed Systems (TPDS):

- Tsung-Wei Huang, Dian-Lun Lin, Chun-Xun Lin, and Yibo Lin, "Taskflow: A Lightweight Parallel and Heterogeneous Task Graph Computing System," IEEE Transactions on Parallel and Distributed Systems (TPDS), vol. 33, no. 6, pp. 1303-1320, June 2022

# Execute a Taskflow

tf::Executor provides a set of `run_*` methods, tf::Executor::run, tf::Executor::run_n, and tf::Executor::run_until to run a taskflow for one time, multiple times, or until a given predicate evaluates to true. All methods accept an optional callback to invoke after the execution completes, and return a tf::Future for users to access the execution status. The code below shows several ways to run a taskflow.

// Declare an executor and a taskflow
tf::Executor executor;
tf::Taskflow taskflow;
// Add three tasks into the taskflow
tf::Task A = taskflow.emplace([] () { std::cout << "This is TaskA\n"; });
tf::Task B = taskflow.emplace([] () { std::cout << "This is TaskB\n"; });
tf::Task C = taskflow.emplace([] () { std::cout << "This is TaskC\n"; });
// Build precedence between tasks
A.precede(B, C);
tf::Future<void> fu = executor.run(taskflow);
fu.wait(); // block until the execution completes
executor.run(taskflow, [](){ std::cout << "end of 1 run"; }).wait();
executor.run_n(taskflow, 4);
executor.wait_for_all(); // block until all associated executions finish
executor.run_n(taskflow, 4, [](){ std::cout << "end of 4 runs"; }).wait();
executor.run_until(taskflow, [cnt=0] () mutable { return ++cnt == 10; });
tf::Executor::run_until
tf::Future< void > run_until(Taskflow &taskflow, P &&pred)
runs a taskflow multiple times until the predicate becomes true
tf::Executor::run
tf::Future< void > run(Taskflow &taskflow)
runs a taskflow once
tf::Executor::run_n
tf::Future< void > run_n(Taskflow &taskflow, size_t N)
runs a taskflow for N times
tf::Executor::wait_for_all
void wait_for_all()
waits for all tasks to complete
tf::FlowBuilder::emplace
Task emplace(C &&callable)
creates a static task
Definition flow_builder.hpp:1738
tf::Future
class to access the result of an execution
Definition taskflow.hpp:640
tf::Task
class to create a task handle over a taskflow node
Definition task.hpp:569
tf::Task::precede
Task & precede(Ts &&... tasks)
adds precedence links from this to other tasks
Definition task.hpp:1305
tf::Taskflow
class to create a taskflow object
Definition taskflow.hpp:64

Here, we first create a taskflow consisting of three tasks, A, B, and C. Then, we execute the taskflow once and block until its completion through `std::future::wait`. Next, we use the `run` series to run the taskflow once, four times, and until the given predicate return `true`.

# Understand the Execution Order

While it is possible to submit the same taskflow to an executor multiple times, these multiple runs will be synchronized to a sequential chain of executions in the order of their submissions. For example, the three submissions below will finish in order, where execution #1 completes before #2, and execution #2 completes before execution #3:

executor.run(taskflow); // execution #1
executor.run_n(taskflow, 10); // execution #2
executor.run(taskflow); // execution #3
executor.wait_for_all(); // execution #1 -> execution #2 -> execution #3

However, there is no deterministic order for different taskflows submitted simultaneously. For example, the three submissions below may finish in an arbitrary order, since the three taskflows are distinct:

executor.run(taskflow1); // execution 1
executor.run_n(taskflow2, 10); // execution 2
executor.run(taskflow3); // execution 3
executor.wait_for_all(); // no order guarantee among the three taskflows

# Understand the Ownership

A running taskflow must remain alive for the duration of its execution because the executor does not take ownership of the taskflow. It is your responsibility to ensure that a taskflow is not destroyed while it is still running. For example, the code below can result in undefined behavior.

tf::Executor executor; // create an executor
// create a taskflow whose lifetime is restricted by the block statement
{
tf::Taskflow taskflow;
// adds tasks to the taskflow
// ...
// runs the taskflow once
executor.run(taskflow);
} // leaving the block statement will destroy taskflow while it is running,
// resulting in undefined behavior

Similarly, you should avoid modifying a taskflow while it is running:

tf::Taskflow taskflow;
// Add tasks into the taskflow
// ...
// Declare an executor
tf::Executor executor;
tf::Future<void> future = executor.run(taskflow);
// alter the taskflow while running leads to undefined behavior
taskflow.emplace([](){ std::cout << "Add a new task\n"; });

You must always keep a taskflow alive and must not modify it while it is running on an executor.

# Execute a Taskflow with Transferred Ownership

You can transfer the ownership of a taskflow to an executor and run it without wrangling with the lifetime issue of that taskflow. Each `run_*` method discussed in the previous section comes with an overload that takes a moved taskflow object.

tf::Taskflow taskflow;
tf::Executor executor;
taskflow.emplace([](){});
// let the executor manage the lifetime of the submitted taskflow
executor.run(std::move(taskflow));
// now taskflow has no tasks
assert(taskflow.num_tasks() == 0);
tf::Taskflow::num_tasks
size_t num_tasks() const
queries the number of tasks in this taskflow
Definition taskflow.hpp:376

However, you should avoid moving a running taskflow which can result in undefined behavior.

tf::Taskflow taskflow;
tf::Executor executor;
taskflow.emplace([](){});
// executor does not manage the lifetime of taskflow
executor.run(taskflow);
// error! you cannot move a taskflow while it is running
executor.run(std::move(taskflow));

The correct way to submit a taskflow with moved ownership to an executor is to ensure all previous runs have completed. The executor will automatically release the resources of a moved taskflow right after its execution completes.

// submit the taskflow and wait until it completes
executor.run(taskflow).wait();
// now it's safe to move the taskflow to the executor and run it
executor.run(std::move(taskflow));

Likewise, you cannot move a taskflow that is running on an executor. You must wait until all the previous fires of runs on that taskflow complete before calling move.

// submit the taskflow and wait until it completes
executor.run(taskflow).wait();
// now it's safe to move the taskflow to another
tf::Taskflow moved_taskflow(std::move(taskflow));

# Execute a Taskflow from an Internal Worker Cooperatively

Each run variant of tf::Executor returns a tf::Future object that allows you to wait for the associated execution to complete. When `tf::Future::wait` is called, the caller blocks without making any progress until the underlying state becomes ready. However, this blocking design can lead to potential deadlocks, particularly when multiple taskflows are launched from within the internal workers of the same executor. For example, the following code creates a taskflow of 1,000 tasks, where each task runs another taskflow of 500 tasks in a blocking manner, leading to a potential deadlock problem:

tf::Executor executor(2);
tf::Taskflow taskflow;
std::array<tf::Taskflow, 1000> others;
for(size_t n=0; n<1000; n++) {
for(size_t i=0; i<500; i++) {
others[n].emplace([&](){});
}
taskflow.emplace([&executor, &tf=others[n]](){
// Blocking a worker can cause deadlock if all workers are waiting
// for their taskflows to complete without making any progress
executor.run(tf).wait();
});
}
executor.run(taskflow).wait();
tf
taskflow namespace
Definition small_vector.hpp:20

To avoid this deadlock issue, tf::Executor provides a cooperative execution method, tf::Executor::corun, which allows a worker to execute a taskflow cooperatively with other workers within the same executor. Specifically, although the worker is blocking until corun finishes, it is cooperatively blocked and continues to execute the taskflow alongside other tasks in the executor's work-stealing loop. For instance, the following code with tf::Executor::corun is deadlock-free:

tf::Executor executor(2);
tf::Taskflow taskflow;
std::array<tf::Taskflow, 1000> others;
std::atomic<size_t> counter{0};
for(size_t n=0; n<1000; n++) {
for(size_t i=0; i<500; i++) {
others[n].emplace([&](){ counter++; });
}
taskflow.emplace([&executor, &tf=others[n]](){
// calling worker coruns the taskflow cooperatively with other workers
executor.corun(tf);
});
}
executor.run(taskflow).wait();
tf::Executor::corun
void corun(T &target)
runs a target graph and waits until it completes using an internal worker of this executor

Similar to tf::Executor::corun, tf::Executor::corun_until provides a more general mechanism for cooperative execution. It allows the calling worker to run tasks cooperatively with other workers until a specified condition becomes true. This function is especially useful for implementing custom synchronization or polling conditions, where the worker continues making progress by executing available tasks in the executor while periodically checking for the termination condition.

taskflow.emplace([&](){
auto fu = std::async([](){ std::sleep(100s); });
executor.corun_until([](){
return fu.wait_for(std::chrono::seconds(0)) == future_status::ready;
});
});
tf::Executor::corun_until
void corun_until(P &&predicate)
keeps running the work-stealing loop until the predicate returns true

Note
  You must call tf::Executor::corun_until and tf::Executor::corun from a worker of the calling executor or an exception will be thrown.

# Thread Safety of Executor

All `run_*` methods of tf::Executor are thread-safe. You can safely call these methods from multiple threads to execute different taskflows concurrently. For instance, the code below launches ten threads, each executing a different taskflow concurrently on the same executor, and waits for all executions to complete:

tf::Executor executor;
std::array<tf::Taskflow, 10> taskflows;
for(int i=0; i<10; ++i) {
std::thread([i, &](){
executor.run(taskflows[i]); // thread i runs taskflow i
}).detach();
}
executor.wait_for_all();

# Query the Worker ID

Each worker thread in a tf::Executor is assigned a unique integer identifier in the range `[0, N)`, where `N` is the number of worker threads in the executor. You can query the identifier of the calling thread using tf::Executor::this_worker_id. If the calling thread is not a worker of the executor, the method returns -1. This functionality is particularly useful for establishing a one-to-one mapping between worker threads and application-specific data structures.

std::vector<int> data[8]; // worker-specific data vector
tf::Taskflow taskflow;
tf::Executor executor(8); // an executor of eight workers
assert(executor.this_worker_id() == -1); // main thread is not a worker
taskflow.emplace([&](){
int id = executor.this_worker_id(); // in the range [0, 8)
auto& vec = data[id]; // worker id process data[id]
});
tf::Executor::this_worker_id
int this_worker_id() const
queries the id of the caller thread within this executor

# Observe Thread Activities

You can observe thread activities in an executor when a worker thread participates in executing a task and leaves the execution using tf::ObserverInterface – an interface class that provides a set of methods for you to define what to do when a thread enters and leaves the execution context of a task.

class ObserverInterface {
virtual ~ObserverInterface() = default;
virtual void set_up(size_t num_workers) = 0;
virtual void on_entry(tf::WorkerView worker_view, tf::TaskView task_view) = 0;
virtual void on_exit(tf::WorkerView worker_view, tf::TaskView task_view) = 0;
};
tf::TaskView
class to access task information from the observer interface
Definition task.hpp:1613
tf::WorkerView
class to create an immutable view of a worker
Definition worker.hpp:119

There are three methods you must define in your derived class, tf::ObserverInterface::set_up, tf::ObserverInterface::on_entry, and tf::ObserverInterface::on_exit. The method, tf::ObserverInterface::set_up, is a constructor-like method that will be called by the executor when the observer is constructed. It passes an argument of the number of workers to observer in the executor. You may use it to preallocate or initialize data storage, e.g., an independent vector for each worker. The methods, tf::ObserverInterface::on_entry and tf::ObserverInterface::on_exit, are called by a worker thread before and after the execution context of a task, respectively. Both methods provide immutable access to the underlying worker and the running task using tf::WorkerView and tf::TaskView. You may use them to record timepoints and calculate the elapsed time of a task.

You can associate an executor with one or multiple observers using tf::Executor::make_observer. We use std::shared_ptr to manage the ownership of an observer. The executor loops through each observer and invoke the corresponding methods accordingly.

#include <taskflow/taskflow.hpp>
struct MyObserver : public tf::ObserverInterface {
MyObserver(const std::string& name) {
std::cout << "constructing observer " << name << '\n';
}
void set_up(size_t num_workers) override final {
std::cout << "setting up observer with " << num_workers << " workers\n";
}
void on_entry(tf::WorkerView w, tf::TaskView tv) override final {
std::ostringstream oss;
oss << "worker " << w.id() << " ready to run " << tv.name() << '\n';
std::cout << oss.str();
}
void on_exit(tf::WorkerView w, tf::TaskView tv) override final {
std::ostringstream oss;
oss << "worker " << w.id() << " finished running " << tv.name() << '\n';
std::cout << oss.str();
}
};
int main(){
tf::Executor executor(4);
// Create a taskflow of eight tasks
tf::Taskflow taskflow;
auto A = taskflow.emplace([] () { std::cout << "1\n"; }).name("A");
auto B = taskflow.emplace([] () { std::cout << "2\n"; }).name("B");
auto C = taskflow.emplace([] () { std::cout << "3\n"; }).name("C");
auto D = taskflow.emplace([] () { std::cout << "4\n"; }).name("D");
auto E = taskflow.emplace([] () { std::cout << "5\n"; }).name("E");
auto F = taskflow.emplace([] () { std::cout << "6\n"; }).name("F");
auto G = taskflow.emplace([] () { std::cout << "7\n"; }).name("G");
auto H = taskflow.emplace([] () { std::cout << "8\n"; }).name("H");
// create an observer
std::shared_ptr<MyObserver> observer = executor.make_observer<MyObserver>(
"MyObserver"
);
// run the taskflow
executor.run(taskflow).get();
// remove the observer (optional)
executor.remove_observer(std::move(observer));
return 0;
}
tf::Executor::remove_observer
void remove_observer(std::shared_ptr< Observer > observer)
removes an observer from the executor
tf::Executor::make_observer
std::shared_ptr< Observer > make_observer(ArgsT &&... args)
constructs an observer to inspect the activities of worker threads
tf::ObserverInterface
class to derive an executor observer
Definition interface.hpp:81
tf::ObserverInterface::set_up
virtual void set_up(size_t num_workers)=0
constructor-like method to call when the executor observer is fully created
tf::ObserverInterface::on_entry
virtual void on_entry(WorkerView wv, TaskView task_view)=0
method to call before a worker thread executes a closure
tf::ObserverInterface::on_exit
virtual void on_exit(WorkerView wv, TaskView task_view)=0
method to call after a worker thread executed a closure

The above code produces the following output:

constructing observer MyObserver
setting up observer with 4 workers
worker 2 ready to run A
1
worker 2 finished running A
worker 2 ready to run B
2
worker 1 ready to run C
worker 2 finished running B
3
worker 2 ready to run D
worker 3 ready to run E
worker 1 finished running C
4
5
worker 1 ready to run F
worker 2 finished running D
worker 3 finished running E
6
worker 2 ready to run G
worker 3 ready to run H
worker 1 finished running F
7
8
worker 2 finished running G
worker 3 finished running H

It is expected each line of std::cout interleaves with each other as there are four workers participating in task scheduling. However, the ready message always appears before the corresponding task message (e.g., numbers) and then the finished message.

# Modify Worker Property

You can change the property of each worker thread from its executor, such as assigning thread-processor affinity before the worker enters the scheduler loop and post-processing additional information after the worker leaves the scheduler loop, by passing an instance derived from tf::WorkerInterface to the executor. The example demonstrates the usage of tf::WorkerInterface to affine a worker to a specific CPU core equal to its id on a Linux platform:

// affine the given thread to the given core index (linux-specific)
bool affine(std::thread& thread, unsigned int core_id) {
cpu_set_t cpuset;
CPU_ZERO(&cpuset);
CPU_SET(core_id, &cpuset);
pthread_t native_handle = thread.native_handle();
return pthread_setaffinity_np(native_handle, sizeof(cpu_set_t), &cpuset) == 0;
}
class CustomWorkerBehavior : public tf::WorkerInterface {
public:
// to call before the worker enters the scheduling loop
void scheduler_prologue(tf::Worker& w) override {
printf("worker %lu prepares to enter the work-stealing loop\n", w.id());
// now affine the worker to a particular CPU core equal to its id
if(affine(w.thread(), w.id())) {
printf("successfully affines worker %lu to CPU core %lu\n", w.id(), w.id());
}
else {
printf("failed to affine worker %lu to CPU core %lu\n", w.id(), w.id());
}
}
// to call after the worker leaves the scheduling loop
void scheduler_epilogue(tf::Worker& w, std::exception_ptr) override {
printf("worker %lu left the work-stealing loop\n", w.id());
}
};
int main() {
tf::Executor executor(4, tf::make_worker_interface<CustomWorkerBehavior>());
return 0;
}
tf::WorkerInterface
class to configure worker behavior in an executor
Definition worker.hpp:277
tf::WorkerInterface::scheduler_epilogue
virtual void scheduler_epilogue(Worker &worker, std::exception_ptr ptr)=0
method to call after a worker leaves the scheduling loop
tf::WorkerInterface::scheduler_prologue
virtual void scheduler_prologue(Worker &worker)=0
method to call before a worker enters the scheduling loop
tf::Worker
class to create a worker in an executor
Definition worker.hpp:55
tf::Worker::id
size_t id() const
queries the worker id associated with its parent executor
Definition worker.hpp:76
tf::Worker::thread
std::thread & thread()
acquires the associated thread
Definition worker.hpp:92
tf::make_worker_interface
std::shared_ptr< T > make_worker_interface(ArgsT &&... args)
helper function to create an instance derived from tf::WorkerInterface
Definition worker.hpp:316

When running the program, we see the following one possible output:

worker 3 prepares to enter the work-stealing loop
successfully affines worker 3 to CPU core 3
worker 3 left the work-stealing loop
worker 0 prepares to enter the work-stealing loop
successfully affines worker 0 to CPU core 0
worker 0 left the work-stealing loop
worker 1 prepares to enter the work-stealing loop
worker 2 prepares to enter the work-stealing loop
successfully affines worker 1 to CPU core 1
worker 1 left the work-stealing loop
successfully affines worker 2 to CPU core 2
worker 2 left the work-stealing loop

When you create an executor, it spawns a set of worker threads to run tasks using a work-stealing scheduling algorithm. The execution logic of the scheduler and its interaction with each spawned worker via tf::WorkerInterface is given below:

for(size_t n=0; n<num_workers; n++) {
create_thread([](Worker& worker){
// enter the scheduling loop
// Here, WorkerInterface::scheduler_prologue is invoked, if any
worker_interface->scheduler_prologue(worker);
try {
while(1) {
perform_work_stealing_algorithm();
if(stop) {
break;
}
}
} catch(...) {
exception_ptr = std::current_exception();
}
// leaves the scheduling loop and joins this worker thread
// Here, WorkerInterface::scheduler_epilogue is invoked, if any
worker_interface->scheduler_epilogue(worker, exception_ptr);
});
}

Note
  tf::WorkerInterface::scheduler_prologue and tf::WorkerInterface::scheduler_epilogue are invoked by each worker simultaneously. It is your responsibility to ensure that no data race can occur during their invocation.