Task Scheduling and Executors

This section covers the task concept, coroutine-based pipeline processing, async/await patterns Across languages, structured concurrency with when_all/when_anyA complete Task class wrapping A coroutine, and a thread pool executor for scheduling coroutines across threads.

Task Concept

A task is a coroutine that produces a result asynchronously. Unlike a generator (which produces Many values), a task produces exactly one result upon completion. The task coroutine is Lazy — it does not begin executing until someone calls resume() or an executor schedules it.

The minimal interface for a task is:

Awaitable: the task can be co_awaitEd, suspending the awaiting coroutine until the task completes.
Result access: once the task completes, its result is available.
Exception propagation: if the task throws, the exception is rethrown at the co_await point.

Cooperative Scheduling with Coroutines

Proof: Cooperative Scheduling Avoids Data Races

Claim: A cooperative scheduler that runs only one coroutine per thread at any given time, and Only switches between coroutines at explicit suspension points (co_await), cannot introduce data Races on non-atomic variables.

Proof:

A data race requires two conflicting accesses from different threads that are not ordered by happens-before [N4950 §6.9.4.1].
In a cooperative scheduler, each thread runs at most one coroutine at a time. There is no preemption — a coroutine runs until it explicitly suspends.
Within a single coroutine, all accesses are sequenced (the coroutine is a single thread of execution).
Two coroutines running on different threads access shared data only through explicit synchronization (mutexes, atomics) because the scheduler provides no implicit sharing mechanism.
If shared data is accessed without synchronization, the accesses are from different threads and are not ordered by happens-before — this is a data race. But this is a programmer error, not a scheduler error.
The scheduler itself does not introduce concurrency between coroutines on the same thread, so it does not introduce data races.

$\square$

The key insight is that cooperative scheduling on a single thread is equivalent to single-threaded Execution with voluntary context switches. Data races require parallel access from multiple Threads, which cooperative scheduling does not create.

Comparison with Thread-Based Concurrency

Property	Thread-Based	Cooperative Coroutines
Context switch cost	~1-10 $\mu$ S (kernel)	~10-100 ns (user-space)
Stack size per task	~1-8 MB	~100-1000 bytes (frame)
Data race risk	High (preemptive)	Low (cooperative)
Deadlock risk	Possible	Possible (less likely)
Memory usage	High (stack per thread)	Low (frame per coroutine)
Debugging	Harder (timing-dependent)	Easier (deterministic order)
CPU-bound work	Good (parallel)	Poor (single thread)
I/O-bound work	Good (blocked thread freed)	Excellent (zero-cost suspend)

Coroutine-Based Pipeline Processing

Coroutines enable natural expression of data pipelines where each stage can suspend and resume Independently:

\mathrm{source \xrightarrow{\mathrm{co\_await} \mathrm{transform_1 \xrightarrow{\mathrm{co\_await} \mathrm{transform_2 \xrightarrow{\mathrm{co\_await} \mathrm{sink

Each stage is a coroutine that reads from the previous stage and writes to the next, with suspension Occurring whenever data is not yet available.

Pipeline Implementation

#include <coroutine>
#include <iostream>
#include <vector>
#include <queue>
#include <mutex>
#include <condition_variable>
#include <optional>

template<typename T>
class Channel {
public:
    void push(T value) {
        {
            std::lock_guard<std::mutex> lock(mtx_);
            queue_.push(std::move(value));
        }
        cv_.notify_one();
    }

    std::optional<T> try_pop() {
        std::lock_guard<std::mutex> lock(mtx_);
        if (queue_.empty()) return std::nullopt;
        T val = std::move(queue_.front());
        queue_.pop();
        return val;
    }

    T pop() {
        std::unique_lock<std::mutex> lock(mtx_);
        cv_.wait(lock, [this] { return !queue_.empty() || closed_; });
        if (queue_.empty()) throw std::runtime_error{"channel closed"};
        T val = std::move(queue_.front());
        queue_.pop();
        return val;
    }

    void close() {
        {
            std::lock_guard<std::mutex> lock(mtx_);
            closed_ = true;
        }
        cv_.notify_all();
    }

    bool is_closed() const {
        std::lock_guard<std::mutex> lock(mtx_);
        return closed_ && queue_.empty();
    }

private:
    mutable std::mutex mtx_;
    std::condition_variable cv_;
    std::queue<T> queue_;
    bool closed_ = false;
};

template<typename T>
class Generator {
public:
    struct promise_type {
        Channel<T>* output_{};

        std::suspend_never initial_suspend() noexcept { return {}; }
        std::suspend_always final_suspend() noexcept { return {}; }
        void return_void() {}
        void unhandled_exception() { std::terminate(); }

        Generator get_return_object() {
            return Generator{std::coroutine_handle<promise_type>::from_promise(*this)};
        }

        auto yield_value(T value) {
            if (output_) output_->push(std::move(value));
            return std::suspend_always{};
        }
    };

    std::coroutine_handle<promise_type> handle;

    Generator(std::coroutine_handle<promise_type> h) : handle(h) {}
    ~Generator() { if (handle) handle.destroy(); }
    Generator(const Generator&) = delete;
    Generator& operator=(const Generator&) = delete;

    void set_output(Channel<T>& ch) { handle.promise().output_ = &ch; }

    void run() {
        while (!handle.done()) {
            handle.resume();
        }
    }
};

void pipeline_demo() {
    Channel<int> ch1;
    Channel<int> ch2;

    Generator<int> source([&ch1]() -> Generator<int> {
        for (int i = 1; i <= 5; ++i) {
            co_yield i;
        }
    }());

    source.set_output(ch1);

    Generator<int> transform([&ch1, &ch2]() -> Generator<int> {
        while (true) {
            auto val = ch1.try_pop();
            if (!val) break;
            co_yield val.value() * 10;
        }
    }());

    transform.set_output(ch2);

    std::thread t1([&] { source.run(); });
    std::thread t2([&] { transform.run(); });

    while (!ch2.is_closed()) {
        auto val = ch2.try_pop();
        if (val) std::cout << "  result: " << val.value() << "\n";
    }

    t1.join();
    t2.join();
}

int main() {
    pipeline_demo();
    return 0;
}

Async/Await Patterns Across Languages

Language	Keyword(s)	Execution model	Cancellation	Error handling
C++20	co_await``co_return``co_yield	Stackless, manual scheduling	`std::stop_token`	Exception propagation
JavaScript	async``await	Event loop (single-threaded)	`AbortController`	`try/catch`
Python	async def``await	Event loop (`asyncio`)	`asyncio.Task.cancel()`	`try/except`
Rust	`.await`	Async runtime (tokio, async-std)	`CancellationToken`	`?` operator
C#	async``await	ThreadPool / IOCP	`CancellationToken`	`try/catch`

C++ is unique in providing no built-in executor or event loop. The coroutine machinery is Deliberately low-level — the standard provides only the suspension/resumption primitives, and Scheduling is entirely the programmer”s or library’s responsibility.

Structured Concurrency: `when_all` / `when_any`

Structured concurrency is the principle that every concurrent operation should have a Well-defined lifetime — all child tasks must complete (or be cancelled) before the parent scope Exits. C++ does not yet have a standard when_all or when_any primitive, but these are common Library patterns.

when_all(tasks...): returns when all tasks have completed. The result is a tuple of results.
when_any(tasks...): returns when any task completes, cancelling the rest. The result identifies which task finished first.

The complexity of when_all for $n$ tasks is $\mathcal{O}(n)$ in terms of coroutine handles that Must be tracked and resumed.

`when_all` Implementation

#include <coroutine>
#include <iostream>
#include <vector>
#include <atomic>
#include <exception>

struct WhenAllTask {
    struct promise_type {
        std::exception_ptr exception_{};

        std::suspend_always initial_suspend() noexcept { return {}; }
        std::suspend_always final_suspend() noexcept { return {}; }
        void return_void() {}
        void unhandled_exception() { exception_ = std::current_exception(); }

        WhenAllTask get_return_object() {
            return WhenAllTask{std::coroutine_handle<promise_type>::from_promise(*this)};
        }
    };

    std::coroutine_handle<promise_type> handle;

    WhenAllTask(std::coroutine_handle<promise_type> h) : handle(h) {}
    WhenAllTask(WhenAllTask&& o) noexcept : handle(std::exchange(o.handle, nullptr)) {}
    ~WhenAllTask() { if (handle) handle.destroy(); }
    WhenAllTask(const WhenAllTask&) = delete;
    WhenAllTask& operator=(const WhenAllTask&) = delete;
};

struct WhenAll {
    struct promise_type {
        std::atomic<int> remaining_{0};
        std::coroutine_handle<> continuation_{};
        std::exception_ptr exception_{};

        std::suspend_always initial_suspend() noexcept { return {}; }

        struct FinalAwaiter {
            bool await_ready() const noexcept { return false; }
            std::coroutine_handle<> await_suspend(
                std::coroutine_handle<promise_type> h) noexcept {
                if (--h.promise().remaining_ == 0) {
                    return h.promise().continuation_;
                }
                return std::noop_coroutine();
            }
            void await_resume() const noexcept {}
        };

        std::suspend_always final_suspend() noexcept { return {}; }
        void return_void() {}
        void unhandled_exception() { exception_ = std::current_exception(); }

        WhenAll get_return_object() {
            return WhenAll{std::coroutine_handle<promise_type>::from_promise(*this)};
        }
    };

    std::coroutine_handle<promise_type> handle;
    std::coroutine_handle<> continuation_{};

    WhenAll(std::coroutine_handle<promise_type> h) : handle(h) {}
    ~WhenAll() { if (handle) handle.destroy(); }
    WhenAll(const WhenAll&) = delete;
    WhenAll& operator=(const WhenAll&) = delete;
};

Complete Example: Task Class Wrapping a Coroutine

#include <coroutine>
#include <iostream>
#include <utility>
#include <exception>
#include <functional>

struct Task {
    struct promise_type {
        std::exception_ptr exception_{};
        bool ready_{false};

        promise_type() = default;
        promise_type(const promise_type&) = delete;
        promise_type& operator=(const promise_type&) = delete;
        ~promise_type() = default;

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

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

        void return_void() { ready_ = true; }

        void unhandled_exception() {
            exception_ = std::current_exception();
            ready_ = true;
        }

        struct FinalAwaiter {
            bool await_ready() const noexcept { return false; }
            void await_suspend(std::coroutine_handle<promise_type> h) const noexcept {
                if (h.promise().continuation_)
                    h.promise().continuation_.resume();
            }
            void await_resume() const noexcept {}
        };

        std::coroutine_handle<> continuation_{};
    };

    struct TaskAwaiter {
        std::coroutine_handle<promise_type> handle_;

        bool await_ready() const noexcept {
            return handle_.done();
        }

        std::coroutine_handle<> await_suspend(
            std::coroutine_handle<> caller) const noexcept {
            handle_.promise().continuation_ = caller;
            return handle_;
        }

        void await_resume() const {
            if (handle_.promise().exception_)
                std::rethrow_exception(handle_.promise().exception_);
        }
    };

    std::coroutine_handle<promise_type> handle;

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

    Task(Task&& other) noexcept
        : handle(std::exchange(other.handle, nullptr)) {}
    Task(const Task&) = delete;
    Task& operator=(Task&& other) noexcept {
        if (this != &other) {
            if (handle) handle.destroy();
            handle = std::exchange(other.handle, nullptr);
        }
        return *this;
    }
    Task& operator=(const Task&) = delete;

    ~Task() { if (handle) handle.destroy(); }

    void start() {
        if (handle && !handle.done())
            handle.resume();
    }

    bool done() const { return !handle || handle.done(); }

    auto operator co_await() && {
        return TaskAwaiter{handle};
    }

    auto operator co_await() & {
        return TaskAwaiter{handle};
    }
};

Task async_add(int a, int b) {
    co_await std::suspend_always{};
    std::cout << "  async_add(" << a << ", " << b << ") resuming\n";
    co_return;
}

Task chained_computation() {
    std::cout << "  starting chained_computation\n";
    co_await std::suspend_always{};
    std::cout << "  chained_computation step 1 done\n";
    co_await std::suspend_always{};
    std::cout << "  chained_computation step 2 done\n";
    co_return;
}

Task runner() {
    std::cout << "runner: launching tasks\n";
    auto t1 = async_add(1, 2);
    auto t2 = async_add(3, 4);
    auto t3 = chained_computation();

    t1.start();
    t2.start();
    t3.start();

    co_await std::move(t1);
    co_await std::move(t2);
    co_await std::move(t3);

    std::cout << "runner: all tasks done\n";
    co_return;
}

int main() {
    std::cout << "main: starting runner\n";
    auto r = runner();
    while (!r.done()) {
        std::cout << "main: pumping event loop\n";
        r.handle.resume();
    }
    std::cout << "main: done\n";
}

Complete Example: Simple Thread Pool Executor

#include <coroutine>
#include <iostream>
#include <thread>
#include <vector>
#include <queue>
#include <mutex>
#include <condition_variable>
#include <functional>
#include <atomic>
#include <memory>

class ThreadPool {
public:
    explicit ThreadPool(std::size_t num_threads = std::thread::hardware_concurrency())
        : stop_(false) {
        for (std::size_t i = 0; i < num_threads; ++i) {
            workers_.emplace_back([this] { worker_loop(); });
        }
    }

    ~ThreadPool() {
        {
            std::lock_guard<std::mutex> lock(mutex_);
            stop_ = true;
        }
        cv_.notify_all();
        for (auto& t : workers_) {
            if (t.joinable()) t.join();
        }
    }

    void schedule(std::function<void()> task) {
        {
            std::lock_guard<std::mutex> lock(mutex_);
            queue_.push(std::move(task));
        }
        cv_.notify_one();
    }

    static ThreadPool& instance() {
        static ThreadPool pool;
        return pool;
    }

private:
    void worker_loop() {
        while (true) {
            std::function<void()> task;
            {
                std::unique_lock<std::mutex> lock(mutex_);
                cv_.wait(lock, [this] { return stop_ || !queue_.empty(); });
                if (stop_ && queue_.empty()) return;
                task = std::move(queue_.front());
                queue_.pop();
            }
            task();
        }
    }

    std::vector<std::thread> workers_;
    std::queue<std::function<void()>> queue_;
    std::mutex mutex_;
    std::condition_variable cv_;
    bool stop_;
};

struct ThreadPoolTask {
    struct promise_type {
        std::exception_ptr exception_{};

        promise_type() = default;
        promise_type(const promise_type&) = delete;
        promise_type& operator=(const promise_type&) = delete;
        ~promise_type() = default;

        std::suspend_never initial_suspend() noexcept { return {}; }
        struct FinalAwaiter {
            bool await_ready() const noexcept { return false; }
            void await_suspend(std::coroutine_handle<promise_type> h) const noexcept {
                if (h.promise().continuation_)
                    h.promise().continuation_.resume();
            }
            void await_resume() const noexcept {}
        };
        std::suspend_always final_suspend() noexcept { return {}; }

        ThreadPoolTask get_return_object() {
            return ThreadPoolTask{
                std::coroutine_handle<promise_type>::from_promise(*this)};
        }

        void return_void() {}
        void unhandled_exception() {
            exception_ = std::current_exception();
        }

        std::coroutine_handle<> continuation_{};
    };

    struct ThreadPoolAwaiter {
        std::coroutine_handle<promise_type> handle_;

        bool await_ready() const noexcept { return handle_.done(); }

        void await_suspend(std::coroutine_handle<> caller) const {
            handle_.promise().continuation_ = caller;
            ThreadPool::instance().schedule([h = handle_] { h.resume(); });
        }

        void await_resume() const {
            if (handle_.promise().exception_)
                std::rethrow_exception(handle_.promise().exception_);
        }
    };

    std::coroutine_handle<promise_type> handle;

    explicit ThreadPoolTask(std::coroutine_handle<promise_type> h) : handle(h) {}
    ThreadPoolTask(ThreadPoolTask&& other) noexcept
        : handle(std::exchange(other.handle, nullptr)) {}
    ThreadPoolTask(const ThreadPoolTask&) = delete;
    ThreadPoolTask& operator=(ThreadPoolTask&&) = delete;
    ThreadPoolTask& operator=(const ThreadPoolTask&) = delete;

    ~ThreadPoolTask() { if (handle) handle.destroy(); }

    auto operator co_await() {
        return ThreadPoolAwaiter{handle};
    }
};

ThreadPoolTask compute_on_thread(int id, int iterations) {
    std::cout << "  task " << id << " starting on thread "
              << std::this_thread::get_id() << "\n";
    for (int i = 0; i < iterations; ++i) {
        co_await std::suspend_always{};
    }
    std::cout << "  task " << id << " finished on thread "
              << std::this_thread::get_id() << "\n";
}

ThreadPoolTask run_all() {
    std::cout << "run_all on thread " << std::this_thread::get_id() << "\n";
    auto t1 = compute_on_thread(1, 3);
    auto t2 = compute_on_thread(2, 2);
    auto t3 = compute_on_thread(3, 4);

    co_await std::move(t1);
    co_await std::move(t2);
    co_await std::move(t3);

    std::cout << "run_all: all tasks completed\n";
}

int main() {
    std::cout << "main on thread " << std::this_thread::get_id() << "\n";

    auto task = run_all();

    while (!task.handle.done()) {
        std::this_thread::sleep_for(std::chrono::milliseconds(10));
    }

    std::cout << "main: done\n";
}

:::caution This thread pool executor is a simplified educational example. A production executor must Handle: work stealing, priority queues, thread affinity, shutdown semantics, exception aggregation Across when_allAnd proper cancellation propagation. Libraries like libunifex (now std::execution proposal, P2300) provide production-grade executors.

Work-Stealing Concepts

Work stealing is a scheduling strategy where idle threads steal tasks from the queues of busy Threads. It provides automatic load balancing without centralized coordination:

Each thread has a local deque of tasks (double-ended queue).
A thread pops tasks from the bottom of its own deque (LIFO. Good for cache locality and depth-first traversal of task trees).
An idle thread steals tasks from the top of another thread’s deque (FIFO. Good for breadth and reducing contention with the owner).

The work-stealing algorithm has provably optimal time bounds: the expected execution time of a fully Strict (fork-join) computation with $P$ processors and work $T_1$ is $\mathcal{O}(T_1 / P + T_{\infty})$ , where $T_{\infty}$ is the span (critical path length) [Blumofe And Leiserson, 1999].

#include <deque>
#include <mutex>
#include <thread>
#include <vector>

class WorkStealingPool {
    struct WorkerDeque {
        std::mutex mtx;
        std::deque<std::function<void()>> tasks;

        void push_bottom(std::function<void()> task) {
            std::lock_guard<std::mutex> lock(mtx);
            tasks.push_back(std::move(task));
        }

        std::function<void()> pop_bottom() {
            std::lock_guard<std::mutex> lock(mtx);
            if (tasks.empty()) return nullptr;
            auto task = std::move(tasks.back());
            tasks.pop_back();
            return task;
        }

        std::function<void()> steal_top() {
            std::lock_guard<std::mutex> lock(mtx);
            if (tasks.empty()) return nullptr;
            auto task = std::move(tasks.front());
            tasks.pop_front();
            return task;
        }
    };

    std::vector<WorkerDeque> deques_;
    std::vector<std::thread> workers_;
    std::atomic<bool> stop_{false};
    std::atomic<std::size_t> next_victim_{0};

    std::function<void()> try_steal(std::size_t thief) {
        std::size_t n = deques_.size();
        for (std::size_t i = 0; i < n; ++i) {
            std::size_t victim = (thief + i + 1) % n;
            auto task = deques_[victim].steal_top();
            if (task) return task;
        }
        return nullptr;
    }

    void worker_loop(std::size_t id) {
        while (!stop_) {
            auto task = deques_[id].pop_bottom();
            if (!task) task = try_steal(id);
            if (task) {
                task();
            } else {
                std::this_thread::yield();
            }
        }
    }

public:
    explicit WorkStealingPool(std::size_t n = std::thread::hardware_concurrency())
        : deques_(n) {
        for (std::size_t i = 0; i < n; ++i) {
            workers_.emplace_back(&WorkStealingPool::worker_loop, this, i);
        }
    }

    void submit(std::function<void()> task, std::size_t target) {
        deques_[target % deques_.size()].push_bottom(std::move(task));
    }

    ~WorkStealingPool() {
        stop_ = true;
        for (auto& w : workers_) {
            if (w.joinable()) w.join();
        }
    }
};

Cancellation with `std::stop_token`

C++ coroutines lack built-in cancellation. The std::stop_token mechanism [N4950 §32.4] provides Cooperative cancellation that integrates with coroutine loops via periodic checks at suspension Points:

#include <iostream>
#include <thread>
#include <stop_token>
#include <latch>

void cancellation_demo() {
    std::latch started{1};

    std::jthread worker([&started](std::stop_token st) {
        int count = 0;
        started.count_down();
        while (!st.stop_requested()) {
            std::cout << "working " << count++ << "\n";
            std::this_thread::sleep_for(std::chrono::milliseconds(50));
        }
        std::cout << "stopped after " << count << " iterations\n";
    });

    started.wait();
    std::this_thread::sleep_for(std::chrono::milliseconds(200));
    worker.request_stop();
}

In a coroutine-based system, the stop token is stored in the promise type and checked at each co_await suspension point. An awaiter that checks the stop token before suspending allows the Coroutine to exit cleanly:

struct CancellableAwaiter {
    std::stop_token token;

    bool await_ready() const noexcept {
        return token.stop_requested();
    }

    bool await_suspend(std::coroutine_handle<>) const noexcept {
        return !token.stop_requested();
    }

    void await_resume() const noexcept {}
};

When await_ready() returns true (stop requested), the coroutine skips suspension and the loop Condition exits. When it returns falseThe coroutine suspends normally and resumes when the Scheduler decides, at which point the loop checks the stop token again. This pattern is cooperative: The coroutine must reach a cancellation point to observe the request. Long-running non-suspending Computation cannot be cancelled until it reaches the next co_await.

Common Pitfalls

Forgetting to resume() after suspend_always. If a coroutine suspends with suspend_always and no scheduler ever calls resume()The coroutine leaks — its frame is never destroyed. Always pair lazy coroutines with a scheduler or manual resume loop.
Symmetric transfer vs direct resume. Using handle.resume() inside await_suspend can cause stack overflow on deep coroutine chains. Use symmetric transfer (return handle;) when the awaiting coroutine is on a different thread or when chain depth is unbounded.
Dangling continuation handles. If a coroutine is destroyed while another coroutine holds its handle as a continuation, resuming that continuation will access freed memory. Use suspend_never for final_suspend when the coroutine is always at the tail of a chain, or reference counting if the handle may outlive the coroutine.
Thread affinity with TLS. A coroutine suspended on one thread and resumed on another must not rely on thread-local storage without careful synchronization. The coroutine frame itself is heap-allocated and thread-safe, but any TLS access in the coroutine body is thread-affine.
Blocking in coroutines. Calling blocking operations (e.g., std::this_thread::sleep_for synchronous I/O) inside a coroutine defeats the purpose of cooperative scheduling. The entire thread is blocked, not just the coroutine. Use asynchronous I/O or suspension-based timers instead.
Unbounded task queues. If tasks are submitted faster than they are consumed, the queue grows without bound. Implement backpressure (e.g., bounded channels, semaphores) to prevent out-of-memory conditions.

Summary

This topic covers the essential concepts and techniques related to task scheduling and executors, including key principles and practical applications.

Key concepts include:

core concepts and definitions
key principles and frameworks
practical applications
common techniques and methods
evaluation and critical analysis

A thorough understanding of these concepts, combined with regular practice and review, is essential for mastery of this topic.

Worked Examples

Worked examples demonstrating the application of key concepts are covered in the detailed sub-pages linked above.

:::

Mark this page as reviewed

Task Scheduling and Executors

Task Scheduling and Executors

Task Concept

Cooperative Scheduling with Coroutines

Proof: Cooperative Scheduling Avoids Data Races

Comparison with Thread-Based Concurrency

Coroutine-Based Pipeline Processing

Pipeline Implementation

Async/Await Patterns Across Languages

Structured Concurrency: when_all / when_any

when_all Implementation

Complete Example: Task Class Wrapping a Coroutine

Complete Example: Simple Thread Pool Executor

Work-Stealing Concepts

Cancellation with std::stop_token

Common Pitfalls

See Also

Summary

Worked Examples

Structured Concurrency: `when_all` / `when_any`

`when_all` Implementation

Cancellation with `std::stop_token`