| //===-- llvm/Support/ThreadPool.h - A ThreadPool implementation -*- C++ -*-===// |
| // |
| // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions. |
| // See https://llvm.org/LICENSE.txt for license information. |
| // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception |
| // |
| //===----------------------------------------------------------------------===// |
| // |
| // This file defines a crude C++11 based thread pool. |
| // |
| //===----------------------------------------------------------------------===// |
| |
| #ifndef LLVM_SUPPORT_THREADPOOL_H |
| #define LLVM_SUPPORT_THREADPOOL_H |
| |
| #include "llvm/ADT/DenseMap.h" |
| #include "llvm/Config/llvm-config.h" |
| #include "llvm/Support/RWMutex.h" |
| #include "llvm/Support/Threading.h" |
| #include "llvm/Support/thread.h" |
| |
| #include <future> |
| |
| #include <condition_variable> |
| #include <deque> |
| #include <functional> |
| #include <memory> |
| #include <mutex> |
| #include <utility> |
| |
| namespace llvm { |
| |
| class ThreadPoolTaskGroup; |
| |
| /// A ThreadPool for asynchronous parallel execution on a defined number of |
| /// threads. |
| /// |
| /// The pool keeps a vector of threads alive, waiting on a condition variable |
| /// for some work to become available. |
| /// |
| /// It is possible to reuse one thread pool for different groups of tasks |
| /// by grouping tasks using ThreadPoolTaskGroup. All tasks are processed using |
| /// the same queue, but it is possible to wait only for a specific group of |
| /// tasks to finish. |
| /// |
| /// It is also possible for worker threads to submit new tasks and wait for |
| /// them. Note that this may result in a deadlock in cases such as when a task |
| /// (directly or indirectly) tries to wait for its own completion, or when all |
| /// available threads are used up by tasks waiting for a task that has no thread |
| /// left to run on (this includes waiting on the returned future). It should be |
| /// generally safe to wait() for a group as long as groups do not form a cycle. |
| class ThreadPool { |
| public: |
| /// Construct a pool using the hardware strategy \p S for mapping hardware |
| /// execution resources (threads, cores, CPUs) |
| /// Defaults to using the maximum execution resources in the system, but |
| /// accounting for the affinity mask. |
| ThreadPool(ThreadPoolStrategy S = hardware_concurrency()); |
| |
| /// Blocking destructor: the pool will wait for all the threads to complete. |
| ~ThreadPool(); |
| |
| /// Asynchronous submission of a task to the pool. The returned future can be |
| /// used to wait for the task to finish and is *non-blocking* on destruction. |
| template <typename Function, typename... Args> |
| auto async(Function &&F, Args &&...ArgList) { |
| auto Task = |
| std::bind(std::forward<Function>(F), std::forward<Args>(ArgList)...); |
| return async(std::move(Task)); |
| } |
| |
| /// Overload, task will be in the given task group. |
| template <typename Function, typename... Args> |
| auto async(ThreadPoolTaskGroup &Group, Function &&F, Args &&...ArgList) { |
| auto Task = |
| std::bind(std::forward<Function>(F), std::forward<Args>(ArgList)...); |
| return async(Group, std::move(Task)); |
| } |
| |
| /// Asynchronous submission of a task to the pool. The returned future can be |
| /// used to wait for the task to finish and is *non-blocking* on destruction. |
| template <typename Func> |
| auto async(Func &&F) -> std::shared_future<decltype(F())> { |
| return asyncImpl(std::function<decltype(F())()>(std::forward<Func>(F)), |
| nullptr); |
| } |
| |
| template <typename Func> |
| auto async(ThreadPoolTaskGroup &Group, Func &&F) |
| -> std::shared_future<decltype(F())> { |
| return asyncImpl(std::function<decltype(F())()>(std::forward<Func>(F)), |
| &Group); |
| } |
| |
| /// Blocking wait for all the threads to complete and the queue to be empty. |
| /// It is an error to try to add new tasks while blocking on this call. |
| /// Calling wait() from a task would deadlock waiting for itself. |
| void wait(); |
| |
| /// Blocking wait for only all the threads in the given group to complete. |
| /// It is possible to wait even inside a task, but waiting (directly or |
| /// indirectly) on itself will deadlock. If called from a task running on a |
| /// worker thread, the call may process pending tasks while waiting in order |
| /// not to waste the thread. |
| void wait(ThreadPoolTaskGroup &Group); |
| |
| // TODO: misleading legacy name warning! |
| // Returns the maximum number of worker threads in the pool, not the current |
| // number of threads! |
| unsigned getThreadCount() const { return MaxThreadCount; } |
| |
| /// Returns true if the current thread is a worker thread of this thread pool. |
| bool isWorkerThread() const; |
| |
| private: |
| /// Helpers to create a promise and a callable wrapper of \p Task that sets |
| /// the result of the promise. Returns the callable and a future to access the |
| /// result. |
| template <typename ResTy> |
| static std::pair<std::function<void()>, std::future<ResTy>> |
| createTaskAndFuture(std::function<ResTy()> Task) { |
| std::shared_ptr<std::promise<ResTy>> Promise = |
| std::make_shared<std::promise<ResTy>>(); |
| auto F = Promise->get_future(); |
| return { |
| [Promise = std::move(Promise), Task]() { Promise->set_value(Task()); }, |
| std::move(F)}; |
| } |
| static std::pair<std::function<void()>, std::future<void>> |
| createTaskAndFuture(std::function<void()> Task) { |
| std::shared_ptr<std::promise<void>> Promise = |
| std::make_shared<std::promise<void>>(); |
| auto F = Promise->get_future(); |
| return {[Promise = std::move(Promise), Task]() { |
| Task(); |
| Promise->set_value(); |
| }, |
| std::move(F)}; |
| } |
| |
| /// Returns true if all tasks in the given group have finished (nullptr means |
| /// all tasks regardless of their group). QueueLock must be locked. |
| bool workCompletedUnlocked(ThreadPoolTaskGroup *Group) const; |
| |
| /// Asynchronous submission of a task to the pool. The returned future can be |
| /// used to wait for the task to finish and is *non-blocking* on destruction. |
| template <typename ResTy> |
| std::shared_future<ResTy> asyncImpl(std::function<ResTy()> Task, |
| ThreadPoolTaskGroup *Group) { |
| |
| #if LLVM_ENABLE_THREADS |
| /// Wrap the Task in a std::function<void()> that sets the result of the |
| /// corresponding future. |
| auto R = createTaskAndFuture(Task); |
| |
| int requestedThreads; |
| { |
| // Lock the queue and push the new task |
| std::unique_lock<std::mutex> LockGuard(QueueLock); |
| |
| // Don't allow enqueueing after disabling the pool |
| assert(EnableFlag && "Queuing a thread during ThreadPool destruction"); |
| Tasks.emplace_back(std::make_pair(std::move(R.first), Group)); |
| requestedThreads = ActiveThreads + Tasks.size(); |
| } |
| QueueCondition.notify_one(); |
| grow(requestedThreads); |
| return R.second.share(); |
| |
| #else // LLVM_ENABLE_THREADS Disabled |
| |
| // Get a Future with launch::deferred execution using std::async |
| auto Future = std::async(std::launch::deferred, std::move(Task)).share(); |
| // Wrap the future so that both ThreadPool::wait() can operate and the |
| // returned future can be sync'ed on. |
| Tasks.emplace_back(std::make_pair([Future]() { Future.get(); }, Group)); |
| return Future; |
| #endif |
| } |
| |
| #if LLVM_ENABLE_THREADS |
| // Grow to ensure that we have at least `requested` Threads, but do not go |
| // over MaxThreadCount. |
| void grow(int requested); |
| |
| void processTasks(ThreadPoolTaskGroup *WaitingForGroup); |
| #endif |
| |
| /// Threads in flight |
| std::vector<llvm::thread> Threads; |
| /// Lock protecting access to the Threads vector. |
| mutable llvm::sys::RWMutex ThreadsLock; |
| |
| /// Tasks waiting for execution in the pool. |
| std::deque<std::pair<std::function<void()>, ThreadPoolTaskGroup *>> Tasks; |
| |
| /// Locking and signaling for accessing the Tasks queue. |
| std::mutex QueueLock; |
| std::condition_variable QueueCondition; |
| |
| /// Signaling for job completion (all tasks or all tasks in a group). |
| std::condition_variable CompletionCondition; |
| |
| /// Keep track of the number of thread actually busy |
| unsigned ActiveThreads = 0; |
| /// Number of threads active for tasks in the given group (only non-zero). |
| DenseMap<ThreadPoolTaskGroup *, unsigned> ActiveGroups; |
| |
| #if LLVM_ENABLE_THREADS // avoids warning for unused variable |
| /// Signal for the destruction of the pool, asking thread to exit. |
| bool EnableFlag = true; |
| #endif |
| |
| const ThreadPoolStrategy Strategy; |
| |
| /// Maximum number of threads to potentially grow this pool to. |
| const unsigned MaxThreadCount; |
| }; |
| |
| /// A group of tasks to be run on a thread pool. Thread pool tasks in different |
| /// groups can run on the same threadpool but can be waited for separately. |
| /// It is even possible for tasks of one group to submit and wait for tasks |
| /// of another group, as long as this does not form a loop. |
| class ThreadPoolTaskGroup { |
| public: |
| /// The ThreadPool argument is the thread pool to forward calls to. |
| ThreadPoolTaskGroup(ThreadPool &Pool) : Pool(Pool) {} |
| |
| /// Blocking destructor: will wait for all the tasks in the group to complete |
| /// by calling ThreadPool::wait(). |
| ~ThreadPoolTaskGroup() { wait(); } |
| |
| /// Calls ThreadPool::async() for this group. |
| template <typename Function, typename... Args> |
| inline auto async(Function &&F, Args &&...ArgList) { |
| return Pool.async(*this, std::forward<Function>(F), |
| std::forward<Args>(ArgList)...); |
| } |
| |
| /// Calls ThreadPool::wait() for this group. |
| void wait() { Pool.wait(*this); } |
| |
| private: |
| ThreadPool &Pool; |
| }; |
| |
| } // namespace llvm |
| |
| #endif // LLVM_SUPPORT_THREADPOOL_H |