1 //==-- llvm/Support/ThreadPool.cpp - A ThreadPool implementation -*- C++ -*-==// 2 // 3 // The LLVM Compiler Infrastructure 4 // 5 // This file is distributed under the University of Illinois Open Source 6 // License. See LICENSE.TXT for details. 7 // 8 //===----------------------------------------------------------------------===// 9 // 10 // This file implements a crude C++11 based thread pool. 11 // 12 //===----------------------------------------------------------------------===// 13 14 #include "llvm/Support/ThreadPool.h" 15 16 #include "llvm/Config/llvm-config.h" 17 #include "llvm/Support/Threading.h" 18 #include "llvm/Support/raw_ostream.h" 19 20 using namespace llvm; 21 22 #if LLVM_ENABLE_THREADS 23 24 // Default to hardware_concurrency 25 ThreadPool::ThreadPool() : ThreadPool(hardware_concurrency()) {} 26 27 ThreadPool::ThreadPool(unsigned ThreadCount) 28 : ActiveThreads(0), EnableFlag(true) { 29 // Create ThreadCount threads that will loop forever, wait on QueueCondition 30 // for tasks to be queued or the Pool to be destroyed. 31 Threads.reserve(ThreadCount); 32 for (unsigned ThreadID = 0; ThreadID < ThreadCount; ++ThreadID) { 33 Threads.emplace_back([&] { 34 while (true) { 35 PackagedTaskTy Task; 36 { 37 std::unique_lock<std::mutex> LockGuard(QueueLock); 38 // Wait for tasks to be pushed in the queue 39 QueueCondition.wait(LockGuard, 40 [&] { return !EnableFlag || !Tasks.empty(); }); 41 // Exit condition 42 if (!EnableFlag && Tasks.empty()) 43 return; 44 // Yeah, we have a task, grab it and release the lock on the queue 45 46 // We first need to signal that we are active before popping the queue 47 // in order for wait() to properly detect that even if the queue is 48 // empty, there is still a task in flight. 49 { 50 std::unique_lock<std::mutex> LockGuard(CompletionLock); 51 ++ActiveThreads; 52 } 53 Task = std::move(Tasks.front()); 54 Tasks.pop(); 55 } 56 // Run the task we just grabbed 57 Task(); 58 59 { 60 // Adjust `ActiveThreads`, in case someone waits on ThreadPool::wait() 61 std::unique_lock<std::mutex> LockGuard(CompletionLock); 62 --ActiveThreads; 63 } 64 65 // Notify task completion, in case someone waits on ThreadPool::wait() 66 CompletionCondition.notify_all(); 67 } 68 }); 69 } 70 } 71 72 void ThreadPool::wait() { 73 // Wait for all threads to complete and the queue to be empty 74 std::unique_lock<std::mutex> LockGuard(CompletionLock); 75 // The order of the checks for ActiveThreads and Tasks.empty() matters because 76 // any active threads might be modifying the Tasks queue, and this would be a 77 // race. 78 CompletionCondition.wait(LockGuard, 79 [&] { return !ActiveThreads && Tasks.empty(); }); 80 } 81 82 std::shared_future<void> ThreadPool::asyncImpl(TaskTy Task) { 83 /// Wrap the Task in a packaged_task to return a future object. 84 PackagedTaskTy PackagedTask(std::move(Task)); 85 auto Future = PackagedTask.get_future(); 86 { 87 // Lock the queue and push the new task 88 std::unique_lock<std::mutex> LockGuard(QueueLock); 89 90 // Don't allow enqueueing after disabling the pool 91 assert(EnableFlag && "Queuing a thread during ThreadPool destruction"); 92 93 Tasks.push(std::move(PackagedTask)); 94 } 95 QueueCondition.notify_one(); 96 return Future.share(); 97 } 98 99 // The destructor joins all threads, waiting for completion. 100 ThreadPool::~ThreadPool() { 101 { 102 std::unique_lock<std::mutex> LockGuard(QueueLock); 103 EnableFlag = false; 104 } 105 QueueCondition.notify_all(); 106 for (auto &Worker : Threads) 107 Worker.join(); 108 } 109 110 #else // LLVM_ENABLE_THREADS Disabled 111 112 ThreadPool::ThreadPool() : ThreadPool(0) {} 113 114 // No threads are launched, issue a warning if ThreadCount is not 0 115 ThreadPool::ThreadPool(unsigned ThreadCount) 116 : ActiveThreads(0) { 117 if (ThreadCount) { 118 errs() << "Warning: request a ThreadPool with " << ThreadCount 119 << " threads, but LLVM_ENABLE_THREADS has been turned off\n"; 120 } 121 } 122 123 void ThreadPool::wait() { 124 // Sequential implementation running the tasks 125 while (!Tasks.empty()) { 126 auto Task = std::move(Tasks.front()); 127 Tasks.pop(); 128 Task(); 129 } 130 } 131 132 std::shared_future<void> ThreadPool::asyncImpl(TaskTy Task) { 133 // Get a Future with launch::deferred execution using std::async 134 auto Future = std::async(std::launch::deferred, std::move(Task)).share(); 135 // Wrap the future so that both ThreadPool::wait() can operate and the 136 // returned future can be sync'ed on. 137 PackagedTaskTy PackagedTask([Future]() { Future.get(); }); 138 Tasks.push(std::move(PackagedTask)); 139 return Future; 140 } 141 142 ThreadPool::~ThreadPool() { 143 wait(); 144 } 145 146 #endif 147
