lib/Support/ThreadPool.cpp

   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