lib/Support/ThreadPool.cpp

   1 //==-- llvm/Support/ThreadPool.cpp - A ThreadPool implementation -*- C++ -*-==//
   2 //
   3 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
   4 // See https://llvm.org/LICENSE.txt for license information.
   5 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
   6 //
   7 //===----------------------------------------------------------------------===//
   8 //
   9 // This file implements a crude C++11 based thread pool.
  10 //
  11 //===----------------------------------------------------------------------===//
  12
  13 #include "llvm/Support/ThreadPool.h"
  14
  15 #include "llvm/Config/llvm-config.h"
  16 #include "llvm/Support/Threading.h"
  17 #include "llvm/Support/raw_ostream.h"
  18
  19 using namespace llvm;
  20
  21 #if LLVM_ENABLE_THREADS
  22
  23 // Default to hardware_concurrency
  24 ThreadPool::ThreadPool() : ThreadPool(hardware_concurrency()) {}
  25
  26 ThreadPool::ThreadPool(unsigned ThreadCount)
  27     : ActiveThreads(0), EnableFlag(true) {
  28   // Create ThreadCount threads that will loop forever, wait on QueueCondition
  29   // for tasks to be queued or the Pool to be destroyed.
  30   Threads.reserve(ThreadCount);
  31   for (unsigned ThreadID = 0; ThreadID < ThreadCount; ++ThreadID) {
  32     Threads.emplace_back([&] {
  33       while (true) {
  34         PackagedTaskTy Task;
  35         {
  36           std::unique_lock<std::mutex> LockGuard(QueueLock);
  37           // Wait for tasks to be pushed in the queue
  38           QueueCondition.wait(LockGuard,
  39                               [&] { return !EnableFlag || !Tasks.empty(); });
  40           // Exit condition
  41           if (!EnableFlag && Tasks.empty())
  42             return;
  43           // Yeah, we have a task, grab it and release the lock on the queue
  44
  45           // We first need to signal that we are active before popping the queue
  46           // in order for wait() to properly detect that even if the queue is
  47           // empty, there is still a task in flight.
  48           {
  49             std::unique_lock<std::mutex> LockGuard(CompletionLock);
  50             ++ActiveThreads;
  51           }
  52           Task = std::move(Tasks.front());
  53           Tasks.pop();
  54         }
  55         // Run the task we just grabbed
  56         Task();
  57
  58         {
  59           // Adjust `ActiveThreads`, in case someone waits on ThreadPool::wait()
  60           std::unique_lock<std::mutex> LockGuard(CompletionLock);
  61           --ActiveThreads;
  62         }
  63
  64         // Notify task completion, in case someone waits on ThreadPool::wait()
  65         CompletionCondition.notify_all();
  66       }
  67     });
  68   }
  69 }
  70
  71 void ThreadPool::wait() {
  72   // Wait for all threads to complete and the queue to be empty
  73   std::unique_lock<std::mutex> LockGuard(CompletionLock);
  74   // The order of the checks for ActiveThreads and Tasks.empty() matters because
  75   // any active threads might be modifying the Tasks queue, and this would be a
  76   // race.
  77   CompletionCondition.wait(LockGuard,
  78                            [&] { return !ActiveThreads && Tasks.empty(); });
  79 }
  80
  81 std::shared_future<void> ThreadPool::asyncImpl(TaskTy Task) {
  82   /// Wrap the Task in a packaged_task to return a future object.
  83   PackagedTaskTy PackagedTask(std::move(Task));
  84   auto Future = PackagedTask.get_future();
  85   {
  86     // Lock the queue and push the new task
  87     std::unique_lock<std::mutex> LockGuard(QueueLock);
  88
  89     // Don't allow enqueueing after disabling the pool
  90     assert(EnableFlag && "Queuing a thread during ThreadPool destruction");
  91
  92     Tasks.push(std::move(PackagedTask));
  93   }
  94   QueueCondition.notify_one();
  95   return Future.share();
  96 }
  97
  98 // The destructor joins all threads, waiting for completion.
  99 ThreadPool::~ThreadPool() {
 100   {
 101     std::unique_lock<std::mutex> LockGuard(QueueLock);
 102     EnableFlag = false;
 103   }
 104   QueueCondition.notify_all();
 105   for (auto &Worker : Threads)
 106     Worker.join();
 107 }
 108
 109 #else // LLVM_ENABLE_THREADS Disabled
 110
 111 ThreadPool::ThreadPool() : ThreadPool(0) {}
 112
 113 // No threads are launched, issue a warning if ThreadCount is not 0
 114 ThreadPool::ThreadPool(unsigned ThreadCount)
 115     : ActiveThreads(0) {
 116   if (ThreadCount) {
 117     errs() << "Warning: request a ThreadPool with " << ThreadCount
 118            << " threads, but LLVM_ENABLE_THREADS has been turned off\n";
 119   }
 120 }
 121
 122 void ThreadPool::wait() {
 123   // Sequential implementation running the tasks
 124   while (!Tasks.empty()) {
 125     auto Task = std::move(Tasks.front());
 126     Tasks.pop();
 127     Task();
 128   }
 129 }
 130
 131 std::shared_future<void> ThreadPool::asyncImpl(TaskTy Task) {
 132   // Get a Future with launch::deferred execution using std::async
 133   auto Future = std::async(std::launch::deferred, std::move(Task)).share();
 134   // Wrap the future so that both ThreadPool::wait() can operate and the
 135   // returned future can be sync'ed on.
 136   PackagedTaskTy PackagedTask([Future]() { Future.get(); });
 137   Tasks.push(std::move(PackagedTask));
 138   return Future;
 139 }
 140
 141 ThreadPool::~ThreadPool() {
 142   wait();
 143 }
 144
 145 #endif