LLVM  11.0.0git
ThreadPool.cpp
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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 
14 
15 #include "llvm/Config/llvm-config.h"
16 #include "llvm/Support/Threading.h"
18 
19 using namespace llvm;
20 
21 #if LLVM_ENABLE_THREADS
22 
24  : ThreadCount(S.compute_thread_count()) {
25  // Create ThreadCount threads that will loop forever, wait on QueueCondition
26  // for tasks to be queued or the Pool to be destroyed.
27  Threads.reserve(ThreadCount);
28  for (unsigned ThreadID = 0; ThreadID < ThreadCount; ++ThreadID) {
29  Threads.emplace_back([S, ThreadID, this] {
30  S.apply_thread_strategy(ThreadID);
31  while (true) {
32  PackagedTaskTy Task;
33  {
34  std::unique_lock<std::mutex> LockGuard(QueueLock);
35  // Wait for tasks to be pushed in the queue
36  QueueCondition.wait(LockGuard,
37  [&] { return !EnableFlag || !Tasks.empty(); });
38  // Exit condition
39  if (!EnableFlag && Tasks.empty())
40  return;
41  // Yeah, we have a task, grab it and release the lock on the queue
42 
43  // We first need to signal that we are active before popping the queue
44  // in order for wait() to properly detect that even if the queue is
45  // empty, there is still a task in flight.
46  ++ActiveThreads;
47  Task = std::move(Tasks.front());
48  Tasks.pop();
49  }
50  // Run the task we just grabbed
51  Task();
52 
53  bool Notify;
54  {
55  // Adjust `ActiveThreads`, in case someone waits on ThreadPool::wait()
56  std::lock_guard<std::mutex> LockGuard(QueueLock);
57  --ActiveThreads;
58  Notify = workCompletedUnlocked();
59  }
60  // Notify task completion if this is the last active thread, in case
61  // someone waits on ThreadPool::wait().
62  if (Notify)
63  CompletionCondition.notify_all();
64  }
65  });
66  }
67 }
68 
70  // Wait for all threads to complete and the queue to be empty
71  std::unique_lock<std::mutex> LockGuard(QueueLock);
72  CompletionCondition.wait(LockGuard, [&] { return workCompletedUnlocked(); });
73 }
74 
75 std::shared_future<void> ThreadPool::asyncImpl(TaskTy Task) {
76  /// Wrap the Task in a packaged_task to return a future object.
77  PackagedTaskTy PackagedTask(std::move(Task));
78  auto Future = PackagedTask.get_future();
79  {
80  // Lock the queue and push the new task
81  std::unique_lock<std::mutex> LockGuard(QueueLock);
82 
83  // Don't allow enqueueing after disabling the pool
84  assert(EnableFlag && "Queuing a thread during ThreadPool destruction");
85 
86  Tasks.push(std::move(PackagedTask));
87  }
88  QueueCondition.notify_one();
89  return Future.share();
90 }
91 
92 // The destructor joins all threads, waiting for completion.
94  {
95  std::unique_lock<std::mutex> LockGuard(QueueLock);
96  EnableFlag = false;
97  }
98  QueueCondition.notify_all();
99  for (auto &Worker : Threads)
100  Worker.join();
101 }
102 
103 #else // LLVM_ENABLE_THREADS Disabled
104 
105 // No threads are launched, issue a warning if ThreadCount is not 0
107  : ThreadCount(S.compute_thread_count()) {
108  if (ThreadCount != 1) {
109  errs() << "Warning: request a ThreadPool with " << ThreadCount
110  << " threads, but LLVM_ENABLE_THREADS has been turned off\n";
111  }
112 }
113 
114 void ThreadPool::wait() {
115  // Sequential implementation running the tasks
116  while (!Tasks.empty()) {
117  auto Task = std::move(Tasks.front());
118  Tasks.pop();
119  Task();
120  }
121 }
122 
123 std::shared_future<void> ThreadPool::asyncImpl(TaskTy Task) {
124  // Get a Future with launch::deferred execution using std::async
125  auto Future = std::async(std::launch::deferred, std::move(Task)).share();
126  // Wrap the future so that both ThreadPool::wait() can operate and the
127  // returned future can be sync'ed on.
128  PackagedTaskTy PackagedTask([Future]() { Future.get(); });
129  Tasks.push(std::move(PackagedTask));
130  return Future;
131 }
132 
134 
135 #endif
void apply_thread_strategy(unsigned ThreadPoolNum) const
Assign the current thread to an ideal hardware CPU or NUMA node.
This class represents lattice values for constants.
Definition: AllocatorList.h:23
std::packaged_task< void()> PackagedTaskTy
Definition: ThreadPool.h:40
std::function< void()> TaskTy
Definition: ThreadPool.h:39
~ThreadPool()
Blocking destructor: the pool will wait for all the threads to complete.
Definition: ThreadPool.cpp:93
ThreadPool(ThreadPoolStrategy S=hardware_concurrency())
Construct a pool using the hardware strategy S for mapping hardware execution resources (threads...
Definition: ThreadPool.cpp:23
This tells how a thread pool will be used.
Definition: Threading.h:148
raw_fd_ostream & errs()
This returns a reference to a raw_ostream for standard error.
unsigned compute_thread_count() const
Retrieves the max available threads for the current strategy.
Definition: Threading.cpp:86
assert(ImpDefSCC.getReg()==AMDGPU::SCC &&ImpDefSCC.isDef())
void wait()
Blocking wait for all the threads to complete and the queue to be empty.
Definition: ThreadPool.cpp:69