1//===-- llvm/Support/ThreadPool.h - 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 defines a crude C++11 based thread pool.
10//
11//===----------------------------------------------------------------------===//
12
13#ifndef LLVM_SUPPORT_THREADPOOL_H
14#define LLVM_SUPPORT_THREADPOOL_H
15
16#include "llvm/ADT/DenseMap.h"
17#include "llvm/Config/llvm-config.h"
18#include "llvm/Support/RWMutex.h"
19#include "llvm/Support/Threading.h"
20#include "llvm/Support/thread.h"
21
22#include <future>
23
24#include <condition_variable>
25#include <deque>
26#include <functional>
27#include <memory>
28#include <mutex>
29#include <utility>
30
31namespace llvm {
32
33class ThreadPoolTaskGroup;
34
35/// A ThreadPool for asynchronous parallel execution on a defined number of
36/// threads.
37///
38/// The pool keeps a vector of threads alive, waiting on a condition variable
39/// for some work to become available.
40///
41/// It is possible to reuse one thread pool for different groups of tasks
42/// by grouping tasks using ThreadPoolTaskGroup. All tasks are processed using
43/// the same queue, but it is possible to wait only for a specific group of
44/// tasks to finish.
45///
46/// It is also possible for worker threads to submit new tasks and wait for
47/// them. Note that this may result in a deadlock in cases such as when a task
48/// (directly or indirectly) tries to wait for its own completion, or when all
49/// available threads are used up by tasks waiting for a task that has no thread
50/// left to run on (this includes waiting on the returned future). It should be
51/// generally safe to wait() for a group as long as groups do not form a cycle.
52class ThreadPool {
53public:
54 /// Construct a pool using the hardware strategy \p S for mapping hardware
55 /// execution resources (threads, cores, CPUs)
56 /// Defaults to using the maximum execution resources in the system, but
57 /// accounting for the affinity mask.
58 ThreadPool(ThreadPoolStrategy S = hardware_concurrency());
59
60 /// Blocking destructor: the pool will wait for all the threads to complete.
61 ~ThreadPool();
62
63 /// Asynchronous submission of a task to the pool. The returned future can be
64 /// used to wait for the task to finish and is *non-blocking* on destruction.
65 template <typename Function, typename... Args>
66 auto async(Function &&F, Args &&...ArgList) {
67 auto Task =
68 std::bind(std::forward<Function>(F), std::forward<Args>(ArgList)...);
69 return async(std::move(Task));
70 }
71
72 /// Overload, task will be in the given task group.
73 template <typename Function, typename... Args>
74 auto async(ThreadPoolTaskGroup &Group, Function &&F, Args &&...ArgList) {
75 auto Task =
76 std::bind(std::forward<Function>(F), std::forward<Args>(ArgList)...);
77 return async(Group, std::move(Task));
78 }
79
80 /// Asynchronous submission of a task to the pool. The returned future can be
81 /// used to wait for the task to finish and is *non-blocking* on destruction.
82 template <typename Func>
83 auto async(Func &&F) -> std::shared_future<decltype(F())> {
84 return asyncImpl(std::function<decltype(F())()>(std::forward<Func>(F)),
85 nullptr);
86 }
87
88 template <typename Func>
89 auto async(ThreadPoolTaskGroup &Group, Func &&F)
90 -> std::shared_future<decltype(F())> {
91 return asyncImpl(std::function<decltype(F())()>(std::forward<Func>(F)),
92 &Group);
93 }
94
95 /// Blocking wait for all the threads to complete and the queue to be empty.
96 /// It is an error to try to add new tasks while blocking on this call.
97 /// Calling wait() from a task would deadlock waiting for itself.
98 void wait();
99
100 /// Blocking wait for only all the threads in the given group to complete.
101 /// It is possible to wait even inside a task, but waiting (directly or
102 /// indirectly) on itself will deadlock. If called from a task running on a
103 /// worker thread, the call may process pending tasks while waiting in order
104 /// not to waste the thread.
105 void wait(ThreadPoolTaskGroup &Group);
106
107 // TODO: misleading legacy name warning!
108 // Returns the maximum number of worker threads in the pool, not the current
109 // number of threads!
110 unsigned getThreadCount() const { return MaxThreadCount; }
111
112 /// Returns true if the current thread is a worker thread of this thread pool.
113 bool isWorkerThread() const;
114
115private:
116 /// Helpers to create a promise and a callable wrapper of \p Task that sets
117 /// the result of the promise. Returns the callable and a future to access the
118 /// result.
119 template <typename ResTy>
120 static std::pair<std::function<void()>, std::future<ResTy>>
121 createTaskAndFuture(std::function<ResTy()> Task) {
122 std::shared_ptr<std::promise<ResTy>> Promise =
123 std::make_shared<std::promise<ResTy>>();
124 auto F = Promise->get_future();
125 return {
126 [Promise = std::move(Promise), Task]() { Promise->set_value(Task()); },
127 std::move(F)};
128 }
129 static std::pair<std::function<void()>, std::future<void>>
130 createTaskAndFuture(std::function<void()> Task) {
131 std::shared_ptr<std::promise<void>> Promise =
132 std::make_shared<std::promise<void>>();
133 auto F = Promise->get_future();
134 return {[Promise = std::move(Promise), Task]() {
135 Task();
136 Promise->set_value();
137 },
138 std::move(F)};
139 }
140
141 /// Returns true if all tasks in the given group have finished (nullptr means
142 /// all tasks regardless of their group). QueueLock must be locked.
143 bool workCompletedUnlocked(ThreadPoolTaskGroup *Group) const;
144
145 /// Asynchronous submission of a task to the pool. The returned future can be
146 /// used to wait for the task to finish and is *non-blocking* on destruction.
147 template <typename ResTy>
148 std::shared_future<ResTy> asyncImpl(std::function<ResTy()> Task,
149 ThreadPoolTaskGroup *Group) {
150
151#if LLVM_ENABLE_THREADS
152 /// Wrap the Task in a std::function<void()> that sets the result of the
153 /// corresponding future.
154 auto R = createTaskAndFuture(Task);
155
156 int requestedThreads;
157 {
158 // Lock the queue and push the new task
159 std::unique_lock<std::mutex> LockGuard(QueueLock);
160
161 // Don't allow enqueueing after disabling the pool
162 assert(EnableFlag && "Queuing a thread during ThreadPool destruction");
163 Tasks.emplace_back(std::make_pair(std::move(R.first), Group));
164 requestedThreads = ActiveThreads + Tasks.size();
165 }
166 QueueCondition.notify_one();
167 grow(requested: requestedThreads);
168 return R.second.share();
169
170#else // LLVM_ENABLE_THREADS Disabled
171
172 // Get a Future with launch::deferred execution using std::async
173 auto Future = std::async(std::launch::deferred, std::move(Task)).share();
174 // Wrap the future so that both ThreadPool::wait() can operate and the
175 // returned future can be sync'ed on.
176 Tasks.emplace_back(std::make_pair([Future]() { Future.get(); }, Group));
177 return Future;
178#endif
179 }
180
181#if LLVM_ENABLE_THREADS
182 // Grow to ensure that we have at least `requested` Threads, but do not go
183 // over MaxThreadCount.
184 void grow(int requested);
185
186 void processTasks(ThreadPoolTaskGroup *WaitingForGroup);
187#endif
188
189 /// Threads in flight
190 std::vector<llvm::thread> Threads;
191 /// Lock protecting access to the Threads vector.
192 mutable llvm::sys::RWMutex ThreadsLock;
193
194 /// Tasks waiting for execution in the pool.
195 std::deque<std::pair<std::function<void()>, ThreadPoolTaskGroup *>> Tasks;
196
197 /// Locking and signaling for accessing the Tasks queue.
198 std::mutex QueueLock;
199 std::condition_variable QueueCondition;
200
201 /// Signaling for job completion (all tasks or all tasks in a group).
202 std::condition_variable CompletionCondition;
203
204 /// Keep track of the number of thread actually busy
205 unsigned ActiveThreads = 0;
206 /// Number of threads active for tasks in the given group (only non-zero).
207 DenseMap<ThreadPoolTaskGroup *, unsigned> ActiveGroups;
208
209#if LLVM_ENABLE_THREADS // avoids warning for unused variable
210 /// Signal for the destruction of the pool, asking thread to exit.
211 bool EnableFlag = true;
212#endif
213
214 const ThreadPoolStrategy Strategy;
215
216 /// Maximum number of threads to potentially grow this pool to.
217 const unsigned MaxThreadCount;
218};
219
220/// A group of tasks to be run on a thread pool. Thread pool tasks in different
221/// groups can run on the same threadpool but can be waited for separately.
222/// It is even possible for tasks of one group to submit and wait for tasks
223/// of another group, as long as this does not form a loop.
224class ThreadPoolTaskGroup {
225public:
226 /// The ThreadPool argument is the thread pool to forward calls to.
227 ThreadPoolTaskGroup(ThreadPool &Pool) : Pool(Pool) {}
228
229 /// Blocking destructor: will wait for all the tasks in the group to complete
230 /// by calling ThreadPool::wait().
231 ~ThreadPoolTaskGroup() { wait(); }
232
233 /// Calls ThreadPool::async() for this group.
234 template <typename Function, typename... Args>
235 inline auto async(Function &&F, Args &&...ArgList) {
236 return Pool.async(*this, std::forward<Function>(F),
237 std::forward<Args>(ArgList)...);
238 }
239
240 /// Calls ThreadPool::wait() for this group.
241 void wait() { Pool.wait(Group&: *this); }
242
243private:
244 ThreadPool &Pool;
245};
246
247} // namespace llvm
248
249#endif // LLVM_SUPPORT_THREADPOOL_H
250

source code of llvm/include/llvm/Support/ThreadPool.h