[llvm] r255444 - Add a C++11 ThreadPool implementation in LLVM
Teresa Johnson via llvm-commits
llvm-commits at lists.llvm.org
Fri Dec 18 08:15:17 PST 2015
On Fri, Dec 18, 2015 at 7:38 AM, Vedant Kumar via llvm-commits
<llvm-commits at lists.llvm.org> wrote:
> Hi Mehdi,
>
> I have some inline comments below --
>
>> On Dec 12, 2015, at 2:55 PM, Mehdi AMINI via llvm-commits <llvm-commits at lists.llvm.org> wrote:
>>
>> Author: mehdi_amini
>> Date: Sat Dec 12 16:55:25 2015
>> New Revision: 255444
>>
>> URL: http://llvm.org/viewvc/llvm-project?rev=255444&view=rev
>> Log:
>> Add a C++11 ThreadPool implementation in LLVM
>>
>> This is a very simple implementation of a thread pool using C++11
>> thread. It accepts any std::function<void()> for asynchronous
>> execution. Individual task can be synchronize using the returned
>> future, or the client can block on the full queue completion.
>>
>> In case LLVM is configured with Threading disabled, it falls back
>> to sequential execution using std::async with launch:deferred.
>>
>> This is intended to support parallelism for ThinLTO processing in
>> linker plugin, but is generic enough for any other uses.
>>
>> Differential Revision: http://reviews.llvm.org/D15464
>>
>> From: Mehdi Amini <mehdi.amini at apple.com>
>>
>> Added:
>> llvm/trunk/include/llvm/Support/ThreadPool.h
>> llvm/trunk/lib/Support/ThreadPool.cpp
>> llvm/trunk/unittests/Support/ThreadPool.cpp
>> Modified:
>> llvm/trunk/include/llvm/Support/thread.h
>> llvm/trunk/lib/Support/CMakeLists.txt
>> llvm/trunk/unittests/Support/CMakeLists.txt
>>
>> Added: llvm/trunk/include/llvm/Support/ThreadPool.h
>> URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/include/llvm/Support/ThreadPool.h?rev=255444&view=auto
>> ==============================================================================
>> --- llvm/trunk/include/llvm/Support/ThreadPool.h (added)
>> +++ llvm/trunk/include/llvm/Support/ThreadPool.h Sat Dec 12 16:55:25 2015
>> @@ -0,0 +1,95 @@
>> +//===-- llvm/Support/ThreadPool.h - A ThreadPool implementation -*- C++ -*-===//
>> +//
>> +// The LLVM Compiler Infrastructure
>> +//
>> +// This file is distributed under the University of Illinois Open Source
>> +// License. See LICENSE.TXT for details.
>> +//
>> +//===----------------------------------------------------------------------===//
>> +//
>> +// This file defines a crude C++11 based thread pool.
>> +//
>> +//===----------------------------------------------------------------------===//
>> +
>> +#ifndef LLVM_SUPPORT_THREAD_POOL_H
>> +#define LLVM_SUPPORT_THREAD_POOL_H
>> +
>> +#include "llvm/Support/thread.h"
>> +
>> +#include <condition_variable>
>> +#include <functional>
>> +#include <future>
>> +#include <memory>
>> +#include <mutex>
>> +#include <queue>
>> +#include <utility>
>> +
>> +namespace llvm {
>> +
>> +/// A ThreadPool for asynchronous parallel execution on a defined number of
>> +/// threads.
>> +///
>> +/// The pool keeps a vector of threads alive, waiting on a condition variable
>> +/// for some work to become available.
>> +class ThreadPool {
>> +public:
>> + using TaskTy = std::function<void()>;
>> +
>> + /// Construct a pool with the number of core available on the system (or
>> + /// whatever the value returned by std::thread::hardware_concurrency() is).
>> + ThreadPool();
>> +
>> + /// Construct a pool of \p ThreadCount threads
>> + ThreadPool(unsigned ThreadCount);
>> +
>> + /// Blocking destructor: the pool will wait for all the threads to complete.
>> + ~ThreadPool();
>> +
>> + /// Asynchronous submission of a task to the pool. The returned future can be
>> + /// used to wait for the task to finish and is *non-blocking* on destruction.
>> + template <typename Function, typename... Args>
>> + inline std::shared_future<void> async(Function &&F, Args &&... ArgList) {
>> + auto Task =
>> + std::bind(std::forward<Function>(F), std::forward<Args...>(ArgList...));
>> + return asyncImpl(Task);
>> + }
>> +
>> + /// Asynchronous submission of a task to the pool. The returned future can be
>> + /// used to wait for the task to finish and is *non-blocking* on destruction.
>> + template <typename Function>
>> + inline std::shared_future<void> async(Function &&F) {
>> + return asyncImpl(F);
>> + }
>> +
>> + /// Blocking wait for all the threads to complete and the queue to be empty.
>> + /// It is an error to try to add new tasks while blocking on this call.
>> + void wait();
>> +
>> +private:
>> + /// Asynchronous submission of a task to the pool. The returned future can be
>> + /// used to wait for the task to finish and is *non-blocking* on destruction.
>> + std::shared_future<void> asyncImpl(TaskTy f);
>> +
>> + /// Threads in flight
>> + std::vector<llvm::thread> Threads;
>> +
>> + /// Tasks waiting for execution in the pool.
>> + std::queue<std::packaged_task<void()>> Tasks;
>> +
>> + /// Locking and signaling for accessing the Tasks queue.
>> + std::mutex QueueLock;
>> + std::condition_variable QueueCondition;
>> +
>> + /// Locking and signaling for job completion
>> + std::mutex CompletionLock;
>> + std::condition_variable CompletionCondition;
>> +
>> + /// Keep track of the number of thread actually busy
>> + std::atomic<unsigned> ActiveThreads;
>> +
>> + /// Signal for the destruction of the pool, asking thread to exit.
>> + bool EnableFlag;
>> +};
>> +}
>> +
>> +#endif // LLVM_SUPPORT_THREAD_POOL_H
>>
>> Modified: llvm/trunk/include/llvm/Support/thread.h
>> URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/include/llvm/Support/thread.h?rev=255444&r1=255443&r2=255444&view=diff
>> ==============================================================================
>> --- llvm/trunk/include/llvm/Support/thread.h (original)
>> +++ llvm/trunk/include/llvm/Support/thread.h Sat Dec 12 16:55:25 2015
>> @@ -43,6 +43,8 @@ typedef std::thread thread;
>>
>> #else // !LLVM_ENABLE_THREADS
>>
>> +#include <utility>
>> +
>> namespace llvm {
>>
>> struct thread {
>>
>> Modified: llvm/trunk/lib/Support/CMakeLists.txt
>> URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Support/CMakeLists.txt?rev=255444&r1=255443&r2=255444&view=diff
>> ==============================================================================
>> --- llvm/trunk/lib/Support/CMakeLists.txt (original)
>> +++ llvm/trunk/lib/Support/CMakeLists.txt Sat Dec 12 16:55:25 2015
>> @@ -89,6 +89,7 @@ add_llvm_library(LLVMSupport
>> StringRef.cpp
>> SystemUtils.cpp
>> TargetParser.cpp
>> + ThreadPool.cpp
>> Timer.cpp
>> ToolOutputFile.cpp
>> Triple.cpp
>>
>> Added: llvm/trunk/lib/Support/ThreadPool.cpp
>> URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Support/ThreadPool.cpp?rev=255444&view=auto
>> ==============================================================================
>> --- llvm/trunk/lib/Support/ThreadPool.cpp (added)
>> +++ llvm/trunk/lib/Support/ThreadPool.cpp Sat Dec 12 16:55:25 2015
>> @@ -0,0 +1,143 @@
>> +//==-- llvm/Support/ThreadPool.cpp - A ThreadPool implementation -*- C++ -*-==//
>> +//
>> +// The LLVM Compiler Infrastructure
>> +//
>> +// This file is distributed under the University of Illinois Open Source
>> +// License. See LICENSE.TXT for details.
>> +//
>> +//===----------------------------------------------------------------------===//
>> +//
>> +// This file implements a crude C++11 based thread pool.
>> +//
>> +//===----------------------------------------------------------------------===//
>> +
>> +#include "llvm/Support/ThreadPool.h"
>> +
>> +#include "llvm/Config/llvm-config.h"
>> +#include "llvm/Support/raw_ostream.h"
>> +
>> +using namespace llvm;
>> +
>> +#if LLVM_ENABLE_THREADS
>> +
>> +// Default to std::thread::hardware_concurrency
>> +ThreadPool::ThreadPool() : ThreadPool(std::thread::hardware_concurrency()) {}
>> +
>> +ThreadPool::ThreadPool(unsigned ThreadCount)
>> + : ActiveThreads(0), EnableFlag(true) {
>> + // Create ThreadCount threads that will loop forever, wait on QueueCondition
>> + // for tasks to be queued or the Pool to be destroyed.
>> + Threads.reserve(ThreadCount);
>> + for (unsigned ThreadID = 0; ThreadID < ThreadCount; ++ThreadID) {
>> + Threads.emplace_back([&] {
>> + while (true) {
>> + std::packaged_task<void()> Task;
>> + {
>> + std::unique_lock<std::mutex> LockGuard(QueueLock);
>> + // Wait for tasks to be pushed in the queue
>> + QueueCondition.wait(LockGuard,
>> + [&] { return !EnableFlag || !Tasks.empty(); });
>> + // Exit condition
>> + if (!EnableFlag && Tasks.empty())
>> + return;
>> + // Yeah, we have a task, grab it and release the lock on the queue
>> +
>> + // We first need to signal that we are active before popping the queue
>> + // in order for wait() to properly detect that even if the queue is
>> + // empty, there is still a task in flight.
>> + {
>> + ++ActiveThreads;
>> + std::unique_lock<std::mutex> LockGuard(CompletionLock);
>> + }
>> + Task = std::move(Tasks.front());
>> + Tasks.pop();
>> + }
>> + // Run the task we just grabbed
>> + Task();
>> +
>> + {
>> + // Adjust `ActiveThreads`, in case someone waits on ThreadPool::wait()
>> + std::unique_lock<std::mutex> LockGuard(CompletionLock);
>> + --ActiveThreads;
>> + }
>> +
>> + // Notify task completion, in case someone waits on ThreadPool::wait()
>> + CompletionCondition.notify_all();
>> + }
>> + });
>> + }
>> +}
>> +
>> +void ThreadPool::wait() {
>> + // Wait for all threads to complete and the queue to be empty
>> + std::unique_lock<std::mutex> LockGuard(CompletionLock);
>> + CompletionCondition.wait(LockGuard,
>> + [&] { return Tasks.empty() && !ActiveThreads; });
>> +}
>> +
>> +std::shared_future<void> ThreadPool::asyncImpl(TaskTy Task) {
>> + /// Wrap the Task in a packaged_task to return a future object.
>> + std::packaged_task<void()> PackagedTask(std::move(Task));
>> + std::future<void> Future = PackagedTask.get_future();
>> + {
>> + // Lock the queue and push the new task
>> + std::unique_lock<std::mutex> LockGuard(QueueLock);
>> +
>> + // Don't allow enqueueing after disabling the pool
>> + assert(EnableFlag && "Queuing a thread during ThreadPool destruction");
>> +
>> + Tasks.push(std::move(PackagedTask));
>> + }
>> + QueueCondition.notify_one();
>> + return Future.share();
>> +}
>> +
>> +// The destructor joins all threads, waiting for completion.
>> +ThreadPool::~ThreadPool() {
>> + {
>> + std::unique_lock<std::mutex> LockGuard(QueueLock);
>> + EnableFlag = false;
>> + }
>> + QueueCondition.notify_all();
>> + for (auto &Worker : Threads)
>> + Worker.join();
>> +}
>> +
>> +#else // LLVM_ENABLE_THREADS Disabled
>> +
>> +ThreadPool::ThreadPool() : ThreadPool(0) {}
>> +
>> +// No threads are launched, issue a warning if ThreadCount is not 0
>> +ThreadPool::ThreadPool(unsigned ThreadCount)
>> + : ActiveThreads(0), EnableFlag(true) {
>> + if (ThreadCount) {
>> + errs() << "Warning: request a ThreadPool with " << ThreadCount
>> + << " threads, but LLVM_ENABLE_THREADS has been turned off\n";
>> + }
>> +}
>> +
>> +void ThreadPool::wait() {
>> + // Sequential implementation running the tasks
>> + while (!Tasks.empty()) {
>> + auto Task = std::move(Tasks.front());
>> + Tasks.pop();
>> + Task();
>> + }
>> +}
>> +
>> +std::shared_future<void> ThreadPool::asyncImpl(TaskTy Task) {
>> + // Get a Future with launch::deferred execution using std::async
>> + auto Future = std::async(std::launch::deferred, std::move(Task)).share();
>> + // Wrap the future so that both ThreadPool::wait() can operate and the
>> + // returned future can be sync'ed on.
>> + std::packaged_task<void()> PackagedTask([Future]() { Future.get(); });
>> + Tasks.push(std::move(PackagedTask));
>> + return Future;
>> +}
>> +
>> +ThreadPool::~ThreadPool() {
>> + EnableFlag = false;
>> + wait();
>> +}
>> +
>> +#endif
>>
>> Modified: llvm/trunk/unittests/Support/CMakeLists.txt
>> URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/unittests/Support/CMakeLists.txt?rev=255444&r1=255443&r2=255444&view=diff
>> ==============================================================================
>> --- llvm/trunk/unittests/Support/CMakeLists.txt (original)
>> +++ llvm/trunk/unittests/Support/CMakeLists.txt Sat Dec 12 16:55:25 2015
>> @@ -41,6 +41,7 @@ add_llvm_unittest(SupportTests
>> SwapByteOrderTest.cpp
>> TargetRegistry.cpp
>> ThreadLocalTest.cpp
>> + ThreadPool.cpp
>> TimeValueTest.cpp
>> TrailingObjectsTest.cpp
>> UnicodeTest.cpp
>>
>> Added: llvm/trunk/unittests/Support/ThreadPool.cpp
>> URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/unittests/Support/ThreadPool.cpp?rev=255444&view=auto
>> ==============================================================================
>> --- llvm/trunk/unittests/Support/ThreadPool.cpp (added)
>> +++ llvm/trunk/unittests/Support/ThreadPool.cpp Sat Dec 12 16:55:25 2015
>> @@ -0,0 +1,91 @@
>> +//========- unittests/Support/ThreadPools.cpp - ThreadPools.h tests --========//
>> +//
>> +// The LLVM Compiler Infrastructure
>> +//
>> +// This file is distributed under the University of Illinois Open Source
>> +// License. See LICENSE.TXT for details.
>> +//
>> +//===----------------------------------------------------------------------===//
>> +
>> +#include "llvm/Support/ThreadPool.h"
>> +
>> +#include "llvm/ADT/STLExtras.h"
>> +
>> +#include "gtest/gtest.h"
>> +
>> +using namespace llvm;
>> +using namespace std::chrono;
>> +
>> +/// Try best to make this thread not progress faster than the main thread
>> +static void yield() {
>> +#ifdef LLVM_ENABLE_THREADS
>> + std::this_thread::yield();
>> +#endif
>> + std::this_thread::sleep_for(milliseconds(200));
>> +#ifdef LLVM_ENABLE_THREADS
>> + std::this_thread::yield();
>> +#endif
>> +}
>> +
>> +TEST(ThreadPoolTest, AsyncBarrier) {
>> + // test that async & barrier work together properly.
>> +
>> + std::atomic_int checked_in{0};
>> +
>> + ThreadPool Pool;
>> + for (size_t i = 0; i < 5; ++i) {
>> + Pool.async([&checked_in, i] {
>> + yield();
>> + ++checked_in;
>> + });
>> + }
>> + ASSERT_EQ(0, checked_in);
>> + Pool.wait();
>> + ASSERT_EQ(5, checked_in);
>> +}
>> +
>> +TEST(ThreadPoolTest, Async) {
>> + ThreadPool Pool;
>> + std::atomic_int i{0};
>> + // sleep here just to ensure that the not-equal is correct.
>
> I'm not sure that the usage of yield() ensures this... please see my comment at the end of the email.
>
> This is related to https://llvm.org/bugs/show_bug.cgi?id=25829.
>
>> + Pool.async([&i] {
>> + yield();
>> + ++i;
>> + });
>> + Pool.async([&i] { ++i; });
>> + ASSERT_NE(2, i.load());
>> + Pool.wait();
>> + ASSERT_EQ(2, i.load());
>> +}
>> +
>> +TEST(ThreadPoolTest, GetFuture) {
>> + ThreadPool Pool;
>> + std::atomic_int i{0};
>> + // sleep here just to ensure that the not-equal is correct.
>> + Pool.async([&i] {
>> + yield();
>> + ++i;
>> + });
>> + // Force the future using get()
>> + Pool.async([&i] { ++i; }).get();
>> + ASSERT_NE(2, i.load());
>> + Pool.wait();
>> + ASSERT_EQ(2, i.load());
>> +}
>> +
>> +TEST(ThreadPoolTest, PoolDestruction) {
>> + // Test that we are waiting on destruction
>> + std::atomic_int checked_in{0};
>> +
>> + {
>> + ThreadPool Pool;
>> + for (size_t i = 0; i < 5; ++i) {
>> + Pool.async([&checked_in, i] {
>> + yield();
>
> Mehdi, I don't think the usage of yield() here guarantees that no updates to `checked_in` happen before we hit `ASSERT_EQ(0, checked_in)`. Yaron referenced a failure in this spot on the -dev list earlier -- could you take a look?
I'm wondering how important it is to even do the ASSERT_EQ(0,
checked_in) check. We know checked_in was initialized to 0 at the
start of the function, really I think the only check that is needed is
to ensure that when we leave the scope it is 5.
Mehdi, wdyt? I can make the change if you agree (think you are on vacation?).
Teresa
>
>> + ++checked_in;
>> + });
>> + }
>> + ASSERT_EQ(0, checked_in);
>> + }
>> + ASSERT_EQ(5, checked_in);
>> +}
>
> vedant
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