unsupported/Eigen/CXX11/src/ThreadPool/NonBlockingThreadPool.h - platform/external/eigen - Git at Google

 // This file is part of Eigen, a lightweight C++ template library
 // for linear algebra.
 //
 // Copyright (C) 2016 Dmitry Vyukov <[email protected]>
 //
 // This Source Code Form is subject to the terms of the Mozilla
 // Public License v. 2.0. If a copy of the MPL was not distributed
 // with this file, You can obtain one at http://mozilla.org/MPL/2.0/.

 #ifndef EIGEN_CXX11_THREADPOOL_NONBLOCKING_THREAD_POOL_H
 #define EIGEN_CXX11_THREADPOOL_NONBLOCKING_THREAD_POOL_H


 namespace Eigen {

 template <typename Environment>
 class NonBlockingThreadPoolTempl : public Eigen::ThreadPoolInterface {
  public:
   typedef typename Environment::Task Task;
   typedef RunQueue<Task, 1024> Queue;

   NonBlockingThreadPoolTempl(int num_threads, Environment env = Environment())
       : env_(env),
         threads_(num_threads),
         queues_(num_threads),
         coprimes_(num_threads),
         waiters_(num_threads),
         blocked_(0),
         spinning_(0),
         done_(false),
         ec_(waiters_) {
     waiters_.resize(num_threads);

     // Calculate coprimes of num_threads.
     // Coprimes are used for a random walk over all threads in Steal
     // and NonEmptyQueueIndex. Iteration is based on the fact that if we take
     // a walk starting thread index t and calculate num_threads - 1 subsequent
     // indices as (t + coprime) % num_threads, we will cover all threads without
     // repetitions (effectively getting a presudo-random permutation of thread
     // indices).
     for (int i = 1; i <= num_threads; i++) {
       unsigned a = i;
       unsigned b = num_threads;
       // If GCD(a, b) == 1, then a and b are coprimes.
       while (b != 0) {
         unsigned tmp = a;
         a = b;
         b = tmp % b;
       }
       if (a == 1) {
         coprimes_.push_back(i);
       }
     }
     for (int i = 0; i < num_threads; i++) {
       queues_.push_back(new Queue());
     }
     for (int i = 0; i < num_threads; i++) {
       threads_.push_back(env_.CreateThread([this, i]() { WorkerLoop(i); }));
     }
   }

   ~NonBlockingThreadPoolTempl() {
     done_ = true;
     // Now if all threads block without work, they will start exiting.
     // But note that threads can continue to work arbitrary long,
     // block, submit new work, unblock and otherwise live full life.
     ec_.Notify(true);

     // Join threads explicitly to avoid destruction order issues.
     for (size_t i = 0; i < threads_.size(); i++) delete threads_[i];
     for (size_t i = 0; i < threads_.size(); i++) delete queues_[i];
   }

   void Schedule(std::function<void()> fn) {
     Task t = env_.CreateTask(std::move(fn));
     PerThread* pt = GetPerThread();
     if (pt->pool == this) {
       // Worker thread of this pool, push onto the thread's queue.
       Queue* q = queues_[pt->thread_id];
       t = q->PushFront(std::move(t));
     } else {
       // A free-standing thread (or worker of another pool), push onto a random
       // queue.
       Queue* q = queues_[Rand(&pt->rand) % queues_.size()];
       t = q->PushBack(std::move(t));
     }
     // Note: below we touch this after making w available to worker threads.
     // Strictly speaking, this can lead to a racy-use-after-free. Consider that
     // Schedule is called from a thread that is neither main thread nor a worker
     // thread of this pool. Then, execution of w directly or indirectly
     // completes overall computations, which in turn leads to destruction of
     // this. We expect that such scenario is prevented by program, that is,
     // this is kept alive while any threads can potentially be in Schedule.
     if (!t.f)
       ec_.Notify(false);
     else
       env_.ExecuteTask(t);  // Push failed, execute directly.
   }

   int NumThreads() const final {
     return static_cast<int>(threads_.size());
   }

   int CurrentThreadId() const final {
     const PerThread* pt =
         const_cast<NonBlockingThreadPoolTempl*>(this)->GetPerThread();
     if (pt->pool == this) {
       return pt->thread_id;
     } else {
       return -1;
     }
   }

  private:
   typedef typename Environment::EnvThread Thread;

   struct PerThread {
     constexpr PerThread() : pool(NULL), rand(0), thread_id(-1) { }
     NonBlockingThreadPoolTempl* pool;  // Parent pool, or null for normal threads.
     uint64_t rand;  // Random generator state.
     int thread_id;  // Worker thread index in pool.
   };

   Environment env_;
   MaxSizeVector<Thread*> threads_;
   MaxSizeVector<Queue*> queues_;
   MaxSizeVector<unsigned> coprimes_;
   MaxSizeVector<EventCount::Waiter> waiters_;
   std::atomic<unsigned> blocked_;
   std::atomic<bool> spinning_;
   std::atomic<bool> done_;
   EventCount ec_;

   // Main worker thread loop.
   void WorkerLoop(int thread_id) {
     PerThread* pt = GetPerThread();
     pt->pool = this;
     pt->rand = std::hash<std::thread::id>()(std::this_thread::get_id());
     pt->thread_id = thread_id;
     Queue* q = queues_[thread_id];
     EventCount::Waiter* waiter = &waiters_[thread_id];
     for (;;) {
       Task t = q->PopFront();
       if (!t.f) {
         t = Steal();
         if (!t.f) {
           // Leave one thread spinning. This reduces latency.
           // TODO(dvyukov): 1000 iterations is based on fair dice roll, tune it.
           // Also, the time it takes to attempt to steal work 1000 times depends
           // on the size of the thread pool. However the speed at which the user
           // of the thread pool submit tasks is independent of the size of the
           // pool. Consider a time based limit instead.
           if (!spinning_ && !spinning_.exchange(true)) {
             for (int i = 0; i < 1000 && !t.f; i++) {
               t = Steal();
             }
             spinning_ = false;
           }
           if (!t.f) {
             if (!WaitForWork(waiter, &t)) {
               return;
             }
           }
         }
       }
       if (t.f) {
         env_.ExecuteTask(t);
       }
     }
   }

   // Steal tries to steal work from other worker threads in best-effort manner.
   Task Steal() {
     PerThread* pt = GetPerThread();
     const size_t size = queues_.size();
     unsigned r = Rand(&pt->rand);
     unsigned inc = coprimes_[r % coprimes_.size()];
     unsigned victim = r % size;
     for (unsigned i = 0; i < size; i++) {
       Task t = queues_[victim]->PopBack();
       if (t.f) {
         return t;
       }
       victim += inc;
       if (victim >= size) {
         victim -= size;
       }
     }
     return Task();
   }

   // WaitForWork blocks until new work is available (returns true), or if it is
   // time to exit (returns false). Can optionally return a task to execute in t
   // (in such case t.f != nullptr on return).
   bool WaitForWork(EventCount::Waiter* waiter, Task* t) {
     eigen_assert(!t->f);
     // We already did best-effort emptiness check in Steal, so prepare for
     // blocking.
     ec_.Prewait(waiter);
     // Now do a reliable emptiness check.
     int victim = NonEmptyQueueIndex();
     if (victim != -1) {
       ec_.CancelWait(waiter);
       *t = queues_[victim]->PopBack();
       return true;
     }
     // Number of blocked threads is used as termination condition.
     // If we are shutting down and all worker threads blocked without work,
     // that's we are done.
     blocked_++;
     if (done_ && blocked_ == threads_.size()) {
       ec_.CancelWait(waiter);
       // Almost done, but need to re-check queues.
       // Consider that all queues are empty and all worker threads are preempted
       // right after incrementing blocked_ above. Now a free-standing thread
       // submits work and calls destructor (which sets done_). If we don't
       // re-check queues, we will exit leaving the work unexecuted.
       if (NonEmptyQueueIndex() != -1) {
         // Note: we must not pop from queues before we decrement blocked_,
         // otherwise the following scenario is possible. Consider that instead
         // of checking for emptiness we popped the only element from queues.
         // Now other worker threads can start exiting, which is bad if the
         // work item submits other work. So we just check emptiness here,
         // which ensures that all worker threads exit at the same time.
         blocked_--;
         return true;
       }
       // Reached stable termination state.
       ec_.Notify(true);
       return false;
     }
     ec_.CommitWait(waiter);
     blocked_--;
     return true;
   }

   int NonEmptyQueueIndex() {
     PerThread* pt = GetPerThread();
     const size_t size = queues_.size();
     unsigned r = Rand(&pt->rand);
     unsigned inc = coprimes_[r % coprimes_.size()];
     unsigned victim = r % size;
     for (unsigned i = 0; i < size; i++) {
       if (!queues_[victim]->Empty()) {
         return victim;
       }
       victim += inc;
       if (victim >= size) {
         victim -= size;
       }
     }
     return -1;
   }

   static EIGEN_STRONG_INLINE PerThread* GetPerThread() {
     EIGEN_THREAD_LOCAL PerThread per_thread_;
     PerThread* pt = &per_thread_;
     return pt;
   }

   static EIGEN_STRONG_INLINE unsigned Rand(uint64_t* state) {
     uint64_t current = *state;
     // Update the internal state
     *state = current * 6364136223846793005ULL + 0xda3e39cb94b95bdbULL;
     // Generate the random output (using the PCG-XSH-RS scheme)
     return static_cast<unsigned>((current ^ (current >> 22)) >> (22 + (current >> 61)));
   }
 };

 typedef NonBlockingThreadPoolTempl<StlThreadEnvironment> NonBlockingThreadPool;

 }  // namespace Eigen

 #endif  // EIGEN_CXX11_THREADPOOL_NONBLOCKING_THREAD_POOL_H
	// This file is part of Eigen, a lightweight C++ template library
	// for linear algebra.
	//
	// Copyright (C) 2016 Dmitry Vyukov <[email protected]>
	//
	// This Source Code Form is subject to the terms of the Mozilla
	// Public License v. 2.0. If a copy of the MPL was not distributed
	// with this file, You can obtain one at http://mozilla.org/MPL/2.0/.

	#ifndef EIGEN_CXX11_THREADPOOL_NONBLOCKING_THREAD_POOL_H
	#define EIGEN_CXX11_THREADPOOL_NONBLOCKING_THREAD_POOL_H


	namespace Eigen {

	template <typename Environment>
	class NonBlockingThreadPoolTempl : public Eigen::ThreadPoolInterface {
	public:
	typedef typename Environment::Task Task;
	typedef RunQueue<Task, 1024> Queue;

	NonBlockingThreadPoolTempl(int num_threads, Environment env = Environment())
	: env_(env),
	threads_(num_threads),
	queues_(num_threads),
	coprimes_(num_threads),
	waiters_(num_threads),
	blocked_(0),
	spinning_(0),
	done_(false),
	ec_(waiters_) {
	waiters_.resize(num_threads);

	// Calculate coprimes of num_threads.
	// Coprimes are used for a random walk over all threads in Steal
	// and NonEmptyQueueIndex. Iteration is based on the fact that if we take
	// a walk starting thread index t and calculate num_threads - 1 subsequent
	// indices as (t + coprime) % num_threads, we will cover all threads without
	// repetitions (effectively getting a presudo-random permutation of thread
	// indices).
	for (int i = 1; i <= num_threads; i++) {
	unsigned a = i;
	unsigned b = num_threads;
	// If GCD(a, b) == 1, then a and b are coprimes.
	while (b != 0) {
	unsigned tmp = a;
	a = b;
	b = tmp % b;
	}
	if (a == 1) {
	coprimes_.push_back(i);
	}
	}
	for (int i = 0; i < num_threads; i++) {
	queues_.push_back(new Queue());
	}
	for (int i = 0; i < num_threads; i++) {
	threads_.push_back(env_.CreateThread([this, i]() { WorkerLoop(i); }));
	}
	}

	~NonBlockingThreadPoolTempl() {
	done_ = true;
	// Now if all threads block without work, they will start exiting.
	// But note that threads can continue to work arbitrary long,
	// block, submit new work, unblock and otherwise live full life.
	ec_.Notify(true);

	// Join threads explicitly to avoid destruction order issues.
	for (size_t i = 0; i < threads_.size(); i++) delete threads_[i];
	for (size_t i = 0; i < threads_.size(); i++) delete queues_[i];
	}

	void Schedule(std::function<void()> fn) {
	Task t = env_.CreateTask(std::move(fn));
	PerThread* pt = GetPerThread();
	if (pt->pool == this) {
	// Worker thread of this pool, push onto the thread's queue.
	Queue* q = queues_[pt->thread_id];
	t = q->PushFront(std::move(t));
	} else {
	// A free-standing thread (or worker of another pool), push onto a random
	// queue.
	Queue* q = queues_[Rand(&pt->rand) % queues_.size()];
	t = q->PushBack(std::move(t));
	}
	// Note: below we touch this after making w available to worker threads.
	// Strictly speaking, this can lead to a racy-use-after-free. Consider that
	// Schedule is called from a thread that is neither main thread nor a worker
	// thread of this pool. Then, execution of w directly or indirectly
	// completes overall computations, which in turn leads to destruction of
	// this. We expect that such scenario is prevented by program, that is,
	// this is kept alive while any threads can potentially be in Schedule.
	if (!t.f)
	ec_.Notify(false);
	else
	env_.ExecuteTask(t); // Push failed, execute directly.
	}

	int NumThreads() const final {
	return static_cast<int>(threads_.size());
	}

	int CurrentThreadId() const final {
	const PerThread* pt =
	const_cast<NonBlockingThreadPoolTempl*>(this)->GetPerThread();
	if (pt->pool == this) {
	return pt->thread_id;
	} else {
	return -1;
	}
	}

	private:
	typedef typename Environment::EnvThread Thread;

	struct PerThread {
	constexpr PerThread() : pool(NULL), rand(0), thread_id(-1) { }
	NonBlockingThreadPoolTempl* pool; // Parent pool, or null for normal threads.
	uint64_t rand; // Random generator state.
	int thread_id; // Worker thread index in pool.
	};

	Environment env_;
	MaxSizeVector<Thread*> threads_;
	MaxSizeVector<Queue*> queues_;
	MaxSizeVector<unsigned> coprimes_;
	MaxSizeVector<EventCount::Waiter> waiters_;
	std::atomic<unsigned> blocked_;
	std::atomic<bool> spinning_;
	std::atomic<bool> done_;
	EventCount ec_;

	// Main worker thread loop.
	void WorkerLoop(int thread_id) {
	PerThread* pt = GetPerThread();
	pt->pool = this;
	pt->rand = std::hash<std::thread::id>()(std::this_thread::get_id());
	pt->thread_id = thread_id;
	Queue* q = queues_[thread_id];
	EventCount::Waiter* waiter = &waiters_[thread_id];
	for (;;) {
	Task t = q->PopFront();
	if (!t.f) {
	t = Steal();
	if (!t.f) {
	// Leave one thread spinning. This reduces latency.
	// TODO(dvyukov): 1000 iterations is based on fair dice roll, tune it.
	// Also, the time it takes to attempt to steal work 1000 times depends
	// on the size of the thread pool. However the speed at which the user
	// of the thread pool submit tasks is independent of the size of the
	// pool. Consider a time based limit instead.
	if (!spinning_ && !spinning_.exchange(true)) {
	for (int i = 0; i < 1000 && !t.f; i++) {
	t = Steal();
	}
	spinning_ = false;
	}
	if (!t.f) {
	if (!WaitForWork(waiter, &t)) {
	return;
	}
	}
	}
	}
	if (t.f) {
	env_.ExecuteTask(t);
	}
	}
	}

	// Steal tries to steal work from other worker threads in best-effort manner.
	Task Steal() {
	PerThread* pt = GetPerThread();
	const size_t size = queues_.size();
	unsigned r = Rand(&pt->rand);
	unsigned inc = coprimes_[r % coprimes_.size()];
	unsigned victim = r % size;
	for (unsigned i = 0; i < size; i++) {
	Task t = queues_[victim]->PopBack();
	if (t.f) {
	return t;
	}
	victim += inc;
	if (victim >= size) {
	victim -= size;
	}
	}
	return Task();
	}

	// WaitForWork blocks until new work is available (returns true), or if it is
	// time to exit (returns false). Can optionally return a task to execute in t
	// (in such case t.f != nullptr on return).
	bool WaitForWork(EventCount::Waiter* waiter, Task* t) {
	eigen_assert(!t->f);
	// We already did best-effort emptiness check in Steal, so prepare for
	// blocking.
	ec_.Prewait(waiter);
	// Now do a reliable emptiness check.
	int victim = NonEmptyQueueIndex();
	if (victim != -1) {
	ec_.CancelWait(waiter);
	*t = queues_[victim]->PopBack();
	return true;
	}
	// Number of blocked threads is used as termination condition.
	// If we are shutting down and all worker threads blocked without work,
	// that's we are done.
	blocked_++;
	if (done_ && blocked_ == threads_.size()) {
	ec_.CancelWait(waiter);
	// Almost done, but need to re-check queues.
	// Consider that all queues are empty and all worker threads are preempted
	// right after incrementing blocked_ above. Now a free-standing thread
	// submits work and calls destructor (which sets done_). If we don't
	// re-check queues, we will exit leaving the work unexecuted.
	if (NonEmptyQueueIndex() != -1) {
	// Note: we must not pop from queues before we decrement blocked_,
	// otherwise the following scenario is possible. Consider that instead
	// of checking for emptiness we popped the only element from queues.
	// Now other worker threads can start exiting, which is bad if the
	// work item submits other work. So we just check emptiness here,
	// which ensures that all worker threads exit at the same time.
	blocked_--;
	return true;
	}
	// Reached stable termination state.
	ec_.Notify(true);
	return false;
	}
	ec_.CommitWait(waiter);
	blocked_--;
	return true;
	}

	int NonEmptyQueueIndex() {
	PerThread* pt = GetPerThread();
	const size_t size = queues_.size();
	unsigned r = Rand(&pt->rand);
	unsigned inc = coprimes_[r % coprimes_.size()];
	unsigned victim = r % size;
	for (unsigned i = 0; i < size; i++) {
	if (!queues_[victim]->Empty()) {
	return victim;
	}
	victim += inc;
	if (victim >= size) {
	victim -= size;
	}
	}
	return -1;
	}

	static EIGEN_STRONG_INLINE PerThread* GetPerThread() {
	EIGEN_THREAD_LOCAL PerThread per_thread_;
	PerThread* pt = &per_thread_;
	return pt;
	}

	static EIGEN_STRONG_INLINE unsigned Rand(uint64_t* state) {
	uint64_t current = *state;
	// Update the internal state
	state = current 6364136223846793005ULL + 0xda3e39cb94b95bdbULL;
	// Generate the random output (using the PCG-XSH-RS scheme)
	return static_cast<unsigned>((current ^ (current >> 22)) >> (22 + (current >> 61)));
	}
	};

	typedef NonBlockingThreadPoolTempl<StlThreadEnvironment> NonBlockingThreadPool;

	} // namespace Eigen

	#endif // EIGEN_CXX11_THREADPOOL_NONBLOCKING_THREAD_POOL_H