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 // This file is part of Eigen, a lightweight C++ template library
 // for linear algebra.
 //
 // Copyright (C) 2016 Dmitry Vyukov <dvyukov@google.com>
 //
 // 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 ThreadPoolTempl : public Eigen::ThreadPoolInterface {
  public:
   typedef typename Environment::Task Task;
   typedef RunQueue<Task, 1024> Queue;
 
   ThreadPoolTempl(int num_threads, Environment env = Environment())
       : ThreadPoolTempl(num_threads, true, env) {}
 
   ThreadPoolTempl(int num_threads, bool allow_spinning,
                   Environment env = Environment())
       : env_(env),
         num_threads_(num_threads),
         allow_spinning_(allow_spinning),
         thread_data_(num_threads),
         all_coprimes_(num_threads),
         waiters_(num_threads),
         global_steal_partition_(EncodePartition(0, num_threads_)),
         blocked_(0),
         spinning_(0),
         done_(false),
         cancelled_(false),
         ec_(waiters_) {
     waiters_.resize(num_threads_);
     // Calculate coprimes of all numbers [1, num_threads].
     // Coprimes are used for random walks over all threads in Steal
     // and NonEmptyQueueIndex. Iteration is based on the fact that if we take
     // a random 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).
     eigen_plain_assert(num_threads_ < kMaxThreads);
     for (int i = 1; i <= num_threads_; ++i) {
       all_coprimes_.emplace_back(i);
       ComputeCoprimes(i, &all_coprimes_.back());
     }
 #ifndef EIGEN_THREAD_LOCAL
     init_barrier_.reset(new Barrier(num_threads_));
 #endif
     thread_data_.resize(num_threads_);
     for (int i = 0; i < num_threads_; i++) {
       SetStealPartition(i, EncodePartition(0, num_threads_));
       thread_data_[i].thread.reset(
           env_.CreateThread([this, i]() { WorkerLoop(i); }));
     }
 #ifndef EIGEN_THREAD_LOCAL
     // Wait for workers to initialize per_thread_map_. Otherwise we might race
     // with them in Schedule or CurrentThreadId.
     init_barrier_->Wait();
 #endif
   }
 
   ~ThreadPoolTempl() {
     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.
     if (!cancelled_) {
       ec_.Notify(true);
     } else {
       // Since we were cancelled, there might be entries in the queues.
       // Empty them to prevent their destructor from asserting.
       for (size_t i = 0; i < thread_data_.size(); i++) {
         thread_data_[i].queue.Flush();
       }
     }
     // Join threads explicitly (by destroying) to avoid destruction order within
     // this class.
     for (size_t i = 0; i < thread_data_.size(); ++i)
       thread_data_[i].thread.reset();
   }
 
   void SetStealPartitions(const std::vector<std::pair<unsigned, unsigned>>& partitions) {
     eigen_plain_assert(partitions.size() == static_cast<std::size_t>(num_threads_));
 
     // Pass this information to each thread queue.
     for (int i = 0; i < num_threads_; i++) {
       const auto& pair = partitions[i];
       unsigned start = pair.first, end = pair.second;
       AssertBounds(start, end);
       unsigned val = EncodePartition(start, end);
       SetStealPartition(i, val);
     }
   }
 
   void Schedule(std::function<void()> fn) EIGEN_OVERRIDE {
     ScheduleWithHint(std::move(fn), 0, num_threads_);
   }
 
   void ScheduleWithHint(std::function<void()> fn, int start,
                         int limit) override {
     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 = thread_data_[pt->thread_id].queue;
       t = q.PushFront(std::move(t));
     } else {
       // A free-standing thread (or worker of another pool), push onto a random
       // queue.
       eigen_plain_assert(start < limit);
       eigen_plain_assert(limit <= num_threads_);
       int num_queues = limit - start;
       int rnd = Rand(&pt->rand) % num_queues;
       eigen_plain_assert(start + rnd < limit);
       Queue& q = thread_data_[start + rnd].queue;
       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.
     }
   }
 
   void Cancel() EIGEN_OVERRIDE {
     cancelled_ = true;
     done_ = true;
 
     // Let each thread know it's been cancelled.
 #ifdef EIGEN_THREAD_ENV_SUPPORTS_CANCELLATION
     for (size_t i = 0; i < thread_data_.size(); i++) {
       thread_data_[i].thread->OnCancel();
     }
 #endif
 
     // Wake up the threads without work to let them exit on their own.
     ec_.Notify(true);
   }
 
   int NumThreads() const EIGEN_FINAL { return num_threads_; }
 
   int CurrentThreadId() const EIGEN_FINAL {
     const PerThread* pt = const_cast<ThreadPoolTempl*>(this)->GetPerThread();
     if (pt->pool == this) {
       return pt->thread_id;
     } else {
       return -1;
     }
   }
 
  private:
   // Create a single atomic<int> that encodes start and limit information for
   // each thread.
   // We expect num_threads_ < 65536, so we can store them in a single
   // std::atomic<unsigned>.
   // Exposed publicly as static functions so that external callers can reuse
   // this encode/decode logic for maintaining their own thread-safe copies of
   // scheduling and steal domain(s).
   static const int kMaxPartitionBits = 16;
   static const int kMaxThreads = 1 << kMaxPartitionBits;
 
   inline unsigned EncodePartition(unsigned start, unsigned limit) {
     return (start << kMaxPartitionBits) | limit;
   }
 
   inline void DecodePartition(unsigned val, unsigned* start, unsigned* limit) {
     *limit = val & (kMaxThreads - 1);
     val >>= kMaxPartitionBits;
     *start = val;
   }
 
   void AssertBounds(int start, int end) {
     eigen_plain_assert(start >= 0);
     eigen_plain_assert(start < end);  // non-zero sized partition
     eigen_plain_assert(end <= num_threads_);
   }
 
   inline void SetStealPartition(size_t i, unsigned val) {
     thread_data_[i].steal_partition.store(val, std::memory_order_relaxed);
   }
 
   inline unsigned GetStealPartition(int i) {
     return thread_data_[i].steal_partition.load(std::memory_order_relaxed);
   }
 
   void ComputeCoprimes(int N, MaxSizeVector<unsigned>* coprimes) {
     for (int i = 1; i <= N; i++) {
       unsigned a = i;
       unsigned b = N;
       // 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);
       }
     }
   }
 
   typedef typename Environment::EnvThread Thread;
 
   struct PerThread {
     constexpr PerThread() : pool(NULL), rand(0), thread_id(-1) {}
     ThreadPoolTempl* pool;  // Parent pool, or null for normal threads.
     uint64_t rand;          // Random generator state.
     int thread_id;          // Worker thread index in pool.
 #ifndef EIGEN_THREAD_LOCAL
     // Prevent false sharing.
     char pad_[128];
 #endif
   };
 
   struct ThreadData {
     constexpr ThreadData() : thread(), steal_partition(0), queue() {}
     std::unique_ptr<Thread> thread;
     std::atomic<unsigned> steal_partition;
     Queue queue;
   };
 
   Environment env_;
   const int num_threads_;
   const bool allow_spinning_;
   MaxSizeVector<ThreadData> thread_data_;
   MaxSizeVector<MaxSizeVector<unsigned>> all_coprimes_;
   MaxSizeVector<EventCount::Waiter> waiters_;
   unsigned global_steal_partition_;
   std::atomic<unsigned> blocked_;
   std::atomic<bool> spinning_;
   std::atomic<bool> done_;
   std::atomic<bool> cancelled_;
   EventCount ec_;
 #ifndef EIGEN_THREAD_LOCAL
   std::unique_ptr<Barrier> init_barrier_;
   std::mutex per_thread_map_mutex_;  // Protects per_thread_map_.
   std::unordered_map<uint64_t, std::unique_ptr<PerThread>> per_thread_map_;
 #endif
 
   // Main worker thread loop.
   void WorkerLoop(int thread_id) {
 #ifndef EIGEN_THREAD_LOCAL
     std::unique_ptr<PerThread> new_pt(new PerThread());
     per_thread_map_mutex_.lock();
     bool insertOK = per_thread_map_.emplace(GlobalThreadIdHash(), std::move(new_pt)).second;
     eigen_plain_assert(insertOK);
     EIGEN_UNUSED_VARIABLE(insertOK);
     per_thread_map_mutex_.unlock();
     init_barrier_->Notify();
     init_barrier_->Wait();
 #endif
     PerThread* pt = GetPerThread();
     pt->pool = this;
     pt->rand = GlobalThreadIdHash();
     pt->thread_id = thread_id;
     Queue& q = thread_data_[thread_id].queue;
     EventCount::Waiter* waiter = &waiters_[thread_id];
     // TODO(dvyukov,rmlarsen): The time spent in NonEmptyQueueIndex() is
     // proportional to num_threads_ and we assume that new work is scheduled at
     // a constant rate, so we set spin_count to 5000 / num_threads_. The
     // constant was picked based on a fair dice roll, tune it.
     const int spin_count =
         allow_spinning_ && num_threads_ > 0 ? 5000 / num_threads_ : 0;
     if (num_threads_ == 1) {
       // For num_threads_ == 1 there is no point in going through the expensive
       // steal loop. Moreover, since NonEmptyQueueIndex() calls PopBack() on the
       // victim queues it might reverse the order in which ops are executed
       // compared to the order in which they are scheduled, which tends to be
       // counter-productive for the types of I/O workloads the single thread
       // pools tend to be used for.
       while (!cancelled_) {
         Task t = q.PopFront();
         for (int i = 0; i < spin_count && !t.f; i++) {
           if (!cancelled_.load(std::memory_order_relaxed)) {
             t = q.PopFront();
           }
         }
         if (!t.f) {
           if (!WaitForWork(waiter, &t)) {
             return;
           }
         }
         if (t.f) {
           env_.ExecuteTask(t);
         }
       }
     } else {
       while (!cancelled_) {
         Task t = q.PopFront();
         if (!t.f) {
           t = LocalSteal();
           if (!t.f) {
             t = GlobalSteal();
             if (!t.f) {
               // Leave one thread spinning. This reduces latency.
               if (allow_spinning_ && !spinning_ && !spinning_.exchange(true)) {
                 for (int i = 0; i < spin_count && !t.f; i++) {
                   if (!cancelled_.load(std::memory_order_relaxed)) {
                     t = GlobalSteal();
                   } else {
                     return;
                   }
                 }
                 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 the range [start,
   // limit) in best-effort manner.
   Task Steal(unsigned start, unsigned limit) {
     PerThread* pt = GetPerThread();
     const size_t size = limit - start;
     unsigned r = Rand(&pt->rand);
     // Reduce r into [0, size) range, this utilizes trick from
     // https://lemire.me/blog/2016/06/27/a-fast-alternative-to-the-modulo-reduction/
     eigen_plain_assert(all_coprimes_[size - 1].size() < (1<<30));
     unsigned victim = ((uint64_t)r * (uint64_t)size) >> 32;
     unsigned index = ((uint64_t) all_coprimes_[size - 1].size() * (uint64_t)r) >> 32;
     unsigned inc = all_coprimes_[size - 1][index];
 
     for (unsigned i = 0; i < size; i++) {
       eigen_plain_assert(start + victim < limit);
       Task t = thread_data_[start + victim].queue.PopBack();
       if (t.f) {
         return t;
       }
       victim += inc;
       if (victim >= size) {
         victim -= size;
       }
     }
     return Task();
   }
 
   // Steals work within threads belonging to the partition.
   Task LocalSteal() {
     PerThread* pt = GetPerThread();
     unsigned partition = GetStealPartition(pt->thread_id);
     // If thread steal partition is the same as global partition, there is no
     // need to go through the steal loop twice.
     if (global_steal_partition_ == partition) return Task();
     unsigned start, limit;
     DecodePartition(partition, &start, &limit);
     AssertBounds(start, limit);
 
     return Steal(start, limit);
   }
 
   // Steals work from any other thread in the pool.
   Task GlobalSteal() {
     return Steal(0, num_threads_);
   }
 
 
   // 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_plain_assert(!t->f);
     // We already did best-effort emptiness check in Steal, so prepare for
     // blocking.
     ec_.Prewait();
     // Now do a reliable emptiness check.
     int victim = NonEmptyQueueIndex();
     if (victim != -1) {
       ec_.CancelWait();
       if (cancelled_) {
         return false;
       } else {
         *t = thread_data_[victim].queue.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_++;
     // TODO is blocked_ required to be unsigned?
     if (done_ && blocked_ == static_cast<unsigned>(num_threads_)) {
       ec_.CancelWait();
       // 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();
     // We intentionally design NonEmptyQueueIndex to steal work from
     // anywhere in the queue so threads don't block in WaitForWork() forever
     // when all threads in their partition go to sleep. Steal is still local.
     const size_t size = thread_data_.size();
     unsigned r = Rand(&pt->rand);
     unsigned inc = all_coprimes_[size - 1][r % all_coprimes_[size - 1].size()];
     unsigned victim = r % size;
     for (unsigned i = 0; i < size; i++) {
       if (!thread_data_[victim].queue.Empty()) {
         return victim;
       }
       victim += inc;
       if (victim >= size) {
         victim -= size;
       }
     }
     return -1;
   }
 
   static EIGEN_STRONG_INLINE uint64_t GlobalThreadIdHash() {
     return std::hash<std::thread::id>()(std::this_thread::get_id());
   }
 
   EIGEN_STRONG_INLINE PerThread* GetPerThread() {
 #ifndef EIGEN_THREAD_LOCAL
     static PerThread dummy;
     auto it = per_thread_map_.find(GlobalThreadIdHash());
     if (it == per_thread_map_.end()) {
       return &dummy;
     } else {
       return it->second.get();
     }
 #else
     EIGEN_THREAD_LOCAL PerThread per_thread_;
     PerThread* pt = &per_thread_;
     return pt;
 #endif
   }
 
   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 ThreadPoolTempl<StlThreadEnvironment> ThreadPool;
 
 }  // namespace Eigen
 
 #endif  // EIGEN_CXX11_THREADPOOL_NONBLOCKING_THREAD_POOL_H

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