runtime/thread_pool.cc

/*
 * Copyright (C) 2012 The Android Open Source Project
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *      http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

#include "thread_pool.h"

#include "base/casts.h"
#include "base/stl_util.h"
#include "runtime.h"
#include "thread-inl.h"

namespace art {

static constexpr bool kMeasureWaitTime = false;

ThreadPoolWorker::ThreadPoolWorker(ThreadPool* thread_pool, const std::string& name,
                                   size_t stack_size)
    : thread_pool_(thread_pool),
      name_(name) {
  std::string error_msg;
  stack_.reset(MemMap::MapAnonymous(name.c_str(), nullptr, stack_size, PROT_READ | PROT_WRITE,
                                    false, &error_msg));
  CHECK(stack_.get() != nullptr) << error_msg;
  const char* reason = "new thread pool worker thread";
  pthread_attr_t attr;
  CHECK_PTHREAD_CALL(pthread_attr_init, (&attr), reason);
  CHECK_PTHREAD_CALL(pthread_attr_setstack, (&attr, stack_->Begin(), stack_->Size()), reason);
  CHECK_PTHREAD_CALL(pthread_create, (&pthread_, &attr, &Callback, this), reason);
  CHECK_PTHREAD_CALL(pthread_attr_destroy, (&attr), reason);
}

ThreadPoolWorker::~ThreadPoolWorker() {
  CHECK_PTHREAD_CALL(pthread_join, (pthread_, nullptr), "thread pool worker shutdown");
}

void ThreadPoolWorker::Run() {
  Thread* self = Thread::Current();
  Task* task = nullptr;
  thread_pool_->creation_barier_.Wait(self);
  while ((task = thread_pool_->GetTask(self)) != nullptr) {
    task->Run(self);
    task->Finalize();
  }
}

void* ThreadPoolWorker::Callback(void* arg) {
  ThreadPoolWorker* worker = reinterpret_cast<ThreadPoolWorker*>(arg);
  Runtime* runtime = Runtime::Current();
  CHECK(runtime->AttachCurrentThread(worker->name_.c_str(), true, nullptr, false));
  // Do work until its time to shut down.
  worker->Run();
  runtime->DetachCurrentThread();
  return nullptr;
}

void ThreadPool::AddTask(Thread* self, Task* task) {
  MutexLock mu(self, task_queue_lock_);
  tasks_.push_back(task);
  // If we have any waiters, signal one.
  if (started_ && waiting_count_ != 0) {
    task_queue_condition_.Signal(self);
  }
}

ThreadPool::ThreadPool(const char* name, size_t num_threads)
  : name_(name),
    task_queue_lock_("task queue lock"),
    task_queue_condition_("task queue condition", task_queue_lock_),
    completion_condition_("task completion condition", task_queue_lock_),
    started_(false),
    shutting_down_(false),
    waiting_count_(0),
    start_time_(0),
    total_wait_time_(0),
    // Add one since the caller of constructor waits on the barrier too.
    creation_barier_(num_threads + 1),
    max_active_workers_(num_threads) {
  Thread* self = Thread::Current();
  while (GetThreadCount() < num_threads) {
    const std::string name = StringPrintf("%s worker thread %zu", name_.c_str(), GetThreadCount());
    threads_.push_back(new ThreadPoolWorker(this, name, ThreadPoolWorker::kDefaultStackSize));
  }
  // Wait for all of the threads to attach.
  creation_barier_.Wait(self);
}

void ThreadPool::SetMaxActiveWorkers(size_t threads) {
  MutexLock mu(Thread::Current(), task_queue_lock_);
  CHECK_LE(threads, GetThreadCount());
  max_active_workers_ = threads;
}

ThreadPool::~ThreadPool() {
  {
    Thread* self = Thread::Current();
    MutexLock mu(self, task_queue_lock_);
    // Tell any remaining workers to shut down.
    shutting_down_ = true;
    // Broadcast to everyone waiting.
    task_queue_condition_.Broadcast(self);
    completion_condition_.Broadcast(self);
  }
  // Wait for the threads to finish.
  STLDeleteElements(&threads_);
}

void ThreadPool::StartWorkers(Thread* self) {
  MutexLock mu(self, task_queue_lock_);
  started_ = true;
  task_queue_condition_.Broadcast(self);
  start_time_ = NanoTime();
  total_wait_time_ = 0;
}

void ThreadPool::StopWorkers(Thread* self) {
  MutexLock mu(self, task_queue_lock_);
  started_ = false;
}

Task* ThreadPool::GetTask(Thread* self) {
  MutexLock mu(self, task_queue_lock_);
  while (!IsShuttingDown()) {
    const size_t thread_count = GetThreadCount();
    // Ensure that we don't use more threads than the maximum active workers.
    const size_t active_threads = thread_count - waiting_count_;
    // <= since self is considered an active worker.
    if (active_threads <= max_active_workers_) {
      Task* task = TryGetTaskLocked();
      if (task != nullptr) {
        return task;
      }
    }

    ++waiting_count_;
    if (waiting_count_ == GetThreadCount() && tasks_.empty()) {
      // We may be done, lets broadcast to the completion condition.
      completion_condition_.Broadcast(self);
    }
    const uint64_t wait_start = kMeasureWaitTime ? NanoTime() : 0;
    task_queue_condition_.Wait(self);
    if (kMeasureWaitTime) {
      const uint64_t wait_end = NanoTime();
      total_wait_time_ += wait_end - std::max(wait_start, start_time_);
    }
    --waiting_count_;
  }

  // We are shutting down, return nullptr to tell the worker thread to stop looping.
  return nullptr;
}

Task* ThreadPool::TryGetTask(Thread* self) {
  MutexLock mu(self, task_queue_lock_);
  return TryGetTaskLocked();
}

Task* ThreadPool::TryGetTaskLocked() {
  if (started_ && !tasks_.empty()) {
    Task* task = tasks_.front();
    tasks_.pop_front();
    return task;
  }
  return nullptr;
}

void ThreadPool::Wait(Thread* self, bool do_work, bool may_hold_locks) {
  if (do_work) {
    Task* task = nullptr;
    while ((task = TryGetTask(self)) != nullptr) {
      task->Run(self);
      task->Finalize();
    }
  }
  // Wait until each thread is waiting and the task list is empty.
  MutexLock mu(self, task_queue_lock_);
  while (!shutting_down_ && (waiting_count_ != GetThreadCount() || !tasks_.empty())) {
    if (!may_hold_locks) {
      completion_condition_.Wait(self);
    } else {
      completion_condition_.WaitHoldingLocks(self);
    }
  }
}

size_t ThreadPool::GetTaskCount(Thread* self) {
  MutexLock mu(self, task_queue_lock_);
  return tasks_.size();
}

WorkStealingWorker::WorkStealingWorker(ThreadPool* thread_pool, const std::string& name,
                                       size_t stack_size)
    : ThreadPoolWorker(thread_pool, name, stack_size), task_(nullptr) {}

void WorkStealingWorker::Run() {
  Thread* self = Thread::Current();
  Task* task = nullptr;
  WorkStealingThreadPool* thread_pool = down_cast<WorkStealingThreadPool*>(thread_pool_);
  while ((task = thread_pool_->GetTask(self)) != nullptr) {
    WorkStealingTask* stealing_task = down_cast<WorkStealingTask*>(task);

    {
      CHECK(task_ == nullptr);
      MutexLock mu(self, thread_pool->work_steal_lock_);
      // Register that we are running the task
      ++stealing_task->ref_count_;
      task_ = stealing_task;
    }
    stealing_task->Run(self);
    // Mark ourselves as not running a task so that nobody tries to steal from us.
    // There is a race condition that someone starts stealing from us at this point. This is okay
    // due to the reference counting.
    task_ = nullptr;

    bool finalize;

    // Steal work from tasks until there is none left to steal. Note: There is a race, but
    // all that happens when the race occurs is that we steal some work instead of processing a
    // task from the queue.
    while (thread_pool->GetTaskCount(self) == 0) {
      WorkStealingTask* steal_from_task  = nullptr;

      {
        MutexLock mu(self, thread_pool->work_steal_lock_);
        // Try finding a task to steal from.
        steal_from_task = thread_pool->FindTaskToStealFrom();
        if (steal_from_task != nullptr) {
          CHECK_NE(stealing_task, steal_from_task)
              << "Attempting to steal from completed self task";
          steal_from_task->ref_count_++;
        } else {
          break;
        }
      }

      if (steal_from_task != nullptr) {
        // Task which completed earlier is going to steal some work.
        stealing_task->StealFrom(self, steal_from_task);

        {
          // We are done stealing from the task, lets decrement its reference count.
          MutexLock mu(self, thread_pool->work_steal_lock_);
          finalize = !--steal_from_task->ref_count_;
        }

        if (finalize) {
          steal_from_task->Finalize();
        }
      }
    }

    {
      MutexLock mu(self, thread_pool->work_steal_lock_);
      // If nobody is still referencing task_ we can finalize it.
      finalize = !--stealing_task->ref_count_;
    }

    if (finalize) {
      stealing_task->Finalize();
    }
  }
}

WorkStealingWorker::~WorkStealingWorker() {}

WorkStealingThreadPool::WorkStealingThreadPool(const char* name, size_t num_threads)
    : ThreadPool(name, 0),
      work_steal_lock_("work stealing lock"),
      steal_index_(0) {
  while (GetThreadCount() < num_threads) {
    const std::string name = StringPrintf("Work stealing worker %zu", GetThreadCount());
    threads_.push_back(new WorkStealingWorker(this, name, ThreadPoolWorker::kDefaultStackSize));
  }
}

WorkStealingTask* WorkStealingThreadPool::FindTaskToStealFrom() {
  const size_t thread_count = GetThreadCount();
  for (size_t i = 0; i < thread_count; ++i) {
    // TODO: Use CAS instead of lock.
    ++steal_index_;
    if (steal_index_ >= thread_count) {
      steal_index_-= thread_count;
    }

    WorkStealingWorker* worker = down_cast<WorkStealingWorker*>(threads_[steal_index_]);
    WorkStealingTask* task = worker->task_;
    if (task) {
      // Not null, we can probably steal from this worker.
      return task;
    }
  }
  // Couldn't find something to steal.
  return nullptr;
}

WorkStealingThreadPool::~WorkStealingThreadPool() {}

}  // namespace art