fixed_thread_pool.h

/* Copyright 2018 Google Inc. All Rights Reserved.
 
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.
==============================================================================*/
 
#ifndef TENSORFLOW_SERVING_UTIL_NET_HTTP_INTERNAL_FIXED_THREAD_POOL_H_
#define TENSORFLOW_SERVING_UTIL_NET_HTTP_INTERNAL_FIXED_THREAD_POOL_H_
 
#include <cassert>
#include <functional>
#include <queue>
#include <thread>  // NOLINT(build/c++11)
#include <vector>
 
#include "absl/base/thread_annotations.h"
#include "absl/synchronization/mutex.h"
 
namespace tensorflow {
namespace serving {
namespace net_http {
 
// A simple fixed-size ThreadPool implementation for tests.
// The initial version is copied from
// absl/synchronization/internal/thread_pool.h
class FixedThreadPool {
 public:
  explicit FixedThreadPool(int num_threads) {
    for (int i = 0; i < num_threads; ++i) {
      threads_.push_back(std::thread(&FixedThreadPool::WorkLoop, this));
    }
  }
 
  FixedThreadPool(const FixedThreadPool &) = delete;
  FixedThreadPool &operator=(const FixedThreadPool &) = delete;
 
  ~FixedThreadPool() {
    {
      absl::MutexLock l(&mu_);
      for (int i = 0; i < threads_.size(); ++i) {
        queue_.push(nullptr);  // Shutdown signal.
      }
    }
    for (auto &t : threads_) {
      t.join();
    }
  }
 
  // Schedule a function to be run on a ThreadPool thread immediately.
  void Schedule(std::function<void()> func) {
    assert(func != nullptr);
    absl::MutexLock l(&mu_);
    queue_.push(std::move(func));
  }
 
 private:
  bool WorkAvailable() const ABSL_EXCLUSIVE_LOCKS_REQUIRED(mu_) {
    return !queue_.empty();
  }
 
  void WorkLoop() {
    while (true) {
      std::function<void()> func;
      {
        absl::MutexLock l(&mu_);
        mu_.Await(absl::Condition(this, &FixedThreadPool::WorkAvailable));
        func = std::move(queue_.front());
        queue_.pop();
      }
      if (func == nullptr) {  // Shutdown signal.
        break;
      }
      func();
    }
  }
 
  absl::Mutex mu_;
  std::queue<std::function<void()>> queue_ ABSL_GUARDED_BY(mu_);
  std::vector<std::thread> threads_;
};
 
}  // namespace net_http
}  // namespace serving
}  // namespace tensorflow
 
#endif  // TENSORFLOW_SERVING_UTIL_NET_HTTP_INTERNAL_FIXED_THREAD_POOL_H_