1// Copyright (c) 2012 The Chromium Authors. All rights reserved.
2// Use of this source code is governed by a BSD-style license that can be
3// found in the LICENSE file.
4//
5// The thread pool used in the POSIX implementation of WorkerPool dynamically
6// adds threads as necessary to handle all tasks.  It keeps old threads around
7// for a period of time to allow them to be reused.  After this waiting period,
8// the threads exit.  This thread pool uses non-joinable threads, therefore
9// worker threads are not joined during process shutdown.  This means that
10// potentially long running tasks (such as DNS lookup) do not block process
11// shutdown, but also means that process shutdown may "leak" objects.  Note that
12// although PosixDynamicThreadPool spawns the worker threads and manages the
13// task queue, it does not own the worker threads.  The worker threads ask the
14// PosixDynamicThreadPool for work and eventually clean themselves up.  The
15// worker threads all maintain scoped_refptrs to the PosixDynamicThreadPool
16// instance, which prevents PosixDynamicThreadPool from disappearing before all
17// worker threads exit.  The owner of PosixDynamicThreadPool should likewise
18// maintain a scoped_refptr to the PosixDynamicThreadPool instance.
19//
20// NOTE: The classes defined in this file are only meant for use by the POSIX
21// implementation of WorkerPool.  No one else should be using these classes.
22// These symbols are exported in a header purely for testing purposes.
23
24#ifndef BASE_THREADING_WORKER_POOL_POSIX_H_
25#define BASE_THREADING_WORKER_POOL_POSIX_H_
26
27#include <queue>
28#include <string>
29
30#include "base/basictypes.h"
31#include "base/callback_forward.h"
32#include "base/location.h"
33#include "base/memory/ref_counted.h"
34#include "base/memory/scoped_ptr.h"
35#include "base/pending_task.h"
36#include "base/synchronization/condition_variable.h"
37#include "base/synchronization/lock.h"
38#include "base/threading/platform_thread.h"
39#include "base/tracked_objects.h"
40
41class Task;
42
43namespace base {
44
45class BASE_EXPORT PosixDynamicThreadPool
46    : public RefCountedThreadSafe<PosixDynamicThreadPool> {
47 public:
48  class PosixDynamicThreadPoolPeer;
49
50  // All worker threads will share the same |name_prefix|.  They will exit after
51  // |idle_seconds_before_exit|.
52  PosixDynamicThreadPool(const std::string& name_prefix,
53                         int idle_seconds_before_exit);
54
55  // Indicates that the thread pool is going away.  Stops handing out tasks to
56  // worker threads.  Wakes up all the idle threads to let them exit.
57  void Terminate();
58
59  // Adds |task| to the thread pool.
60  void PostTask(const tracked_objects::Location& from_here,
61                const Closure& task);
62
63  // Worker thread method to wait for up to |idle_seconds_before_exit| for more
64  // work from the thread pool.  Returns NULL if no work is available.
65  PendingTask WaitForTask();
66
67 private:
68  friend class RefCountedThreadSafe<PosixDynamicThreadPool>;
69  friend class PosixDynamicThreadPoolPeer;
70
71  ~PosixDynamicThreadPool();
72
73  // Adds pending_task to the thread pool.  This function will clear
74  // |pending_task->task|.
75  void AddTask(PendingTask* pending_task);
76
77  const std::string name_prefix_;
78  const int idle_seconds_before_exit_;
79
80  Lock lock_;  // Protects all the variables below.
81
82  // Signal()s worker threads to let them know more tasks are available.
83  // Also used for Broadcast()'ing to worker threads to let them know the pool
84  // is being deleted and they can exit.
85  ConditionVariable pending_tasks_available_cv_;
86  int num_idle_threads_;
87  TaskQueue pending_tasks_;
88  bool terminated_;
89  // Only used for tests to ensure correct thread ordering.  It will always be
90  // NULL in non-test code.
91  scoped_ptr<ConditionVariable> num_idle_threads_cv_;
92
93  DISALLOW_COPY_AND_ASSIGN(PosixDynamicThreadPool);
94};
95
96}  // namespace base
97
98#endif  // BASE_THREADING_WORKER_POOL_POSIX_H_
99