Revive voronoi multi-threaded demo.

native_client_sdk/src/examples/demo/voronoi/threadpool.cc

+// Copyright (c) 2013 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#if !defined(DISABLE_THREADS)
+
+#include "threadpool.h"
+
+#include <pthread.h>
+#include <semaphore.h>
+#include <stdio.h>
+#include <stdlib.h>
+
+// TODO(nfullagar): Switch DecCounter to use atomic decrement.
+// TODO(nfullagar): Large sets of fine grained tasks may benefit from
+// pthread_condvar_* over sem_*
+
+// Initializes mutex, semaphores and a pool of threads.
+ThreadPool::ThreadPool(const int num_threads)

[noelallen1, 2013/05/25 00:21:11]

const?  Wasn't this non-const in your header?   Maybe assign num_threads to
num_threads_ in the const int constructor?

[nfullagar1, 2013/05/28 23:11:50]

Done.

+    : threads_(NULL), counter_(0), num_threads_(num_threads), exiting_(false),
+      user_data_(NULL), user_work_function_(NULL) {
+  if (num_threads_ > 1) {

[noelallen1, 2013/05/25 00:21:11]

Shouldn't a thread pool of 1 work?

[nfullagar1, 2013/05/28 23:11:50]

changed so num_threads_ of 0 does all tasks on dispatch thread.

+    int status;
+    status = pthread_mutex_init(&mutex_, NULL);
+    if (0 != status) {
+      fprintf(stderr, "Failed to initialize mutex!\n");
+      exit(-1);
+    }
+    status = sem_init(&work_tasks_, 0, 0);
+    if (-1 == status) {
+      fprintf(stderr, "Failed to initialize semaphore!\n");
+      exit(-1);
+    }
+    status = sem_init(&done_tasks_, 0, 0);
+    if (-1 == status) {
+      fprintf(stderr, "Failed to initialize semaphore!\n");
+      exit(-1);
+    }
+    threads_ = new pthread_t[num_threads_];
+    for (int i = 0; i < num_threads_; i++) {
+      status = pthread_create(&threads_[i], NULL, WorkerThreadEntry, this);
+      if (0 != status) {
+        fprintf(stderr, "Failed to create thread!\n");
+        exit(-1);
+      }
+    }
+  }
+}
+
+// Post exit request, wait for all threads to join, and cleanup.
+ThreadPool::~ThreadPool() {
+  if (num_threads_ > 1) {
+    PostExitAndJoinAll();
+    delete[] threads_;
+    sem_destroy(&done_tasks_);
+    sem_destroy(&work_tasks_);
+    pthread_mutex_destroy(&mutex_);
+  }
+}
+
+// Setup work parameters.  This function is called from the dispatch thread,
+// when all worker threads are sleeping.
+void ThreadPool::Setup(int counter, WorkFunction work, void *data) {
+  counter_ = counter;
+  user_work_function_ = work;
+  user_data_ = data;
+}
+
+// Decrement and get the value of the mutex protected counter.  This function
+// can be called from multiple threads at any given time.
+int ThreadPool::DecCounter() {
+  int v;
+  pthread_mutex_lock(&mutex_);
+  {
+    v = --counter_;
+  }
+  pthread_mutex_unlock(&mutex_);
+  return v;
+}
+
+// Set exit flag, post and join all the threads in the pool.  This function is
+// called only from the dispatch thread, and only when all worker threads are
+// sleeping.
+void ThreadPool::PostExitAndJoinAll() {
+  exiting_ = true;
+  // Wake up all the sleeping worker threads.
+  for (int i = 0; i < num_threads_; ++i)
+    sem_post(&work_tasks_);
+  void *retval;
+  for (int i = 0; i < num_threads_; ++i)
+    pthread_join(threads_[i], &retval);
+}
+
+// Main work loop - one for each worker thread.
+void ThreadPool::WorkLoop() {
+  while (true) {
+    // Wait for work. If no work is availble, this thread will sleep.
+    sem_wait(&work_tasks_);
+    // Workers wake up from PostWork() issued from the dispatch thread.
+    if (exiting_) break;
+    // Grab a task index to work on from the counter.
+    int task_index = DecCounter();
+    if (task_index < 0) {
+      // This indicates we're not sync'ing properly.
+      fprintf(stderr, "Task index went negative!\n");
+      exit(-1);
+    }
+    user_work_function_(task_index, user_data_);
+    // Post to dispatch thread that a task completed.
+    sem_post(&done_tasks_);
+  }
+}
+
+// pthread entry point for a worker thread.
+void* ThreadPool::WorkerThreadEntry(void *thiz) {
+  static_cast<ThreadPool *>(thiz)->WorkLoop();
+  return NULL;
+}
+
+// MultiThread() will dispatch a set of tasks across multiple worker threads.
+// Note: This function will block until all work has completed.
+void ThreadPool::MultiThread(int num_tasks, WorkFunction work, void *data) {
+  // On entry, all worker threads are sleeping.
+  Setup(num_tasks, work, data);
+
+  // Wake up the worker threads & have them process the tasks.
+  for (int i = 0; i < num_tasks; i++)
+    sem_post(&work_tasks_);
+
+  // Worker threads are now awake and busy.
+
+  // This dispatch thread will now sleep-wait on the done_tasks_ semaphore,
+  // waiting for the worker threads to finish all tasks.
+  for (int i = 0; i < num_tasks; i++)
+    sem_wait(&done_tasks_);
+
+  // Make sure the counter is where we expect.
+  int c = DecCounter();
+  if (-1 != c) {
+    fprintf(stderr, "We're not syncing correctly! (%d)\n", c);
+    exit(-1);
+  }
+  // On exit, all tasks are done and all worker threads are sleeping again.
+}
+
+// SingleThread() will dispatch all work on this thread.

[noelallen1, 2013/05/25 00:21:11]

You already check for >1 elsewhere, why not make this one function?

[nfullagar1, 2013/05/28 23:11:50]

see other changes

+void ThreadPool::SingleThread(int num_tasks, WorkFunction work, void *data) {
+  for (int i = 0; i < num_tasks; i++)

[noelallen1, 2013/05/25 00:21:11]

Use of "single thread" is confusing.   In the case that ThreadPool is
constructed with only a single thread, that thread would never be used.  So
either you create 2-N threads which will do all the work outside of your main
thread, or you create no threads and must call SingleThread.  There is no case
for create 1 thread to do the work off your main thread.

[nfullagar1, 2013/05/28 23:11:50]

changed so 0 = dispatch thread, 1 = one thread off dispatch thread 

+    work(i, data);
+}
+
+// Dispatch() will invoke the user supplied work function across
+// one or more threads for each task.
+// Note: This function will block until all work has completed.
+void ThreadPool::Dispatch(int num_tasks, WorkFunction work, void *data) {
+  if (num_threads_ > 1)
+    MultiThread(num_tasks, work, data);
+  else
+    SingleThread(num_tasks, work, data);
+}
+#endif