gthreadpool.c

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/* GLIB - Library of useful routines for C programming
 * Copyright (C) 1995-1997  Peter Mattis, Spencer Kimball and Josh MacDonald
 *
 * GAsyncQueue: thread pool implementation.
 * Copyright (C) 2000 Sebastian Wilhelmi; University of Karlsruhe
 *
 * This library is free software; you can redistribute it and/or
 * modify it under the terms of the GNU Lesser General Public
 * License as published by the Free Software Foundation; either
 * version 2 of the License, or (at your option) any later version.
 *
 * This library is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
 * Lesser General Public License for more details.
 *
 * You should have received a copy of the GNU Lesser General Public
 * License along with this library; if not, write to the
 * Free Software Foundation, Inc., 59 Temple Place - Suite 330,
 * Boston, MA 02111-1307, USA.
 */

/*
 * MT safe
 */

#include "glib.h"

typedef struct _GRealThreadPool GRealThreadPool;

struct _GRealThreadPool
{
  GThreadPool pool;
  GAsyncQueue* queue;
  gint max_threads;
  gint num_threads;
  gboolean running;
  gboolean immediate;
  gboolean waiting;
};

/* The following is just an address to mark the stop order for a
 * thread, it could be any address (as long, as it isn't a valid
 * GThreadPool address) */
static const gpointer stop_this_thread_marker = (gpointer) &g_thread_pool_new;

/* Here all unused threads are waiting  */
static GAsyncQueue *unused_thread_queue;
static gint unused_threads = 0;
static gint max_unused_threads = 0;
G_LOCK_DEFINE_STATIC (unused_threads);

static GMutex *inform_mutex = NULL;
static GCond *inform_cond = NULL;

static void     g_thread_pool_free_internal (GRealThreadPool* pool);
static gpointer g_thread_pool_thread_proxy (gpointer data);
static void     g_thread_pool_start_thread (GRealThreadPool* pool, 
					    GError **error);
static void     g_thread_pool_wakeup_and_stop_all (GRealThreadPool* pool);

#define g_thread_should_run(pool, len) \
  ((pool)->running || (!(pool)->immediate && (len) > 0))

static gpointer 
g_thread_pool_thread_proxy (gpointer data)
{
  GRealThreadPool *pool = data;
  gboolean watcher = FALSE;

  g_async_queue_lock (pool->queue);
  while (TRUE)
    {
      gpointer task; 
      gboolean goto_global_pool = !pool->pool.exclusive;
      gint len = g_async_queue_length_unlocked (pool->queue);
      
      if (g_thread_should_run (pool, len))
	{
	  if (watcher)
	    {
	      /* This thread is actually not needed here, but it waits
               * for some time anyway. If during that time a new
               * request arrives, this saves process
               * swicthes. Otherwise the thread will go to the global
               * pool afterwards */
	      GTimeVal end_time;
	      g_get_current_time (&end_time);
	      g_time_val_add (&end_time, G_USEC_PER_SEC / 2); /* 1/2 second */
	      task = g_async_queue_timed_pop_unlocked (pool->queue, &end_time);
	    }
	  else
	    task = g_async_queue_pop_unlocked (pool->queue);
	  
	  if (task)
	    {
	      watcher = FALSE;
	      if (pool->num_threads > pool->max_threads && 
		  pool->max_threads != -1)
		/* We are in fact a superfluous threads, so we go to
		 * the global pool and just hand the data further to
		 * the next one waiting in the queue */
		{
		  g_async_queue_push_unlocked (pool->queue, task);
		  goto_global_pool = TRUE;
		}
	      else if (pool->running || !pool->immediate)
		{
		  g_async_queue_unlock (pool->queue);
		  pool->pool.func (task, pool->pool.user_data);
		  g_async_queue_lock (pool->queue);
		}
	    }
	  len = g_async_queue_length_unlocked (pool->queue);
	}

      if (!g_thread_should_run (pool, len))
	{
	  g_cond_broadcast (inform_cond);
	  goto_global_pool = TRUE;
	}
      else if (len > 0)
	{
	  /* At this pool there are no threads waiting, but tasks are. */
	  goto_global_pool = FALSE; 
	}
      else if (len == 0 && !watcher && !pool->pool.exclusive)
	{
	  /* Here neither threads nor tasks are queued and we didn't
	   * just return from a timed wait. We now wait for a limited
	   * time at this pool for new tasks to avoid costly context
	   * switches. */
	  goto_global_pool = FALSE;
	  watcher = TRUE;
	}

      if (goto_global_pool)
	{
	  pool->num_threads--;

	  if (!pool->running && !pool->waiting)
	    {
	      if (pool->num_threads == 0)
		{
		  g_async_queue_unlock (pool->queue);
		  g_thread_pool_free_internal (pool);
		}		
	      else 
		{
		  if (len == - pool->num_threads)
		    g_thread_pool_wakeup_and_stop_all (pool);

		  g_async_queue_unlock (pool->queue);
		}
	    }
	  else
	    g_async_queue_unlock (pool->queue);
	  
	  g_async_queue_lock (unused_thread_queue);

	  G_LOCK (unused_threads);
	  if ((unused_threads >= max_unused_threads && 
	       max_unused_threads != -1))
	    {
	      G_UNLOCK (unused_threads);
	      g_async_queue_unlock (unused_thread_queue);
	      /* Stop this thread */
	      return NULL;      
	    }
	  unused_threads++;
	  G_UNLOCK (unused_threads);

	  pool = g_async_queue_pop_unlocked (unused_thread_queue);

	  G_LOCK (unused_threads);
	  unused_threads--;
	  G_UNLOCK (unused_threads);

	  g_async_queue_unlock (unused_thread_queue);
	  
	  if (pool == stop_this_thread_marker)
	    /* Stop this thread */
	    return NULL;
	  
	  g_async_queue_lock (pool->queue);

	  /* pool->num_threads++ is not done here, but in
           * g_thread_pool_start_thread to make the new started thread
           * known to the pool, before itself can do it. */
	}
    }
  return NULL;
}

static void
g_thread_pool_start_thread (GRealThreadPool  *pool, 
			    GError          **error)
{
  gboolean success = FALSE;
  
  if (pool->num_threads >= pool->max_threads && pool->max_threads != -1)
    /* Enough threads are already running */
    return;

  g_async_queue_lock (unused_thread_queue);

  if (g_async_queue_length_unlocked (unused_thread_queue) < 0)
    {
      g_async_queue_push_unlocked (unused_thread_queue, pool);
      success = TRUE;
    }

  g_async_queue_unlock (unused_thread_queue);

  if (!success)
    {
      GError *local_error = NULL;
      /* No thread was found, we have to start a new one */
      g_thread_create (g_thread_pool_thread_proxy, pool, FALSE, &local_error);
      
      if (local_error)
	{
	  g_propagate_error (error, local_error);
	  return;
	}
    }

  /* See comment in g_thread_pool_thread_proxy as to why this is done
   * here and not there */
  pool->num_threads++;
}

/**
 * g_thread_pool_new: 
 * @func: a function to execute in the threads of the new thread pool
 * @user_data: user data that is handed over to @func every time it 
 *   is called
 * @max_threads: the maximal number of threads to execute concurrently in 
 *   the new thread pool, -1 means no limit
 * @exclusive: should this thread pool be exclusive?
 * @error: return location for error
 *
 * This function creates a new thread pool.
 *
 * Whenever you call g_thread_pool_push(), either a new thread is
 * created or an unused one is reused. At most @max_threads threads
 * are running concurrently for this thread pool. @max_threads = -1
 * allows unlimited threads to be created for this thread pool. The
 * newly created or reused thread now executes the function @func with
 * the two arguments. The first one is the parameter to
 * g_thread_pool_push() and the second one is @user_data.
 *
 * The parameter @exclusive determines, whether the thread pool owns
 * all threads exclusive or whether the threads are shared
 * globally. If @exclusive is %TRUE, @max_threads threads are started
 * immediately and they will run exclusively for this thread pool until
 * it is destroyed by g_thread_pool_free(). If @exclusive is %FALSE,
 * threads are created, when needed and shared between all
 * non-exclusive thread pools. This implies that @max_threads may not
 * be -1 for exclusive thread pools.
 *
 * @error can be %NULL to ignore errors, or non-%NULL to report
 * errors. An error can only occur when @exclusive is set to %TRUE and
 * not all @max_threads threads could be created.
 *
 * Return value: the new #GThreadPool
 **/
GThreadPool* 
g_thread_pool_new (GFunc            func,
		   gpointer         user_data,
		   gint             max_threads,
		   gboolean         exclusive,
		   GError         **error)
{
  GRealThreadPool *retval;
  G_LOCK_DEFINE_STATIC (init);

  g_return_val_if_fail (func, NULL);
  g_return_val_if_fail (!exclusive || max_threads != -1, NULL);
  g_return_val_if_fail (max_threads >= -1, NULL);
  g_return_val_if_fail (g_thread_supported (), NULL);

  retval = g_new (GRealThreadPool, 1);

  retval->pool.func = func;
  retval->pool.user_data = user_data;
  retval->pool.exclusive = exclusive;
  retval->queue = g_async_queue_new ();
  retval->max_threads = max_threads;
  retval->num_threads = 0;
  retval->running = TRUE;

  G_LOCK (init);
  
  if (!inform_mutex)
    {
      inform_mutex = g_mutex_new ();
      inform_cond = g_cond_new ();
      unused_thread_queue = g_async_queue_new ();
    }

  G_UNLOCK (init);

  if (retval->pool.exclusive)
    {
      g_async_queue_lock (retval->queue);
  
      while (retval->num_threads < retval->max_threads)
	{
	  GError *local_error = NULL;
	  g_thread_pool_start_thread (retval, &local_error);
	  if (local_error)
	    {
	      g_propagate_error (error, local_error);
	      break;
	    }
	}

      g_async_queue_unlock (retval->queue);
    }

  return (GThreadPool*) retval;
}

/**
 * g_thread_pool_push:
 * @pool: a #GThreadPool
 * @data: a new task for @pool
 * @error: return location for error
 * 
 * Inserts @data into the list of tasks to be executed by @pool. When
 * the number of currently running threads is lower than the maximal
 * allowed number of threads, a new thread is started (or reused) with
 * the properties given to g_thread_pool_new (). Otherwise @data stays
 * in the queue until a thread in this pool finishes its previous task
 * and processes @data. 
 *
 * @error can be %NULL to ignore errors, or non-%NULL to report
 * errors. An error can only occur when a new thread couldn't be
 * created. In that case @data is simply appended to the queue of work