gstreamer/docs/manual/advanced-threads.xml

<chapter id="chapter-threads">
  <title>Threads</title>
  <para> 
    &GStreamer; is inherently multi-threaded, and is fully thread-safe.
    Most threading internals are hidden from the application, which should
    make application development easier. However, in some cases, applications
    may want to have influence on some parts of those. &GStreamer; allows
    applications to force the use of multiple threads over some parts of
    a pipeline.
    See <xref linkend="section-threads-uses"/>.
  </para>
  <para> 
    &GStreamer; can also notify you when threads are created so that you can
    configure things such as the thread priority or the threadpool to use.
    See <xref linkend="section-threads-status"/>.
  </para>

  <sect1 id="section-threads-scheduling">
    <title>Scheduling in &GStreamer;</title>
    <para>
      Each element in the &GStreamer; pipeline decides how it is going to
      be scheduled. Elements can choose if their pads are to be scheduled
      push-based or pull-based. An element can, for example, choose to start
      a thread to start pulling from the sink pad or/and start pushing on
      the source pad. An element can also choose to use the upstream or
      downstream thread for its data processing in push and pull mode
      respectively. &GStreamer; does not pose any restrictions on how the
      element chooses to be scheduled. See the Plugin Writer Guide for more
      details.
    </para>
    <para>
      What will happen in any case is that some elements will start a thread
      for their data processing, called the <quote>streaming threads</quote>. 
      The streaming threads, or <classname>GstTask</classname> objects, are
      created from a <classname>GstTaskPool</classname> when the element
      needs to make a streaming thread. In the next section we see how we
      can receive notifications of the tasks and pools.
    </para>
  </sect1>

  <sect1 id="section-threads-status">
    <title>Configuring Threads in &GStreamer;</title>
    <para>
      A STREAM_STATUS message is posted on the bus to inform you about the
      status of the streaming threads. You will get the following information
      from the message:
      <itemizedlist>
        <listitem>
          <para>
            When a new thread is about to be created, you will be notified
            of this with a GST_STREAM_STATUS_TYPE_CREATE type. It is then
            possible to configure a <classname>GstTaskPool</classname> in
            the <classname>GstTask</classname>. The custom taskpool will
            provide custom threads for the task to implement the streaming
            threads.
          </para>
          <para>
            This message needs to be handled synchronously if you want to
            configure a custom taskpool. If you don't configure the taskpool
            on the task when this message returns, the task will use its
            default pool.
          </para>
        </listitem>
        <listitem>
          <para>
            When a thread is entered or left. This is the moment where you
            could configure thread priorities. You also get a notification
            when a thread is destroyed.
          </para>
        </listitem>
        <listitem>
          <para>
            You get messages when the thread starts, pauses and stops. This
            could be used to visualize the status of streaming threads in
            a gui application.
          </para>
        </listitem>
      </itemizedlist>
    </para>
    <para>
    </para>
    <para>
      We will now look at some examples in the next sections.
    </para>

    <sect2 id="section-threads-rt">
      <title>Boost priority of a thread</title>
      <programlisting>
        .----------.    .----------.
        | faksesrc |    | fakesink |
        |         src->sink        |
        '----------'    '----------'
      </programlisting>
      <para>
        Let's look at the simple pipeline above. We would like to boost
        the priority of the streaming thread.
        It will be the fakesrc element that starts the streaming thread for
        generating the fake data pushing them to the peer fakesink.
        The flow for changing the priority would go like this:
      </para>
      <itemizedlist>
        <listitem>
          <para>
            When going from READY to PAUSED state, fakesrc will require a
            streaming thread for pushing data into the fakesink. It will
            post a STREAM_STATUS message indicating its requirement for a
            streaming thread.
          </para>
        </listitem>
        <listitem>
          <para>
            The application will react to the STREAM_STATUS messages with a
            sync bus handler. It will then configure a custom
            <classname>GstTaskPool</classname> on the
            <classname>GstTask</classname> inside the message. The custom
            taskpool is responsible for creating the threads. In this
            example we will make a thread with a higher priority.
          </para>
        </listitem>
        <listitem>
          <para>
            Alternatively, since the sync message is called in the thread
            context, you can use thread ENTER/LEAVE notifications to
            change the priority or scheduling pollicy of the current thread.
          </para>
        </listitem>
      </itemizedlist>
      <para>
        In a first step we need to implement a custom
        <classname>GstTaskPool</classname> that we can configure on the task.
        Below is the implementation of a <classname>GstTaskPool</classname>
        subclass that uses pthreads to create a SCHED_RR real-time thread.
        Note that creating real-time threads might require extra priveleges.
      </para>
      <programlisting>
<!-- example-begin testrtpool.c a -->
<!--
#include <gst/gst.h>

#define TEST_TYPE_RT_POOL             (test_rt_pool_get_type ())
#define TEST_RT_POOL(pool)            (G_TYPE_CHECK_INSTANCE_CAST ((pool), TEST_TYPE_RT_POOL, TestRTPool))
#define TEST_IS_RT_POOL(pool)         (G_TYPE_CHECK_INSTANCE_TYPE ((pool), TEST_TYPE_RT_POOL))
#define TEST_RT_POOL_CLASS(pclass)    (G_TYPE_CHECK_CLASS_CAST ((pclass), TEST_TYPE_RT_POOL, TestRTPoolClass))
#define TEST_IS_RT_POOL_CLASS(pclass) (G_TYPE_CHECK_CLASS_TYPE ((pclass), TEST_TYPE_RT_POOL))
#define TEST_RT_POOL_GET_CLASS(pool)  (G_TYPE_INSTANCE_GET_CLASS ((pool), TEST_TYPE_RT_POOL, TestRTPoolClass))
#define TEST_RT_POOL_CAST(pool)       ((TestRTPool*)(pool))

typedef struct _TestRTPool TestRTPool;
typedef struct _TestRTPoolClass TestRTPoolClass;

struct _TestRTPool {
  GstTaskPool    object;
};

struct _TestRTPoolClass {
  GstTaskPoolClass parent_class;
};

GType           test_rt_pool_get_type    (void);

GstTaskPool *   test_rt_pool_new         (void);

-->
<!-- example-end testrtpool.c a-->
<!-- example-begin testrtpool.c b -->
<![CDATA[
#include <pthread.h>

typedef struct
{
  pthread_t thread;
} TestRTId;

G_DEFINE_TYPE (TestRTPool, test_rt_pool, GST_TYPE_TASK_POOL);

static void
default_prepare (GstTaskPool * pool, GError ** error)
{
  /* we don't do anything here. We could construct a pool of threads here that
   * we could reuse later but we don't */
}

static void
default_cleanup (GstTaskPool * pool)
{
}

static gpointer
default_push (GstTaskPool * pool, GstTaskPoolFunction func, gpointer data,
    GError ** error)
{
  TestRTId *tid;
  gint res;
  pthread_attr_t attr;
  struct sched_param param;

  tid = g_slice_new0 (TestRTId);

  pthread_attr_init (&attr);
  if ((res = pthread_attr_setschedpolicy (&attr, SCHED_RR)) != 0)
    g_warning ("setschedpolicy: failure: %p", g_strerror (res));

  param.sched_priority = 50;
  if ((res = pthread_attr_setschedparam (&attr, &param)) != 0)
    g_warning ("setschedparam: failure: %p", g_strerror (res));

  if ((res = pthread_attr_setinheritsched (&attr, PTHREAD_EXPLICIT_SCHED)) != 0)
    g_warning ("setinheritsched: failure: %p", g_strerror (res));

  res = pthread_create (&tid->thread, &attr, (void *(*)(void *)) func, data);

  if (res != 0) {
    g_set_error (error, G_THREAD_ERROR, G_THREAD_ERROR_AGAIN,
        "Error creating thread: %s", g_strerror (res));
    g_slice_free (TestRTId, tid);
    tid = NULL;
  }

  return tid;
}

static void
default_join (GstTaskPool * pool, gpointer id)
{
  TestRTId *tid = (TestRTId *) id;

  pthread_join (tid->thread, NULL);

  g_slice_free (TestRTId, tid);
}

static void
test_rt_pool_class_init (TestRTPoolClass * klass)
{
  GstTaskPoolClass *gsttaskpool_class;

  gsttaskpool_class = (GstTaskPoolClass *) klass;

  gsttaskpool_class->prepare = default_prepare;
  gsttaskpool_class->cleanup = default_cleanup;
  gsttaskpool_class->push = default_push;
  gsttaskpool_class->join = default_join;
}

static void
test_rt_pool_init (TestRTPool * pool)
{
}

GstTaskPool *
test_rt_pool_new (void)
{
  GstTaskPool *pool;

  pool = g_object_new (TEST_TYPE_RT_POOL, NULL);

  return pool;
}
]]>
<!-- example-end testrtpool.c b -->
      </programlisting>
      <para>
        The important function to implement when writing an taskpool is the
        <quote>push</quote> function. The implementation should start a thread
        that calls the given function. More involved implementations might
        want to keep some threads around in a pool because creating and
        destroying threads is not always the fastest operation.
      </para>
      <para>
        In a next step we need to actually configure the custom taskpool when
        the fakesrc needs it. For this we intercept the STREAM_STATUS messages
        with a sync handler.
      </para>
      <programlisting>
<!-- example-begin testrtpool.c c -->
<![CDATA[
static GMainLoop* loop;

static void
on_stream_status (GstBus     *bus,
                  GstMessage *message,
                  gpointer    user_data)
{
  GstStreamStatusType type;
  GstElement *owner;
  const GValue *val;
  GstTask *task = NULL;

  gst_message_parse_stream_status (message, &type, &owner);

  val = gst_message_get_stream_status_object (message);

  /* see if we know how to deal with this object */
  if (G_VALUE_TYPE (val) == GST_TYPE_TASK) {
    task = g_value_get_object (val);
  }

  switch (type) {
    case GST_STREAM_STATUS_TYPE_CREATE:
      if (task) {
        GstTaskPool *pool;

        pool = test_rt_pool_new();

        gst_task_set_pool (task, pool);
      }
      break;
    default:
      break;
  }
}

static void
on_error (GstBus     *bus,
          GstMessage *message,
          gpointer    user_data)
{
  g_message ("received ERROR");
  g_main_loop_quit (loop);
}

static void
on_eos (GstBus     *bus,
        GstMessage *message,
        gpointer    user_data)
{
  g_main_loop_quit (loop);
}

int
main (int argc, char *argv[])
{
  GstElement *bin, *fakesrc, *fakesink;
  GstBus *bus;
  GstStateChangeReturn ret;
  
  gst_init (&argc, &argv);

  /* create a new bin to hold the elements */
  bin = gst_pipeline_new ("pipeline");
  g_assert (bin);

  /* create a source */
  fakesrc = gst_element_factory_make ("fakesrc", "fakesrc");
  g_assert (fakesrc);
  g_object_set (fakesrc, "num-buffers", 50, NULL);

  /* and a sink */
  fakesink = gst_element_factory_make ("fakesink", "fakesink");
  g_assert (fakesink);

  /* add objects to the main pipeline */
  gst_bin_add_many (GST_BIN (bin), fakesrc, fakesink, NULL);

  /* link the elements */
  gst_element_link (fakesrc, fakesink);

  loop = g_main_loop_new (NULL, FALSE);

  /* get the bus, we need to install a sync handler */
  bus = gst_pipeline_get_bus (GST_PIPELINE (bin));
  gst_bus_enable_sync_message_emission (bus);
  gst_bus_add_signal_watch (bus);

  g_signal_connect (bus, "sync-message::stream-status",
      (GCallback) on_stream_status, NULL);
  g_signal_connect (bus, "message::error",
      (GCallback) on_error, NULL);
  g_signal_connect (bus, "message::eos",
      (GCallback) on_eos, NULL);

  /* start playing */
  ret = gst_element_set_state (bin, GST_STATE_PLAYING);
  if (ret != GST_STATE_CHANGE_SUCCESS) {
    g_message ("failed to change state");
    return -1;
  }

  /* Run event loop listening for bus messages until EOS or ERROR */
  g_main_loop_run (loop);

  /* stop the bin */
  gst_element_set_state (bin, GST_STATE_NULL);
  gst_object_unref (bus);
  g_main_loop_unref (loop);

  return 0;
}
]]>
<!-- example-end testrtpool.c c -->
      </programlisting>
      <para>
        Note that this program likely needs root permissions in order to
        create real-time threads. When the thread can't be created, the
        state change function will fail, which we catch in the application
        above.
      </para>
      <para>
        When there are multiple threads in the pipeline, you will receive
        multiple STREAM_STATUS messages. You should use the owner of the
        message, which is likely the pad or the element that starts the
        thread, to figure out what the function of this thread is in the
        context of the application.
      </para>
    </sect2>
  </sect1>

  <sect1 id="section-threads-uses">
    <title>When would you want to force a thread?</title>
    <para>
      We have seen that threads are created by elements but it is also
      possible to insert elements in the pipeline for the sole purpose of
      forcing a new thread in the pipeline.
    </para>
    <para>
      There are several reasons to force the use of threads. However,
      for performance reasons, you never want to use one thread for every
      element out there, since that will create some overhead.
      Let's now list some situations where threads can be particularly
      useful:
    </para>
    <itemizedlist>
      <listitem>
        <para>
          Data buffering, for example when dealing with network streams or
          when recording data from a live stream such as a video or audio
          card. Short hickups elsewhere in the pipeline will not cause data
          loss. See also <xref linkend="section-buffering-stream"/> about network
          buffering with queue2.
        </para>
	<figure float="1" id="section-thread-buffering-img">
	  <title>Data buffering, from a networked source</title>
	  <mediaobject>
            <imageobject>
              <imagedata scale="75" fileref="images/thread-buffering.&image;" format="&IMAGE;"/>
            </imageobject>
	  </mediaobject>
	</figure>

      </listitem>
      <listitem>
        <para>
          Synchronizing output devices, e.g. when playing a stream containing
          both video and audio data. By using threads for both outputs, they
          will run independently and their synchronization will be better.
        </para>
	<figure float="1" id="section-thread-synchronizing-img">
	  <title>Synchronizing audio and video sinks</title>
	  <mediaobject>
            <imageobject>
              <imagedata scale="75" fileref="images/thread-synchronizing.&image;" format="&IMAGE;"/>
            </imageobject>
	  </mediaobject>
	</figure>
      </listitem>
    </itemizedlist>


    <para>
      Above, we've mentioned the <quote>queue</quote> element several times
      now. A queue is the thread boundary element through which you can
      force the use of threads. It does so by using a classic
      provider/consumer model as learned in threading classes at
      universities all around the world. By doing this, it acts both as a
      means to make data throughput between threads threadsafe, and it can
      also act as a buffer. Queues have several <classname>GObject</classname>
      properties to be configured for specific uses. For example, you can set
      lower and upper thresholds for the element. If there's less data than
      the lower threshold (default: disabled), it will block output. If
      there's more data than the upper threshold, it will block input or
      (if configured to do so) drop data.
    </para>
    <para>
      To use a queue (and therefore force the use of two distinct threads
      in the pipeline), one can simply create a <quote>queue</quote> element
      and put this in as part of the pipeline. &GStreamer; will take care of
      all threading details internally.
    </para>
  </sect1>

</chapter>