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.
  </para>

  <sect1 id="section-threads-uses">
    <title>When would you want to force a thread?</title>
    <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 <xref linkend="section-queues-img"/> for a visualization
          of this idea.
        </para>
      </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>
      </listitem>
    </itemizedlist>
    <figure float="1" id="section-queues-img">
      <title>a two-threaded decoder with a queue</title>
      <mediaobject>
        <imageobject>
          <imagedata fileref="images/queue.&image;" format="&IMAGE;"/>
        </imageobject>
      </mediaobject>
    </figure>
    <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/receiver 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 tresholds for the element. If there's less data than
      the lower treshold (default: disabled), it will block output. If
      there's more data than the upper treshold, it will block input or
      (if configured to do so) drop data.
    </para>
    <para>
      To use a queues (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>

  <sect1 id="section-threads-scheduling">
    <title>Scheduling in &GStreamer;</title>

    <para>
      Scheduling of pipelines in &GStreamer; is done by using a thread for
      each <quote>group</quote>, where a group is a set of elements separated
      by <quote>queue</quote> elements. Within such a group, scheduling is
      either push-based or pull-based, depending on which mode is supported
      by the particular element. If elements support random access to data,
      such as file sources, then elements downstream in the pipeline become
      the entry point of this group (i.e. the element controlling the
      scheduling of other elements). The entry point pulls data from upstream
      and pushes data downstream, thereby calling data handling functions on
      either type of element.
    </para>
    <para>
      In practice, most elements in &GStreamer;, such as decoders, encoders,
      etc. only support push-based scheduling, which means that in practice,
      &GStreamer; uses a push-based scheduling model.
    </para>
  </sect1>
</chapter>