gstreamer/docs/manual/cothreads.xml

<chapter id="chapter-cothreads">
  <title>Cothreads</title>
  <para>
    Cothreads are user-space threads that greatly reduce context switching overhead introduced by
    regular kernel threads. Cothreads are also used to handle the more complex elements. They differ
    from other user-space threading libraries in that they are scheduled explictly by GStreamer.
  </para>
  <para> 
    A cothread is created by a <classname>GstBin</classname> whenever an element is found
    inside the bin that has one or more of the following properties:
    <itemizedlist>
      <listitem>
        <para>
	  The element is loop-based instead of chain-based
        </para>
      </listitem>
      <listitem>
        <para>
	  The element has multiple input pads
        </para>
      </listitem>
      <listitem>
        <para>
	  The element has the MULTI_IN flag set
        </para>
      </listitem>
    </itemizedlist>
    The <classname>GstBin</classname> will create a cothread context for all the elements
    in the bin so that the elements will interact in cooperative
    multithreading.
  </para>
  <para> 
    Before proceding to the concept of loop-based elements we will first
    explain the chain-based elements.
  </para>

  <sect1 id="section-chain-based">
    <title>Chain-based elements</title>
    <para>
      Chain based elements receive a buffer of data and are supposed
      to handle the data and perform a gst_pad_push.
    </para>
    <para>
      The basic main function of a chain-based element is like:
    </para>
    <programlisting>
static void 
chain_function (GstPad *pad, GstBuffer *buffer)
{
  GstBuffer *outbuffer;

  ....
  // process the buffer, create a new outbuffer
  ...

  gst_pad_push (srcpad, outbuffer);
}
    </programlisting>
    <para>
      Chain based function are mainly used for elements that have a one to one
      relation between their input and output behaviour. An example of such an 
      element can be a simple video blur filter. The filter takes a buffer in, performs
      the blur operation on it and sends out the resulting buffer. 
    </para>
    <para>
      Another element, for example, is a volume filter. The filter takes audio samples as
      input, performs the volume effect and sends out the resulting buffer.
    </para>

  </sect1>

  <sect1 id="section-loop-based">
    <title>Loop-based elements</title>
    <para>
     As opposed to chain-based elements, loop-based elements enter an 
     infinite loop that looks like this:

    <programlisting>
  GstBuffer *buffer, *outbuffer;
 
  while (1) {
    buffer = gst_pad_pull (sinkpad);
    ...
    // process buffer, create outbuffer
    while (!done) {
      ....
      // optionally request another buffer
      buffer = gst_pad_pull (sinkpad);
      ....
    }
    ...
    gst_pad_push (srcpad, outbuffer);
  }
    </programlisting>

     The loop-based elements request a buffer whenever they need one.
    </para>

    <para>
      When the request for a buffer cannot be immediately satisfied, the control
      will be given to the source element of the loop-based element until it
      performs a push on its source pad. At that time the control is handed
      back to the loop-based element, etc... The execution trace can get
      fairly complex using cothreads when there are multiple input/output
      pads for the loop-based element. Cothread switches are performed within
      the call to gst_pad_pull and gst_pad_push; from the perspective of
      the loop-based element, it just "appears" that gst_pad_push (or _pull)
      might take a long time to return.
    </para>
    <para>
      Loop based elements are mainly used for the more complex elements
      that need a specific amount of data before they can start to produce
      output. An example of such an element is the MPEG video decoder. The
      element will pull a buffer, perform some decoding on it and optionally
      request more buffers to decode, and when a complete video frame has
      been decoded, a buffer is sent out. For example, any plugin using the
      bytestream library will need to be loop-based.
    </para>
    <para>
      There is no problem in putting cothreaded elements into a <classname>GstThread</classname> to
      create even more complex pipelines with both user and kernel space threads.
    </para>

  </sect1>
</chapter>