gstreamer/docs/manual/cothreads.xml

<chapter id="cha-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.
  </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="sec-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="sec-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 immedialty 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 the execution trace can get fairly
      complex using cothreads when there are multiple input/output pads for the
      loop-based element.
    </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,
      performs some decoding on it and optionally requests more buffers to decode, when
      a complete video frame has been decoded, a buffer is send out.
    </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>