gstreamer/docs/manual/schedulers.xml

<chapter id="cha-scheduler">
  <title>Understanding schedulers</title>
  <para> 
    The scheduler is responsible for managing the plugins at runtime. Its
    main responsibilities are:
    <itemizedlist>
      <listitem>
        <para>
	  Preparing the plugins so they can be scheduled. 
        </para>
      </listitem>
      <listitem>
        <para>
	  Monitoring state changes and enabling/disabling the element in the
	  chain.
        </para>
      </listitem>
      <listitem>
        <para>
	  Choosing an element as the entry point for the pipeline.
        </para>
      </listitem>
      <listitem>
        <para>
	  Selecting and distributing the global clock.
        </para>
      </listitem>
    </itemizedlist>
  </para>
  <para> 
    The scheduler is a pluggable component; this means that alternative
    schedulers can be written and plugged into GStreamer. The default scheduler
    uses cothreads to schedule the plugins in a pipeline. Cothreads are fast
    and lightweight user-space threads.
  </para>
  <para> 
    There is usually no need to interact with the scheduler directly, however
    in some cases it is feasible to set a specific clock or force a specific
    plugin as the entry point in the pipeline.
  </para>

</chapter>