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121 lines
3.8 KiB
Text
121 lines
3.8 KiB
Text
<chapter id="cha-cothreads">
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<title>Cothreads</title>
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<para>
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Cothreads are user-space threads that greatly reduce context
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switching overhead introduced by regular kernel threads.
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Cothreads are also used to handle the more complex elements.
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</para>
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<para>
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A cothread is created by a <classname>GstBin</classname> whenever an element is found
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inside the bin that has one or more of the following properties:
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<itemizedlist>
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<listitem>
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<para>
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The element is loop-based instead of chain-based
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</para>
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</listitem>
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<listitem>
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<para>
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The element has multiple input pads
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</para>
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</listitem>
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<listitem>
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<para>
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The element has the MULTI_IN flag set
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</para>
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</listitem>
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</itemizedlist>
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The <classname>GstBin</classname> will create a cothread context for all the elements
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in the bin so that the elements will interact in cooperative
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multithreading.
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</para>
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<para>
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Before proceding to the concept of loop-based elements we will first
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explain the chain-based elements
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</para>
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<sect1 id="sec-chain-based">
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<title>Chain-based elements</title>
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<para>
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Chain based elements receive a buffer of data and are supposed
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to handle the data and perform a gst_pad_push.
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</para>
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<para>
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The basic main function of a chain-based element is like:
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</para>
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<programlisting>
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static void
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chain_function (GstPad *pad, GstBuffer *buffer)
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{
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GstBuffer *outbuffer;
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....
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// process the buffer, create a new outbuffer
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...
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gst_pad_push (srcpad, outbuffer);
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}
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</programlisting>
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<para>
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Chain based function are mainly used for elements that have a one to one
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relation between their input and output behaviour. An example of such an
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element can be a simple video blur filter. The filter takes a buffer in, performs
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the blur operation on it and sends out the resulting buffer.
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</para>
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<para>
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Another element, for example, is a volume filter. The filter takes audio samples as
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input, performs the volume effect and sends out the resulting buffer.
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</para>
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</sect1>
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<sect1 id="sec-loop-based">
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<title>Loop-based elements</title>
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<para>
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As opposed to chain-based elements, Loop-based elements enter an
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infinite loop that looks like this:
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<programlisting>
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GstBuffer *buffer, *outbuffer;
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while (1) {
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buffer = gst_pad_pull (sinkpad);
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...
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// process buffer, create outbuffer
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while (!done) {
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....
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// optionally request another buffer
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buffer = gst_pad_pull (sinkpad);
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....
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}
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...
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gst_pad_push (srcpad, outbuffer);
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}
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</programlisting>
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The loop-based elements request a buffer whenever they need one.
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</para>
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<para>
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When the request for a buffer cannot immedialty satisfied, the control
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will be given to the source element of the loop-based element until it
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performs a push on its source pad. At that time the control is handed back
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to the loop-based element, etc... The the execution trace can get fairly
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complex using cothreads when there are multiple input/output pads for the
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loop-based element.
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</para>
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<para>
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Loop based elements are mainly used for the more complex elements that need a
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specific amount of data before they can start to produce output. An example
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of such an element is the mpeg video decoder. the element will pull a buffer,
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performs some decoding on it and optionally requests more buffers to decode, when
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a complete video frame has been decoded, a buffer is send out.
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</para>
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<para>
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There is no problem in putting cothreaded elements into a
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<classname>GstThread</classname> to create even more complex pipelines with
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both user and kernel space threads.
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</para>
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</sect1>
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</chapter>
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