mirror of
https://gitlab.freedesktop.org/gstreamer/gstreamer.git
synced 2024-12-04 23:46:43 +00:00
ded7ca6616
Original commit message from CVS: Docs updates
121 lines
3.8 KiB
Text
121 lines
3.8 KiB
Text
<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>
|