gstreamer/docs/manual/cothreads.xml

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<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. 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="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 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>