mirror of
https://gitlab.freedesktop.org/gstreamer/gstreamer.git
synced 2024-12-29 19:50:40 +00:00
70cfc6cb4d
Original commit message from CVS: * new parser that uses flex and bison - doesn't do dynamic pipelines yet... * added GErrors to the gst_parse_launch[v] api * added --gst-mask-help command line option * fixed -o option for gst-launch * GstElement api change: - gst_element_get_pad - gst_element_get_request_pad, gst_element_get_static_pad - gst_element_get_compatible_pad - gst_element_get_compatible_static_pad, gst_element_get_compatible_request_pad - gst_element_[dis]connect -> gst_element_[dis]connect_pads - gst_element_[dis]connect_elements -> gst_element_[dis]connect * manual update * example, tool, and doc updates for the api changes - no more plugin docs in the core docs, plugins require a more extensive doc system
121 lines
4.2 KiB
XML
121 lines
4.2 KiB
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. 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 immediatly 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. 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, 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. 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>
|