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235 lines
7.8 KiB
Text
235 lines
7.8 KiB
Text
Scheduling
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----------
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The scheduling in GStreamer is based on pads actively pushing (producing) data or
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pad pulling in data (consuming) from other pads.
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Pushing
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~~~~~~~
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A pad can produce data and push it to the next pad. A pad that behaves this way
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exposes a loop function that will be called repeadedly until it returns false.
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The loop function is allowed to block whenever it wants. When the pad is deactivated
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the loop function should unblock though.
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A pad operating in the push mode can only produce data to a pad that exposes a
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chain function. This chain function will be called with the buffer produced by
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the pushing pad.
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This method of producing data is called the streaming mode since the producer
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produces a constant stream of data.
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Pulling
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~~~~~~~
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Pads that operate in pulling mode can only pull data from a pad that exposes the
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pull_range function. In this case, the sink pad exposes a loop function that will be
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called repeadedly until the task is stopped.
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After pulling data from the peer pad, the loop function will typically call the
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push function to push the result to the peer sinkpad.
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Deciding the scheduling mode
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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When a pad is activated, the _activate() function is called. The pad can then
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choose to activate itself in push or pull mode depending on upstream
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capabilities.
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The GStreamer core will by default activate pads in push mode when there is no
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activate function for the pad.
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The chain function
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~~~~~~~~~~~~~~~~~~
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The chain function will be called when a upstream element performs a _push() on the pad.
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The upstream element can be another chain based element or a pushing source.
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The getrange function
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~~~~~~~~~~~~~~~~~~~~~
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The getrange function is called when a peer pad performs a _pull_range() on the pad. This
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downstream pad can be a pulling element or another _pull_range() based element.
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Scheduling Query
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~~~~~~~~~~~~~~~~
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A sinkpad can ask the upstream srcpad for its scheduling attributes. It does
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this with the SCHEDULING query.
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(out) "pull-mode", G_TYPE_BOOLEAN (default FALSE)
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- if the pad can operate in pull mode, if this flag is not set the pad
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will operate in push mode.
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(out) "random-access", G_TYPE_BOOLEAN (default FALSE)
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- the offset of a pull operation can be specified, if this flag is false,
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the offset should be -1,
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(out) "sequential", G_TYPE_BOOLEAN (default TRUE)
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- suggest sequential access to the data. If random-access is specified,
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seeks are allowed but should be avoided. This is common for network
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streams. (
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(out) "minsize", G_TYPE_INT (default 1)
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- the suggested minimum size of pull requests
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(out) "maxsize", G_TYPE_INT (default -1, unlimited)
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- the suggested maximum size of pull requests
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(out) "align", G_TYPE_INT (default 1)
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- the suggested alignment for the pull requests.
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Plug-in techniques
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~~~~~~~~~~~~~~~~~~
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Multi-sink elements
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^^^^^^^^^^^^^^^^^^^
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Elements with multiple sinks can either expose a loop function on each of the pads to
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actively pull_range data or they can expose a chain function on each pad.
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Implementing a chain function is usually easy and allows for all possible scheduling
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methods.
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Pad select
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----------
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If the chain based sink wants to wait for one of the pads to receive a buffer, just
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implement the action to perform in the chain function. Be aware that the action could
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be performed in different threads and possibly simultaneously so grab the STREAM_LOCK.
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Collect pads
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------------
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If the chain based sink pads all require one buffer before the element can operate on
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the data, collect all the buffers in the chain function and perform the action when
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all chainpads received the buffer.
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In this case you probably also don't want to accept more data on a pad that has a buffer
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queued. This can easily be done with the following code snippet:
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static GstFlowReturn _chain (GstPad *pad, GstBuffer *buffer)
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{
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LOCK (mylock);
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while (pad->store != NULL) {
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WAIT (mycond, mylock);
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}
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pad->store = buffer;
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SIGNAL (mycond);
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UNLOCK (mylock);
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return GST_FLOW_OK;
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}
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static void _pull (GstPad *pad, GstBuffer **buffer)
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{
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LOCK (mylock);
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while (pad->store == NULL) {
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WAIT (mycond, mylock);
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}
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**buffer = pad->store;
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pad->store = NULL;
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SIGNAL (mycond);
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UNLOCK (mylock);
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}
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Cases
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~~~~~
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Inside the braces below the pads is stated what function the
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pad support:
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l: exposes a loop function, so it can act as a pushing source.
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g: exposes a getrange function
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c: exposes a chain function
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following scheduling decisions are made based on the scheduling
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methods exposed by the pads:
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(g) - (l): sinkpad will pull data from src
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(l) - (c): srcpad actively pushes data to sinkpad
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() - (c): srcpad will push data to sinkpad.
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() - () : not schedulable.
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() - (l): not schedulable.
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(g) - () : not schedulable.
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(g) - (c): not schedulable.
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(l) - () : not schedulable.
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(l) - (l): not schedulable
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() - (g): impossible
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(g) - (g): impossible.
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(l) - (g): impossible
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(c) - () : impossible
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(c) - (g): impossible
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(c) - (l): impossible
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(c) - (c): impossible
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+---------+ +------------+ +-----------+
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| filesrc | | mp3decoder | | audiosink |
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| src--sink src--sink |
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+---------+ +------------+ +-----------+
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(l-g) (c) () (c)
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When activating the pads:
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* audiosink has a chain function and the peer pad has no
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loop function, no scheduling is done.
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* mp3decoder and filesrc expose an (l) - (c) connection,
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a thread is created to call the srcpad loop function.
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+---------+ +------------+ +----------+
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| filesrc | | avidemuxer | | fakesink |
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| src--sink src--sink |
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+---------+ +------------+ +----------+
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(l-g) (l) () (c)
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* fakesink has a chain function and the peer pad has no
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loop function, no scheduling is done.
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* avidemuxer and filesrc expose an (g) - (l) connection,
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a thread is created to call the sinkpad loop function.
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+---------+ +----------+ +------------+ +----------+
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| filesrc | | identity | | avidemuxer | | fakesink |
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| src--sink src--sink src--sink |
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+---------+ +----------+ +------------+ +----------+
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(l-g) (c) () (l) () (c)
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* fakesink has a chain function and the peer pad has no
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loop function, no scheduling is done.
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* avidemuxer and identity expose no schedulable connection so
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this pipeline is not schedulable.
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+---------+ +----------+ +------------+ +----------+
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| filesrc | | identity | | avidemuxer | | fakesink |
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| src--sink src--sink src--sink |
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+---------+ +----------+ +------------+ +----------+
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(l-g) (c-l) (g) (l) () (c)
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* fakesink has a chain function and the peer pad has no
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loop function, no scheduling is done.
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* avidemuxer and identity expose an (g) - (l) connection,
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a thread is created to call the sinkpad loop function.
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* identity knows the srcpad is getrange based and uses the
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thread from avidemux to getrange data from filesrc.
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+---------+ +----------+ +------------+ +----------+
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| filesrc | | identity | | oggdemuxer | | fakesink |
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| src--sink src--sink src--sink |
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+---------+ +----------+ +------------+ +----------+
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(l-g) (c) () (l-c) () (c)
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* fakesink has a chain function and the peer pad has no
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loop function, no scheduling is done.
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* oggdemuxer and identity expose an () - (l-c) connection,
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oggdemux has to operate in chain mode.
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* identity chan only work chain based and so filesrc creates
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a thread to push data to identity.
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