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