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152 lines
7.2 KiB
XML
152 lines
7.2 KiB
XML
<chapter id="chapter-scheduler">
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<title>Scheduling</title>
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<para>
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By now, you've seen several example applications. All of them would set
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up a pipeline and call <function>gst_bin_iterate ()</function> to start
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media processing. You might have started wondering what happens during
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pipeline iteration. This whole process of media processing is called
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scheduling. Scheduling is considered one of the most complex parts of
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&GStreamer;. Here, we will do no more than give a global overview of
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scheduling, most of which will be purely informative. It might help in
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understanding the underlying parts of &GStreamer;.
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</para>
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<para>
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The scheduler is responsible for managing the plugins at runtime. Its
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main responsibilities are:
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<itemizedlist>
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<listitem>
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<para>
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Managing data throughput between pads and elements in a pipeline.
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This might sometimes imply temporary data storage between elements.
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</para>
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</listitem>
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<listitem>
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<para>
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Calling functions in elements that do the actual data processing.
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</para>
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</listitem>
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<listitem>
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<para>
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Monitoring state changes and enabling/disabling elements in the
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chain.
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</para>
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</listitem>
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<listitem>
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<para>
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Selecting and distributing the global clock.
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<!-- FIXME: is this still true? -->
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</para>
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</listitem>
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</itemizedlist>
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</para>
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<para>
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The scheduler is a pluggable component; this means that alternative
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schedulers can be written and plugged into GStreamer. There is usually
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no need for interaction in the process of choosing the scheduler, though.
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The default scheduler in &GStreamer; is called <quote>opt</quote>. Some
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of the concepts discussed here are specific to opt.
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</para>
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<sect1 id="section-scheduler-manage">
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<title>Managing elements and data throughput</title>
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<para>
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To understand some specifics of scheduling, it is important to know
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how elements work internally. Largely, there are four types of elements:
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<function>_chain ()</function>-based elements, <function>_loop
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()</function>-based elements, <function>_get ()</function>-based
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elements and decoupled elements. Each of those have a set of features
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and limitations that are important for how they are scheduled.
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</para>
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<itemizedlist>
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<listitem>
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<para>
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<function>_chain ()</function>-based elements are elements that
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have a <function>_chain ()</function>-function defined for each of
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their sinkpads. Those functions will receive data whenever input
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data is available. In those functions, the element can
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<emphasis>push</emphasis> data over its source pad(s) to peer
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elements. <function>_chain ()</function>-based elements cannot
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<emphasis>pull</emphasis> additional data from their sinkpad(s).
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Most elements in &GStreamer; are <function>_chain
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()</function>-based.
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</para>
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</listitem>
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<listitem>
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<para>
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<function>_loop ()</function>-based elements are elements that have
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a <function>_loop ()</function>-function defined for the whole
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element. Inside this function, the element can pull buffers from
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its sink pad(s) and push data over its source pad(s) as it sees fit.
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Such elements usually require specific control over their input.
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Muxers and demuxers are usually <function>_loop ()</function>-based.
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</para>
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</listitem>
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<listitem>
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<para>
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<function>_get ()</function>-based elements are elements with only
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source pads. For each source pad, a <function>_get
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()</function>-function is defined, which is called whenever the peer
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element needs additional input data. Most source elements are, in
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fact, <function>_get ()</function>-based. Such an element cannot
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actively push data.
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</para>
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</listitem>
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<listitem>
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<para>
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Decoupled elements are elements whose source pads are
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<function>_get ()</function>-based and whose sink pads are
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<function>_chain ()</function>-based. The <function>_chain
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()</function>-function cannot push data over its source pad(s),
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however. One such element is the <quote>queue</quote> element,
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which is a thread boundary element. Since only one side of such
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elements are interesting for one particular scheduler, we can
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safely handle those elements as if they were either
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<function>_get ()</function>- or <function>_chain
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()</function>-based. Therefore, we will further omit this type
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of elements in the discussion.
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</para>
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</listitem>
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</itemizedlist>
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<para>
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Obviously, the type of elements that are linked together have
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implications for how the elements will be scheduled. If a get-based
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element is linked to a loop-based element and the loop-based element
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requests data from its sinkpad, we can just call the get-function and
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be done with it. However, if two loop-based elements are linked to
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each other, it's a lot more complicated. Similarly, a loop-based
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element linked to a chain-based element is a lot easier than two
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loop-based elements linked to each other.
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</para>
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<para>
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The default &GStreamer; scheduler, <quote>opt</quote>, uses a concept
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of chains and groups. A group is a series of elements that
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do not require any context switches or intermediate data stores to
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be executed. In practice, this implies zero or one loop-based elements,
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one get-based element (at the beginning) and an infinite amount of
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chain-based elements. If there is a loop-based element, then the
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scheduler will simply call this elements loop-function to iterate.
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If there is no loop-based element, then data will be pulled from the
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get-based element and will be pushed over the chain-based elements.
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</para>
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<para>
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A chain is a series of groups that depend on each other for data.
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For example, two linked loop-based elements would end up in different
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groups, but in the same chain. Whenever the first loop-based element
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pushes data over its source pad, the data will be temporarily stored
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inside the scheduler until the loop-function returns. When it's done,
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the loop-function of the second element will be called to process this
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data. If it pulls data from its sinkpad while no data is available,
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the scheduler will <quote>emulate</quote> a get-function and, in this
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function, iterate the first group until data is available.
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</para>
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<para>
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The above is roughly how scheduling works in &GStreamer;. This has
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some implications for ideal pipeline design. An pipeline would
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ideally contain at most one loop-based element, so that all data
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processing is immediate and no data is stored inside the scheduler
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during group switches. You would think that this decreases overhead
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significantly. In practice, this is not so bad, however. It's something
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to keep in the back of your mind, nothing more.
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</para>
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</sect1>
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</chapter>
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