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>Chapter 11. How scheduling works</H1
><P
> Scheduling is, in short, a method for making sure that every element gets
called once in a while to process data and prepare data for the next
element. Likewise, a kernel has a scheduler to for processes, and your
brain is a very complex scheduler too in a way.
Randomly calling elements' chain functions won't bring us far, however, so
you'll understand that the schedulers in <SPAN
CLASS="application"
>GStreamer</SPAN
> are a bit more complex
than this. However, as a start, it's a nice picture.
<SPAN
CLASS="application"
>GStreamer</SPAN
> currently provides two schedulers: a <SPAN
CLASS="emphasis"
><I
CLASS="emphasis"
>basic</I
></SPAN
>
scheduler and an <SPAN
CLASS="emphasis"
><I
CLASS="emphasis"
>optimal</I
></SPAN
> scheduler. As the name says,
the basic scheduler (<SPAN
CLASS="QUOTE"
>"basic"</SPAN
>) is an unoptimized, but very
complete and simple scheduler. The optimal scheduler (<SPAN
CLASS="QUOTE"
>"opt"</SPAN
>),
on the other hand, is optimized for media processing, but therefore also
more complex.
</P
><P
> Note that schedulers only operate on one thread. If your pipeline contains
multiple threads, each thread will run with a separate scheduler. That is
the reason why two elements running in different threads need a queue-like
element (a <CODE
CLASS="classname"
>DECOUPLED</CODE
> element) in between them.
</P
><DIV
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><H1
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><A
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>11.1. The Basic Scheduler</A
></H1
><P
> The <SPAN
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><I
CLASS="emphasis"
>basic</I
></SPAN
> scheduler assumes that each element is its
own process. We don't use UNIX processes or POSIX threads for this,
however; instead, we use so-called <SPAN
CLASS="emphasis"
><I
CLASS="emphasis"
>co-threads</I
></SPAN
>.
Co-threads are threads that run besides each other, but only one is active
at a time. The advantage of co-threads over normal threads is that they're
lightweight. The disadvantage is that UNIX or POSIX do not provide such a
thing, so we need to include our own co-threads stack for this to run.
</P
><P
> The task of the scheduler here is to control which co-thread runs at what
time. A well-written scheduler based on co-threads will let an element run
until it outputs one piece of data. Upon pushing one piece of data to the
next element, it will let the next element run, and so on. Whenever a
running element requires data from the previous element, the scheduler will
switch to that previous element and run that element until it has provided
data for use in the next element.
</P
><P
> This method of running elements as needed has the disadvantage that a lot
of data will often be queued in between two elements, as the one element
has provided data but the other element hasn't actually used it yet. These
storages of in-between-data are called <SPAN
CLASS="emphasis"
><I
CLASS="emphasis"
>bufpens</I
></SPAN
>, and
they can be visualized as a light <SPAN
CLASS="QUOTE"
>"queue"</SPAN
>.
</P
><P
> Note that since every element runs in its own (co-)thread, this scheduler
is rather heavy on your system for larger pipelines.
</P
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