Chapter 16. Threads

GStreamer is inherently multi-threaded, and is fully thread-safe. Most threading internals are hidden from the application, which should make application development easier. However, in some cases, applications may want to have influence on some parts of those. GStreamer allows applications to force the use of multiple threads over some parts of a pipeline.

16.1. When would you want to force a thread?

There are several reasons to force the use of threads. However, for performance reasons, you never want to use one thread for every element out there, since that will create some overhead. Let's now list some situations where threads can be particularly useful:

Figure 16-1. a two-threaded decoder with a queue

Above, we've mentioned the "queue" element several times now. A queue is the thread boundary element through which you can force the use of threads. It does so by using a classic provider/receiver model as learned in threading classes at universities all around the world. By doing this, it acts both as a means to make data throughput between threads threadsafe, and it can also act as a buffer. Queues have several GObject properties to be configured for specific uses. For example, you can set lower and upper tresholds for the element. If there's less data than the lower treshold (default: disabled), it will block output. If there's more data than the upper treshold, it will block input or (if configured to do so) drop data.

To use a queues (and therefore force the use of two distinct threads in the pipeline), one can simply create a "queue" element and put this in as part of the pipeline. GStreamer will take care of all threading details internally.