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64 lines
2.2 KiB
Markdown
64 lines
2.2 KiB
Markdown
# Queues
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A `queue` can appear almost anywhere in a GStreamer pipeline. Like most elements, it has an input (sink) and output (src). It has two uses:
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* As a thread boundary - i.e. The elements after a queue will run in a different thread to those before it
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* As a buffer, for when different parts of the pipeline may move at different speeds.
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Queues can generally be added anywhere in a pipeline. For example, a test stream:
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```
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gst-launch-1.0 videotestsrc ! autovideosink
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```
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This works just as well with a queue in the middle:
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```
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gst-launch-1.0 videotestsrc ! queue ! autovideosink
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```
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(If you [count the number of threads on the process](https://stackoverflow.com/questions/28047653/osx-how-can-i-see-the-tid-of-all-threads-from-my-process), you will see that this second example, with a queue, has one more.)
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Queues add latency, so the general advice is not to add them unless you need them.
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## Queue2
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Confusingly, `queue2` is not a replacement for `queue`. It's not obvious when to use one or the other.
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Most of the time, `queue2` appears to replace `queue` without issue. For example:
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```
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# Same as above, but with queue2 instead of queue:
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gst-launch-1.0 videotestsrc ! queue2 ! autovideosink
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```
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According to the [GStreamer tutorial](https://gstreamer.freedesktop.org/documentation/tutorials/basic/handy-elements.html), _as a rule of thumb, prefer queue2 over queue whenever network buffering is a concern to you._
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## Multiqueue
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The `multiqueue` can provide multiple queues. If, for example, you have a video and an audio queue, it can handle them both, and do a better job of allowing one to grow if the other is delayed.
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As a simple (pointless) example, it can be used to replace `queue` or `queue2`
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```
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# Same as above, but with multiqueue instead of queue/queue2:
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gst-launch-1.0 videotestsrc ! multiqueue ! autovideosink
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```
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A more realistic example is where there are two queues, such as here, for video and audio:
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```
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gst-launch-1.0 \
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videotestsrc ! queue ! autovideosink \
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audiotestsrc ! queue ! autoaudiosink
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```
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The two queues could be replaced with one multiqueue. Naming it (in this case, `q`) allows it to be referenced later.
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```
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gst-launch-1.0 \
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videotestsrc ! multiqueue name=q ! autovideosink \
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audiotestsrc ! q. q. ! autoaudiosink
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```
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