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41 lines
2 KiB
Text
41 lines
2 KiB
Text
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PUSH vs PULL
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(This is a writedown of ideas I plan to update whenever I find issues. This
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should be fleshed out when doing the threadsafety / buffer passing changes.)
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GStreamer allows 3 different forms of elements:
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- loopbased:
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Loopbased elements may push and pull from every pad however they want. They're
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therefore heaviest on the scheduler and for latency or similar considerations.
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- chainbased:
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Chainbased elements have one sinkpad. Data gets pushed into a function attached
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to the pad. => PUSH based
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- getbased:
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Getbased elements have no sinkpads. Every sourcepad has a getfunction that
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produces data on request. => PULL based
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The current problem is that PUSH and PULL based elements are quite limited in
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what they may or may not do (especially PULL based elements). Some libraries
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elements link to might also expect the architecture around them to work in a
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specific way (libogg requires the ogg input to be pull-based, JACK is PULL-based
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on sinks and PUSH-based on sinks). This often requires costly wrapping.
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Inside a scheduler cothread switches can be avoided (which makes the pipeline
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faster and scheduling less error-prone) if a PUSH-based element is connected on
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the push-side of the pipeline. (Same goes with PULL-based on the pull-side). So
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a pipeline with elements like "pull-pull-loop-push" works without cothread
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switches while a pipeline like "push-loop-pull" needs 2. Even "push-pull" needs
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one.
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I'm under the impression that push-vs-pull is often best decided by the
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application in use. (Video players probably prefer PULL because they display on
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demand, live recorders prefer PUSH.) Since many elements probably don't care if
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they're PUSH or PULL based it would be nice if an element could implement both
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and use whatever suits better. (The code to achieve this should obviously be
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abstracted into superclasses like videofilter.)
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references:
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MPlayer (http://www.mplayerhq.hu) - push-based architecture on version 1,
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switching to PULL-based on version 2.
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