mirror of
https://gitlab.freedesktop.org/gstreamer/gstreamer.git
synced 2024-11-05 17:09:48 +00:00
41 lines
2 KiB
Text
41 lines
2 KiB
Text
|
PUSH vs PULL
|
||
|
|
|
||
|
|
(This is a writedown of ideas I plan to update whenever I find issues. This
|
||
|
|
should be fleshed out when doing the threadsafety / buffer passing changes.)
|
||
|
|
|
||
|
|
GStreamer allows 3 different forms of elements:
|
||
|
|
- loopbased:
|
||
|
|
Loopbased elements may push and pull from every pad however they want. They're
|
||
|
|
therefore heaviest on the scheduler and for latency or similar considerations.
|
||
|
|
- chainbased:
|
||
|
|
Chainbased elements have one sinkpad. Data gets pushed into a function attached
|
||
|
|
to the pad. => PUSH based
|
||
|
|
- getbased:
|
||
|
|
Getbased elements have no sinkpads. Every sourcepad has a getfunction that
|
||
|
|
produces data on request. => PULL based
|
||
|
|
|
||
|
|
The current problem is that PUSH and PULL based elements are quite limited in
|
||
|
|
what they may or may not do (especially PULL based elements). Some libraries
|
||
|
|
elements link to might also expect the architecture around them to work in a
|
||
|
|
specific way (libogg requires the ogg input to be pull-based, JACK is PULL-based
|
||
|
|
on sinks and PUSH-based on sinks). This often requires costly wrapping.
|
||
|
|
|
||
|
|
Inside a scheduler cothread switches can be avoided (which makes the pipeline
|
||
|
|
faster and scheduling less error-prone) if a PUSH-based element is connected on
|
||
|
|
the push-side of the pipeline. (Same goes with PULL-based on the pull-side). So
|
||
|
|
a pipeline with elements like "pull-pull-loop-push" works without cothread
|
||
|
|
switches while a pipeline like "push-loop-pull" needs 2. Even "push-pull" needs
|
||
|
|
one.
|
||
|
|
|
||
|
|
I'm under the impression that push-vs-pull is often best decided by the
|
||
|
|
application in use. (Video players probably prefer PULL because they display on
|
||
|
|
demand, live recorders prefer PUSH.) Since many elements probably don't care if
|
||
|
|
they're PUSH or PULL based it would be nice if an element could implement both
|
||
|
|
and use whatever suits better. (The code to achieve this should obviously be
|
||
|
|
abstracted into superclasses like videofilter.)
|
||
|
|
|
||
|
|
|
||
|
|
references:
|
||
|
|
MPlayer (http://www.mplayerhq.hu) - push-based architecture on version 1,
|
||
|
|
switching to PULL-based on version 2.
|