gstreamer/docs/design/draft-push-pull.txt
Wim Taymans c94f6bf5bf docs/design/: Some more docs.
Original commit message from CVS:
* docs/design/draft-push-pull.txt:
* docs/design/part-MT-refcounting.txt:
* docs/design/part-TODO.txt:
* docs/design/part-caps.txt:
* docs/design/part-events.txt:
* docs/design/part-gstbus.txt:
* docs/design/part-gstpipeline.txt:
* docs/design/part-messages.txt:
* docs/design/part-push-pull.txt:
* docs/design/part-query.txt:
Some more docs.
2005-04-21 09:37:34 +00:00

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DRAFT push-pull scheduling
--------------------------
Observations:
- The main scheduling mode is chain based scheduling where the source
element pushes buffers through the pipeline to the sinks. this is
called the push model
- In the pull model, some plugin pulls buffers from an upstream peer
element before consuming and/or pushing them further downstream.
Usages of pull based scheduling:
- sinks that pull in data, possibly at fixed intervals driven by some
hardware device (audiocard, videodevice, ...).
- Efficient random access to resources. Especially usefull for certain
types of demuxers.
API for pull-based scheduling:
- an element that wants to pull data from a peer element needs to call
the pull_range() method. This methods requires an offset and a size.
It is possible to leave the offset and size at -1, indicating that
any offset or size is acceptable, this of course removes the advantages
of getrange based scheduling.
Types of pull based scheduling:
- some sources can do random access (file source, ...)
- some sources can read a random number of bytes but not at a random
offset. (audio cards, ...)
- some sources can do random access in a range of bytes but not in
another range. (a caching network source).
- some sources can do a fixed size data but without an offset.
(video sources, ...)
Current scheduling decision:
- core selects scheduling type starting on sinks by looking at existence
of loop function on sinkpad and calling _check_pull_region() on the
source pad to activate the pads in push/pull mode.
- element proxies pull mode pad activation to peer pad.
Problems:
- core makes a touch desicion without knowing anything about the
element.
Requirements:
- element should be able to select scheduling method itself based on
how it can use the peer element pull_region. This includes if the
peer can operate with or without offset/size. This also means that
the core does not need to select the scheduling method anymore and
allows for more afficient scheduling methods adjusted for the
particular element.
Proposition:
- pads are activated without the core selecting a method.
- pads queries scheduling mode of peer pad. This query is rather
finegrained and allows the element to know if the peer supports
offsets and sizes in the get_region function.
- pad selects scheduling mode and informs the peer pad of this
decision.