blob: cb33bc7966f0972dcf53cee8f6153bfd97652ddf [file] [log] [blame]
DRAFT push-pull scheduling
DRAFT. DEPRECATED by better current implementation.
- 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 useful 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, ...) Audio cards using a ringbuffer can
however do random access in the ringbuffer.
- 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 and 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_range() on the
source pad to activate the pads in push/pull mode.
- element proxies pull mode pad activation to peer pad.
- core makes a tough desicion without knowing anything about the
element. Some elements are able to deal with a pull_range()
without offset while others need full random access.
- element should be able to select scheduling method itself based on
how it can use the peer element pull_range. 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 efficient scheduling methods adjusted for the
particular element.
- 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_range function. A proposition for
the query is outlined in draft-query.txt.
- pad selects scheduling mode and informs the peer pad of this
Things to query:
- pad can do real random access (downstream peer can ask for offset != -1)
- min offset
- suggest sequential access
- max offset
- align: all offsets should be aligned with this value.
- pad can give ranges from A to B length (peer can ask for A <= length <= B)
- min length
- suggested length
- max length
Use cases:
- An audio source can provide random access to the samples queued in its
DMA buffer, it however suggests sequential access method.
An audio source can provide a random number of samples but prefers
reading from the hardware using a fixed segment size.
- A caching network source would suggest sequential access but is seekable
in the cached region. Applications can query for the already downloaded
portion and update the GUI, a seek can be done in that area.
- a live video source can only provide buffers sequentialy. It exposes
offsets as -1. lengths are also -1.