mirror of
https://gitlab.freedesktop.org/gstreamer/gstreamer.git
synced 2024-12-11 19:06:33 +00:00
115 lines
3.7 KiB
Markdown
115 lines
3.7 KiB
Markdown
# DRAFT push-pull scheduling
|
|
|
|
Status
|
|
|
|
DRAFT. DEPRECATED by better current implementation.
|
|
|
|
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 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 method 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.
|
|
|
|
Problems:
|
|
|
|
- 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.
|
|
|
|
Requirements:
|
|
|
|
- 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.
|
|
|
|
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_range()` function. A proposition for the
|
|
query is outlined in draft-query.txt.
|
|
|
|
- pad selects scheduling mode and informs the peer pad of this
|
|
decision.
|
|
|
|
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.
|