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116 lines
3.7 KiB
Markdown
116 lines
3.7 KiB
Markdown
# DRAFT push-pull scheduling
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Status
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DRAFT. DEPRECATED by better current implementation.
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Observations:
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- The main scheduling mode is chain based scheduling where the source
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element pushes buffers through the pipeline to the sinks. this is
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called the push model
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- In the pull model, some plugin pulls buffers from an upstream peer
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element before consuming and/or pushing them further downstream.
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Usages of pull based scheduling:
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- sinks that pull in data, possibly at fixed intervals driven by some
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hardware device (audiocard, videodevice, …).
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- Efficient random access to resources. Especially useful for certain
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types of demuxers.
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API for pull-based scheduling:
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- an element that wants to pull data from a peer element needs to call
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the `pull_range()` method. This method requires an offset and a
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size. It is possible to leave the offset and size at -1, indicating
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that any offset or size is acceptable, this of course removes the
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advantages of getrange based scheduling.
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Types of pull based scheduling:
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- some sources can do random access (file source, …)
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- some sources can read a random number of bytes but not at a random
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offset. (audio cards, …) Audio cards using a ringbuffer can however
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do random access in the ringbuffer.
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- some sources can do random access in a range of bytes but not in
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another range. (a caching network source).
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- some sources can do a fixed size data and without an offset. (video
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sources, …)
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Current scheduling decision:
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- core selects scheduling type starting on sinks by looking at
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existence of loop function on sinkpad and calling
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`_check_pull_range()` on the source pad to activate the pads in
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push/pull mode.
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- element proxies pull mode pad activation to peer pad.
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Problems:
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- core makes a tough desicion without knowing anything about the
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element. Some elements are able to deal with a `pull_range()` without
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offset while others need full random access.
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Requirements:
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- element should be able to select scheduling method itself based on
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how it can use the peer element `pull_range()`. This includes if the
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peer can operate with or without offset/size. This also means that
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the core does not need to select the scheduling method anymore and
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allows for more efficient scheduling methods adjusted for the
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particular element.
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Proposition:
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- pads are activated without the core selecting a method.
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- pads queries scheduling mode of peer pad. This query is rather
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finegrained and allows the element to know if the peer supports
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offsets and sizes in the `get_range()` function. A proposition for the
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query is outlined in draft-query.txt.
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- pad selects scheduling mode and informs the peer pad of this
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decision.
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Things to query:
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- pad can do real random access (downstream peer can ask for offset `!= -1`)
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- min offset
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- suggest sequential access
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- max offset
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- align: all offsets should be aligned with this value.
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- pad can give ranges from A to B length (peer can ask for `A ⇐ length ⇐ B`)
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- min length
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- suggested length
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- max length
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Use cases:
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- An audio source can provide random access to the samples queued in
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its DMA buffer, it however suggests sequential access method. An
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audio source can provide a random number of samples but prefers
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reading from the hardware using a fixed segment size.
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- A caching network source would suggest sequential access but is
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seekable in the cached region. Applications can query for the
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already downloaded portion and update the GUI, a seek can be done in
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that area.
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- a live video source can only provide buffers sequentialy. It exposes
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offsets as -1. lengths are also -1.
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