3.7 KiB
DRAFT push-pull scheduling
Status
DRAFT. DEPRECATED by better current implementation.
Observations:
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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
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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:
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sinks that pull in data, possibly at fixed intervals driven by some hardware device (audiocard, videodevice, …).
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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:
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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 offset. (audio cards, …) Audio cards using a ringbuffer can however do random access in the ringbuffer.
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some sources can do random access in a range of bytes but not in another range. (a caching network source).
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some sources can do a fixed size data and without an offset. (video sources, …)
Current scheduling decision:
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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:
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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 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:
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pad can do real random access (downstream peer can ask for offset
!= -1) -
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) -
min length
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suggested length
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max length
Use cases:
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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.
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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.
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a live video source can only provide buffers sequentialy. It exposes offsets as -1. lengths are also -1.