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Original commit message from CVS: * docs/design/draft-push-pull.txt: * docs/design/part-overview.txt: * docs/random/TODO-pre-0.9: * docs/random/old/ChangeLog.gstreamer: * gst/base/gstpushsrc.c: * gst/gstclock.c: fixed typos
119 lines
3.8 KiB
Text
119 lines
3.8 KiB
Text
DRAFT push-pull scheduling
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--------------------------
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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 methods requires an offset and a size.
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It is possible to leave the offset and size at -1, indicating that
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any offset or size is acceptable, this of course removes the advantages
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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
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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
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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.
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(video sources, ...)
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Current scheduling decision:
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- core selects scheduling type starting on sinks by looking at existence
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of loop function on sinkpad and calling _check_pull_range() on the
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source pad to activate the pads in 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 touch desicion without knowing anything about the
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element. Some elements are able to deal with a pull_range()
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without 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 afficient 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
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the 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 its
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DMA buffer, it however suggests sequential access method.
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An 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 seekable
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in the cached region. Applications can query for the already downloaded
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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
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offsets as -1. lengths are also -1.
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