gstreamer/markdown/design/draft-push-pull.md

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 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.

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.