[FFmpeg-devel] [PATCH 1/1] lavf/dashdec: Multithreaded DASH initialization

[Lukas Fellechner]

This patch adds multithreading support to DASH initialization. Initializing DASH streams is currently slow, because each individual stream is opened and probed sequentially. With DASH streams often having somewhere between 10-20 substreams, this can easily take up to half a minute on slow connections. This patch adds an "init-threads" option, specifying the max number of threads to use for parallel probing and initialization of substreams. If "init-threads" is set to a value larger than 1, multiple worker threads are spun up to massively bring down init times.

Here is a free DASH stream for testing:

http://www.bok.net/dash/tears_of_steel/cleartext/stream.mpd

It has 7 substreams. I currently get init times of 3.5 seconds without patch. The patch brings it down to about 0.5 seconds, so using 7 threads nearly cut down init times by factor 7. On a slower connection (100mbit), the same stream took 7-8 seconds to initialize, the patch brought it down to just over 1 second.

In the current patch, the behavior is disabled by default (init-threads = 0). But I think it could make sense to enable it by default, maybe with a reasonable value of 8? Not sure though, open for discussion.

Some notes on the actual implementation:
- DASH streams sometimes share a common init section. If this is the case, a mutex and condition is used, to make sure that the first stream reads the common section before the following streams start initialization.
- Only the init and probing part is done in parallel. After all threads are joined, I collect the results and add the AVStreams to the parent AVFormatContext. That is why I split open_demux_for_component() into begin_open_demux_for_component() and end_open_demux_for_component().
- I tried to do this as clean as possible and added multiple comments.
- Multithreading is never simple, so a proper review is needed.

If this gets merged, I might try to do the same for HLS.

This is my first PR by the way, so please be nice :)

Lukas
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