PeerStreamer

FFmpeg multithreading methods

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FFmpeg provides two methods for multithreading codecs.

Slice threading decodes multiple parts of a frame at the same time, using

AVCodecContext execute() and execute2().

Frame threading decodes multiple frames at the same time.

It accepts N future frames and delays decoded pictures by N-1 frames.

The later frames are decoded in separate threads while the user is

displaying the current one.

Restrictions on clients

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Slice threading -

* The client's draw_horiz_band() must be thread-safe according to the comment

  in avcodec.h.

Frame threading -

* Restrictions with slice threading also apply.

* For best performance, the client should set thread_safe_callbacks if it

  provides a thread-safe get_buffer() callback.

* There is one frame of delay added for every thread beyond the first one.

  Clients must be able to handle this; the pkt_dts and pkt_pts fields in

  AVFrame will work as usual.

Restrictions on codec implementations

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Slice threading -

 None except that there must be something worth executing in parallel.

Frame threading -

* Codecs can only accept entire pictures per packet.

* Codecs similar to ffv1, whose streams don't reset across frames,

  will not work because their bitstreams cannot be decoded in parallel.

* The contents of buffers must not be read before ff_thread_await_progress()

  has been called on them. reget_buffer() and buffer age optimizations no longer work.

* The contents of buffers must not be written to after ff_thread_report_progress()

  has been called on them. This includes draw_edges().

Porting codecs to frame threading

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Find all context variables that are needed by the next frame. Move all

code changing them, as well as code calling get_buffer(), up to before

the decode process starts. Call ff_thread_finish_setup() afterwards. If

some code can't be moved, have update_thread_context() run it in the next

thread.

If the codec allocates writable tables in its init(), add an init_thread_copy()

which re-allocates them for other threads.

Add CODEC_CAP_FRAME_THREADS to the codec capabilities. There will be very little

speed gain at this point but it should work.

Call ff_thread_report_progress() after some part of the current picture has decoded.

A good place to put this is where draw_horiz_band() is called - add this if it isn't

called anywhere, as it's useful too and the implementation is trivial when you're

doing this. Note that draw_edges() needs to be called before reporting progress.

Before accessing a reference frame or its MVs, call ff_thread_await_progress().