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Libav multithreading methods
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==============================================
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Libav provides two methods for multithreading codecs.
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Slice threading decodes multiple parts of a frame at the same time, using
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AVCodecContext execute() and execute2().
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Frame threading decodes multiple frames at the same time.
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It accepts N future frames and delays decoded pictures by N-1 frames.
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The later frames are decoded in separate threads while the user is
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displaying the current one.
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Restrictions on clients
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==============================================
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Slice threading -
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* The client's draw_horiz_band() must be thread-safe according to the comment
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  in avcodec.h.
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Frame threading -
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* Restrictions with slice threading also apply.
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* For best performance, the client should set thread_safe_callbacks if it
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  provides a thread-safe get_buffer() callback.
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* There is one frame of delay added for every thread beyond the first one.
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  Clients must be able to handle this; the pkt_dts and pkt_pts fields in
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  AVFrame will work as usual.
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Restrictions on codec implementations
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==============================================
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Slice threading -
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 None except that there must be something worth executing in parallel.
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Frame threading -
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* Codecs can only accept entire pictures per packet.
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* Codecs similar to ffv1, whose streams don't reset across frames,
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  will not work because their bitstreams cannot be decoded in parallel.
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* The contents of buffers must not be read before ff_thread_await_progress()
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  has been called on them. reget_buffer() and buffer age optimizations no longer work.
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* The contents of buffers must not be written to after ff_thread_report_progress()
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  has been called on them. This includes draw_edges().
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Porting codecs to frame threading
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==============================================
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Find all context variables that are needed by the next frame. Move all
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code changing them, as well as code calling get_buffer(), up to before
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the decode process starts. Call ff_thread_finish_setup() afterwards. If
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some code can't be moved, have update_thread_context() run it in the next
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thread.
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If the codec allocates writable tables in its init(), add an init_thread_copy()
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which re-allocates them for other threads.
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Add CODEC_CAP_FRAME_THREADS to the codec capabilities. There will be very little
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speed gain at this point but it should work.
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Call ff_thread_report_progress() after some part of the current picture has decoded.
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A good place to put this is where draw_horiz_band() is called - add this if it isn't
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called anywhere, as it's useful too and the implementation is trivial when you're
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doing this. Note that draw_edges() needs to be called before reporting progress.
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Before accessing a reference frame or its MVs, call ff_thread_await_progress().