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- Libav multithreading methods
- ==============================================
-
- Libav 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
- ==============================================
-
- 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
- ==============================================
-
- 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
- ==============================================
-
- 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().
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