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The current design, where - proper init is called for the first per-thread context - first thread's private data is copied into private data for all the other threads - a "fixup" function is called for all the other threads to e.g. allocate dynamically allocated data is very fragile and hard to follow, so it is abandoned. Instead, the same init function is used to init each per-thread context. Where necessary, AVCodecInternal.is_copy can be used to differentiate between the first thread and the other ones (e.g. for decoding the extradata just once).
69 lines
2.9 KiB
Plaintext
69 lines
2.9 KiB
Plaintext
FFmpeg multithreading methods
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==============================================
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FFmpeg 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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Add AV_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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If there are inter-frame dependencies, so the codec calls
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ff_thread_report/await_progress(), set FF_CODEC_CAP_ALLOCATE_PROGRESS in
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AVCodec.caps_internal and use ff_thread_get_buffer() to allocate frames. The
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frames must then be freed with ff_thread_release_buffer().
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Otherwise decode directly into the user-supplied frames.
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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().
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