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67 lines
2.8 KiB
67 lines
2.8 KiB
FFmpeg multithreading methods |
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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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* Custom get_buffer2() and get_format() callbacks must be thread-safe. |
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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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